Mitochondrial reactive oxygen species production by fish muscle mitochondria: Potential role in acute heat-induced oxidative stress

Heat stress induces oxidative stress and weakens the immune system in catfish Clarias magur: Evidence from physiological, histological, and transcriptomic analyses

Climate change is unequivocal, causing a rise in the Earth's temperature, which ultimately impacts all ecosystems. However, aquatic ecosystems are most severely affected by rising temperatures resulting in huge losses to aquaculture industry. The present study investigated the oxidative stress, histopathological changes, and transcriptomic responses in a freshwater catfish Clarias magur subjected to acute heat stress. Fish were exposed to four different temperatures, i.e., 28, 30, 32, and 34 °C, for 96 h to assess their heat tolerance and adaptation behavior. Fish kept at 26 °C were considered the control group. Elevated levels of key antioxidative enzymes such as catalase, glutathione reductase, and superoxide dismutase, were recorded in vital organs (gills, kidney, liver, and rosette). High rates of lipid peroxidation were also observed in the gills, kidney, liver, and rosette. An analysis of the top 25 differentially expressed genes of the gill transcriptome revealed that 72 percent of the transcripts were represented by innate and adaptive immune response genes. Downregulation of BOLA class I and MHC class I molecules indicated impaired immunity whereas, upregulation of MHC class II beta chain and GTPase IMAP8 suggested a compensatory immune response. These findings were also supported by the observed histoarchitectural alterations, such as disintegration of the skin barrier, hepatic and nephrotic apoptosis, tissue hyperplasia, macrophage infiltration, and development of splenic granulomas. This study provides important insights into physiological and molecular mechanisms underlying acute heat stress responses. Understanding these mechanisms is important for developing mitigation strategies to improve the sustainability and resilience of commercially important catfish under continuously changing climatic conditions.

The cell origin of reactive oxygen species and its implication for evolutionary trade-offs

The allocation of resources in animals is shaped by adaptive trade-offs aimed at maximizing fitness. At the heart of these trade-offs, lies metabolism and the conversion of food resources into energy, a process mostly occurring in mitochondria. Yet, the conversion of nutrients to utilizable energy molecules (adenosine triphosphate) inevitably leads to the by-production of reactive oxygen species (ROS) that may cause damage to important biomolecules such as proteins or lipids. The 'ROS theory of ageing' has thus proposed that the relationship between lifespan and metabolic rate may be mediated by ROS production. However, the relationship is not as straightforward as it may seem: not only are mitochondrial ROS crucial for various cellular functions, but mitochondria are also actually equipped with antioxidant systems, and many extra-mitochondrial sources also produce ROS. In this review, we discuss how viewing the mitochondrion as a regulator of cellular oxidative homeostasis, not merely a ROS producer, may provide new insights into the role of oxidative stress in the reproduction-survival trade-off. We suggest several avenues to test how mitochondrial oxidative buffering capacity might complement current bioenergetic and evolutionary studies.

Harpagifer bispinis, but not Patagonotothen tessellata, appears robust to interactive effects of ocean warming and acidification in southern Patagonia

Ocean warming and acidification challenge marine ectotherms with rapid, multiple and simultaneous environmental changes. As knowledge of these impacts on fish from the sub-Antarctic is scarce, this study seeks to explore the combined effects of warming and acidification on the thermal and metabolic responses of Patagonotothen tessellata and Harpagifer bispinis, two sympatric notothenioid fish from the Beagle Channel. Juveniles were exposed to present-day and near-future summer temperatures (∼10 and 13 °C) and pCO2 levels (∼500 and 1300 μatm) in a full factorial design. Their critical thermal minimum/maximum (CTmin/CTmax) were assessed and their partial thermal tolerance polygons were estimated. Oxygen consumption rates allowed us to calculate fish' aerobic scope (AS) as the difference between the standard and maximum metabolic rates (SMR and MMR). The CTmin of both species were affected by temperature, pCO2 level and their interaction, while the CTmax of P. tessellata was affected by both factors and that of H. bispinis, only by temperature. The partial thermal tolerance polygon of P. tessellata significantly decreased with future pCO2 levels, while no changes were observed for H. bispinis. In P. tessellata, SMR and MMR were affected by temperature and pCO2 levels and the AS by their interaction. Conversely, H. bispinis showed no differences in SMR, MMR and AS under different conditions. The increase in SMR and decrease in AS of P. tessellata with future temperatures and pCO2 levels may explain the changes in its thermal tolerance, while for H. bispinis, either the species has a greater capacity to adapt its metabolic response to warming and acidification, or different physiological processes are responsible for the observed changes in its thermal tolerance. Overall, present information could be a valuable tool for forecasting shifts in habitat suitability across the distribution range of both species and other similar fish in the context of climate change.

Interactive effects of elevated temperature and venlafaxine on mitochondrial respiration and enzymatic capacity in Nile tilapia (Oreochromis niloticus)

Warming events are becoming more frequent and extreme in aquatic environments worldwide. Concurrently, many environments are polluted with biologically active compounds such as pharmaceuticals. Understanding how these challenges interact is critical for understanding the climate crisis, as contaminants may modulate how ectotherms respond to heat stress or vice versa. One potential site for these heat × contaminant interactions is the mitochondrion, which is central to metabolism, implicated in thermal tolerance, and evolutionarily conserved. Using high-resolution respirometry, we investigated how acute warming (to 35 °C, 40 °C, or 45 °C from 25 °C) impacted the respiration, coupling, and metabolic capacity of liver mitochondria isolated from Nile tilapia, and how exposure to environmentally relevant levels of the ubiquitous antidepressant venlafaxine modulated those effects. Mitochondria exposed to hotter temperatures had higher respiration rates and decreased respiratory control ratio compared to mitochondria exposed to cooler temperatures. The depressive effects of venlafaxine on respiration rates through complex I and II or complex II only (State 3 and State 4), as well as complex IV-linked respiration, were mild except in mitochondria exposed to high temperatures, suggesting an interactive effect of warming and contaminant exposure. Finally, we found that the maximal enzyme activity of intact mitochondria (represented by mitochondrial respiration) showed a different pattern of response to warming and venlafaxine compared to its underlying components (as reflected by the activity of succinate dehydrogenase [complex II] and cytochrome c oxidase [complex IV]), demonstrating the value of incorporating both interactive and reductive approaches in understanding how mitochondria cope with anthropogenic changes in the environment.

Exploring the immunological functions of thioredoxin domain-containing protein 17 (TXNDC17) in chub mackerel (Scomber japonicus): Immune response and cellular redox homeostasis

All organisms have evolved sophisticated antioxidant networks and enzymes to counteract reactive radicals, among which thioredoxin (Trx) systems are especially noteworthy. Thioredoxin domain-containing protein 17 (TXNDC17) is a ubiquitously expressed enzyme with oxidoreductase activity belonging to the Trx protein family. This study successfully uncovered and analyzed the TXNDC17 gene in Scomber japonicus (SjTXNDC17). The gene consists of a 372-base-pair coding sequence that encodes a protein of 123 amino acids, with an estimated molecular weight of 14.1 kDa. Structural analysis revealed that SjTXNDC17 contains a TRX-related protein 14 domain with two redox-responsive cysteine residues in the 42WCPDC46 motif. Spatial expression analysis indicated that SjTXNDC17 had the highest constitutive expression in the brain. Stimulation with polyinosinic-polycytidylic acid (poly I:C), Vibrio harveyi, and Streptococcus iniae, significantly upregulated the mRNA levels of SjTXNDC17 in the head kidney. The antioxidant activity of the recombinant SjTXNDC17 protein was evidenced by 2,2-Diphenyl-1-picryl-hydrazyl-hydrate (DPPH) radical scavenging, insulin reduction, and cupric ion-reducing antioxidant capacity assays. SjTXNDC17 overexpression in fathead minnow (FHM) cells significantly reduced reactive oxygen species (ROS) levels and decreased apoptosis. The anti-apoptotic effect was driven by the upregulation of the Bcl2 gene and the downregulation of the Bax gene, as well as the suppression of JNK signaling pathway genes. Moreover, overexpression of SjTXNDC17 facilitated M2 polarization and suppressed nitric oxide production in macrophages. Collectively, these results demonstrate that SjTXNDC17 plays a crucial role in both the immune response and cellular redox balance in Scomber japonicus.

Temperature alters antioxidant status and induces cell damage in the Amazonian fish tambaqui

Since Amazonian fish live close to their maximum thermal limits, this makes them vulnerable to the effects of global warming. The aim of this study was to evaluate the oxidative stress and antioxidant enzymatic and biochemical responses of the plasma, liver and muscle of tambaqui (Colossoma macropomum) exposed to a rising gradient of water temperature. One hundred and twenty (N = 120) juvenile tambaqui were exposed to four temperature levels, these being: the environmental temperature of the season (Tenv - 25.7-30 °C), 31 °C, 34 °C and 37 °C, following a completely randomized design with three replicates for a period of 60 days. Liver and muscle samples were used to determine the levels of the enzymes superoxide dismutase (SOD), catalase (CAT) glutathione peroxidase (GPx) and lipid peroxidation (LPO). Plasma levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were measured. A histopathological damage assessment (HAI) was performed using liver samples and the results showed an increase in lipid peroxidation in the muscle and liver of animals kept at 37 °C in relation to other temperatures. Enzyme responses were tissue-specific in the liver and muscle. In the liver, the reduction of CAT, SOD and GPx levels of the animals was observed at 37 °C compared to those maintained at Tenv and SOD and GPx in relation to animals maintained at 31 and 34 °C. The GPx enzyme showed higher activity at 34 and 37 °C compared to the other evaluated temperatures. At 37 °C, plasma levels of ALT and AST were higher than the other temperatures evaluated, as well as an increase in histopathological damage. In this way, in a scenario of warming of the waters of the Amazon or even of the systems used for rearing of the species, the tambaqui will be able to cope with temperatures of up to 34 °C, without affecting its antioxidant capacity. However, at 37 °C, oxidative stress levels and increased liver damage suggest a reduction in antioxidant capacity due to tissue impairment of the organ and general loss of animal performance as it approaches the upper thermal limit of the species.

Effect of dietary supplementation of selenium-L-methionine on growth, antioxidant capacity and resistance to nitrite stress of spotted seabass (Lateolabrax maculatus) under two rearing water temperatures

A 10-week feeding trial, followed by 24-h nitrite stress, was performed to evaluate the effects of dietary selenium-L-methionine (Se-Met) on growth, Se accumulation, antioxidant capacity, transcripts of selenoproteins and histological changes of muscle as well as resistance to nitrite stress in spotted seabass (Lateolabrax maculatus) reared at optimal (27 °C) and high (33 °C) temperatures. Five experimental diets were formulated to contain 0, 0.9, 1.8, 3.5, and 7.0 mg Se-Met/kg. Each diet was fed to fish (2.60 ± 0.2 g) in two parallel treatments at 27 or 33 °C. The results showed that elevated temperature (33 °C) induced thermal stress in fish, and fish under thermal stress exhibited lower weight gain and hepatosomatic index but a higher condition factor compared to those reared at 27 °C. However, the growth and feed utilisation were promoted in L. maculatus with 0.9 to 3.5 mg/kg Se-Met treatments. The protein and lipid content in the muscle increased with the dietary Se-Met level, and the total Se level in the whole body and muscle showed a linear increase with dietary Se-Met supplementation. Thermal stress changed the histology of the muscle, leading to raised levels of malondialdehyde (MDA), reduced antioxidant parameters in the serum and liver, and a decrease in the transcripts of selenoprotein genes in the muscle. Meanwhile, increased antioxidant capacity of serum and liver and up-regulated transcripts of selenoprotein of muscle were observed in L. maculatus reaching a maximum with 3.5 mg Se-Met/kg treatment. After 24 h of nitrite stress, thermal stress exacerbated oxidative damage caused by nitrite stress in L. maculatus. In contrast, dietary Se-Met enhanced the resistance to nitrite stress of L. maculatus fed with Se-Met enriched diets containing 0.9 to 1.8 mg Se-Met/kg. Based on the effects of dietary Se-Met on the growth, antioxidant capacity and resistance to nitrite stress of L. maculatus, this study suggests that the optimal range of Se-Met supplementation in L. maculatus diets is 1.80 to 2.39 mg Se-Met/kg of diet at 27 °C and 1.80 to 4.46 mg Se-Met/kg of diet at 33 °C.

Heat stress mediates toxicity of rutile titanium dioxide nanoparticles on fertilisation capacity in the broadcast spawning mussel Mytilus galloprovincialis

Titanium dioxide nanoparticle (nTiO2) pollution of marine environments is rapidly increasing with potentially deleterious effects on wildlife. Yet, the impacts of nTiO2 on reproduction remain poorly understood. This is especially the case for broadcast spawners, who are likely to be more severely impacted by environmental disturbances because their gametes are directly exposed to the environment during fertilisation. In addition, it is unclear whether rising water temperatures will further exacerbate the impact of nTiO2 toxicity. Here, in a series of fertilisation trials, we systematically examine the main and interactive effects of nTiO2 exposure and seawater temperature on fertilisation success in the Mediterranean mussel Mytilus galloprovincialis. Specifically, our fertilisation trials explored whether nTiO2 exposure influences fertilisation rates when (i) eggs alone are exposed, (ii) both sperm and eggs are exposed simultaneously, and (iii) whether increases in seawater temperature interact with nTiO2 exposure to influence fertilisation rates. We also ask whether changes in nTiO2 concentrations influence key sperm motility traits using computer-assisted sperm analysis (CASA). In fertilisation trials for treatment groups (i) and (ii), we found no main effects of nTiO2 at environmentally relevant concentrations of 5, 10 and 50 μg L-1 on fertilisation capacity relative to the control. Consistent with these findings, we found no effect of nTiO2 exposure on sperm motility. However, in treatment group (iii), when fertilisation trials were conducted at higher temperatures (+6 °C), exposure of gametes from both sexes to 10 μg L-1 nTiO2 led to a reduction in fertilisation rates that was significantly greater than when gametes were exposed to elevated temperature alone. These interacting effects of nTiO2 exposure and seawater temperature demonstrate the toxic potential of nTiO2 for fertilisation processes in a system that is likely to be impacted heavily by predicted future increases in sea surface temperatures.

Hypoxia Tolerance of Two Killifish Species

Hypoxia tolerance in aquatic ectotherms involves a suite of behavioral and physiological responses at the organismal, tissue, and cellular levels. The current study evaluated two closely related killifish species (Fundulus heteroclitus, Fundulus majalis) to evaluate responses to acute moderate and acute severe hypoxia. Routine metabolic rate and loss of equilibrium were assessed, followed by analysis in skeletal muscle of markers of oxidative damage to proteins (2,4-DNPH), lipids (4-HNE), and DNA (8-OHdG), hypoxia signaling (HIF1α, HIF2α), cellular energy state (p-AMPK: AMPK), and protein degradation (Ubiquitin, LC3B, Calpain 2, Hsp70). Both species had a similar reduction in metabolic rate at low PO2. However, F. heteroclitus was the more hypoxia-tolerant species based on a lower PO2 at which there was loss of equilibrium, perhaps due in part to a lower oxygen demand at all oxygen tensions. Despite the differences in hypoxia tolerance between the species, skeletal muscle molecular markers were largely insensitive to hypoxia, and there were few differences in responses between the species. Thus, the metabolic depression observed at the whole animal level appears to limit perturbations in skeletal muscle in both species during the hypoxia treatments.

Asparagopsis taxiformis as a Novel Antioxidant Ingredient for Climate-Smart Aquaculture: Antioxidant, Metabolic and Digestive Modulation in Juvenile White Seabream (Diplodus sargus) Exposed to a Marine Heatwave

The increasing frequency and duration of marine heatwaves (MHWs) due to climate change pose severe threats to aquaculture, causing drastic physiological and growth impairments in farmed fish, undermining their resilience against additional environmental pressures. To ensure sustainable production that meets the global seafood demand and animal welfare standards, cost-effective and eco-friendly strategies are urgently needed. This study explored the efficacy of the red macroalga Asparagopsis taxiformis on juvenile white seabream Diplodus sargus reared under optimal conditions and upon exposure to a MHW. Fish were fed with four experimental diets (0%, 1.5%, 3% or 6% of dried powdered A. taxiformis) for a prophylactic period of 30 days (T30) and subsequently exposed to a Mediterranean category II MHW for 15 days (T53). Biometric data and samples were collected at T30, T53 and T61 (8 days post-MHW recovery), to assess performance indicators, biomarker responses and histopathological alterations. Results showed that A. taxiformis supplementation improved catalase and glutathione S-transferase activities and reduced lipid peroxidation promoted by the MHW, particularly in fish biofortified with 1.5% inclusion level. No histopathological alterations were observed after 30 days. Additionally, fish biofortified with 1.5% A. taxiformis exhibited increased citrate synthase activity and fish supplemented with 1.5% and 3% showed improved digestive enzyme activities (e.g., pepsin and trypsin activities). Overall, the present findings pointed to 1.5% inclusion as the optimal dosage for aquafeeds biofortification with A. taxiformis, and confirmed that this seaweed species is a promising cost-effective ingredient with functional properties and great potential for usage in a climate-smart context.

Alterations in mitochondrial structure and function in response to environmental temperature changes in Apostichopus japonicus

Global temperatures have risen as a result of climate change, and the resulting warmer seawater will exert physiological stresses on many aquatic animals, including Apostichopus japonicus. It has been suggested that the sensitivity of aquatic poikilothermal animals to climate change is closely related to mitochondrial function. Therefore, understanding the interaction between elevated temperature and mitochondrial functioning is key to characterizing organisms' responses to heat stress. However, little is known about the mitochondrial response to heat stress in A. japonicus. In this work, we investigated the morphological and functional changes of A. japonicus mitochondria under three representative temperatures, control temperature (18 °C), aestivation temperature (25 °C) and heat stress temperature (32 °C) temperatures using transmission electron microscopy (TEM) observation of mitochondrial morphology combined with proteomics and metabolomics techniques. The results showed that the mitochondrial morphology of A. japonicus was altered, with decreases in the number of mitochondrial cristae at 25 °C and mitochondrial lysis, fracture, and vacuolization at 32 °C. Proteomic and metabolomic analyses revealed 103 differentially expressed proteins and 161 differential metabolites at 25 °C. At 32 °C, the levels of 214 proteins and 172 metabolites were significantly altered. These proteins and metabolites were involved in the tricarboxylic acid (TCA) cycle, substance transport, membrane potential homeostasis, anti-stress processes, mitochondrial autophagy, and apoptosis. Furthermore, a hypothetical network of proteins and metabolites in A. japonicus mitochondria in response to temperature changes was constructed based on proteomic and metabolomic data. These results suggest that the dynamic regulation of mitochondrial energy metabolism, resistance to oxidative stress, autophagy, apoptosis, and mitochondrial morphology in A. japonicus may play important roles in the response to elevated temperatures. In summary, this study describes the response of A. japonicus mitochondria to temperature changes from the perspectives of morphology, proteins, and metabolites, which provided a better understanding the mechanisms of mitochondrial regulation under environment stress in marine echinoderms.

Characterization of myosin heavy chain (MYH) genes and their differential expression in white and red muscles of Chinese perch, Siniperca chuatsi

Fish skeletal muscle is composed of two anatomically and functionally different fiber layers, white or fast and red or slow muscles. Myosin, the major structural protein of fish skeletal muscle, contains multiple myosin heavy chain (MYH) isoforms involved in the high plasticity of muscle in response to varying functional demands and/or environmental changes. In this study, we comparatively assayed the cellular and ultrastructural feature of white and red skeletal muscles. Then, a total of 28 class II myosin heavy chain genes were identified in by searching the Chinese perch genome database. Among them, 14 genes code for the fast-muscle-type myosin heavy chain, and 7 genes code for the slow-muscle-type myosin heavy chain. Further, the different isoform gene structures, function domains, phylogenetic relations, and muscle-fiber type-specific expression were characterized. This is the first systematic work on the molecular characterization of class II myosin heavy chain isoforms and the differential analysis of their expression in red and white muscle tissues in Chinese perch Siniperca chuatsi. Our work provided valuable information for a better understanding of myh genes and their molecular characteristics, and the correlations of multiple myosin isoforms with potential functions in response to varying functional demands and/or environmental changes.

Dietary Debaryomyces hansenii promotes skin and skin mucus defensive capacities in a marine fish model

The present study explores the effects of two supplementation levels of Debaryomyces hansenii (1.1% and 2.2%) as a probiotic in a reference low fish meal-based diet on the skin mucosal tissue in Sparus aurata. This study includes the evaluation of fish performance coupled with a holistic study of the skin mucosa: i) a transcriptomic study of the skin tissue, and ii) the evaluation of its secreted mucus both in terms of skin mucosal-associated biomarkers and its defensive capacity by means of co-culture analysis with two pathogenic bacteria. Results showed that after 70 days of diet administration, fish fed the diet supplemented with D. hansenii at 1.1% presented increased somatic growth and a better feed conversion ratio, compared to fish fed the control diet. In contrast, fish fed the diet including 2.2% of the probiotic presented intermediate values. Regarding gene regulation, the probiotic administration at 1.1% resulted in 712 differentially expressed genes (DEGs), among which 53.4% and 46.6% were up- and down-regulated, respectively. In particular, D. hansenii modulated some skin biological processes related to immunity and metabolism. Specifically, D. hansenii administration induced a strong modulation of some immune biological-related processes (61 DEGs), mainly involved in B- and T-cell regulatory pathways. Furthermore, dietary D. hansenii promoted the skin barrier function by the upregulation of anchoring junction genes (23 DEGs), which reinforces the physical defense against potential skin damage. In contrast, the skin showed modulated genes related to extracellular exosome and membrane organization (50 DEGs). This modulated functioning is of great interest, particularly in relation to the increased skin mucus defensive capacity observed in the bacterial co-culture in vitro trials, which could be related to the increased modulation and exudation of the innate immune components from the skin cells into the mucus. In summary, the modulation of innate immune parameters coupled with increased skin barrier function and cell trafficking potentiates the skin's physical barrier and mucus defensive capacity, while maintaining the skin mucosa's homeostatic immune and metabolic status. These findings confirmed the advantages of D. hansenii supplementation in low fish meal-based diets, demonstrating the probiotic benefits on cultured marine species.

Redox state and metabolic responses to severe heat stress in lenok Brachymystax lenok (Salmonidae)

In order to provide new insights into the physiological responses of lenok (Brachymystax lenok: Salmonidae) to acute and severe heat stress (25°C, 48 h), dynamic changes in redox state and metabolic responses are studied combined biochemical index and non-targeted metabolome. Nicotinamide adenine dinucleotide (NAD⁺) consumption causes significant increases in ratio of reduced NADH to NAD⁺ and ratio of reduced nicotinamide adenine dinucleotide phosphate (NADPH) to NADP⁺, which induced the redox imbalance in heat stressed lenok. Lowered reduced glutathione/oxidized glutathione (GSH/GSSG) ratios suggested that more oxidized conditions occurred in heat-stressed lenok, leading to membrane lipid oxidation. The first few hours of heat stress promoted the activity of enzymes involved in anaerobic glycolysis (hexokinase, pyruvate kinase, lactic dehydrogenase) and glutamicpyruvic transaminase and glutamic oxaloacetic transaminase, which might lead to consumption of many carbohydrates and amino acid catabolism. These enzyme activities decreased with time in a possible compensatory strategy to manage anabolic and catabolic metabolism, maintaining the redox homeostasis. After 48 h of recovery, NAD⁺, carbohydrate levels and enzyme activities had returned to control levels, whereas many amino acids were consumed for repair and new synthesis. GSH remained at levels lower than controls, and the more oxidized conditions had not recovered, aggravating oxidative damage. Glutamic acid, glutamine, lysine and arginine may play important roles in survival of heat-stressed lenok.

Enzymatic responses reveal different physiological strategies employed by eurytolerant fish during extreme hot and cold cycling acclimation temperatures

Heat waves and cold snaps are projected to rise in magnitude, duration, interval, and harshness in the coming years. The current literature examining thermal impacts on the physiology of organisms rarely uses chronic, variable thermal acclimations despite the fact that climate change predictions project a more variable environment. If we are to determine species' susceptibility to climate change, chronic and variable lab acclimations should be prioritized. Here, we acclimated the eurytolerant sheepshead minnow (Cyprinodon variegatus) to two extreme cycling thermal regimes: one warm [resting 27 °C with a spike to 33 °C for 8 h daily], one cold [resting 6.5 °C with a spike to 12 °C for 8 h daily], and three chronically stable conditions (10, 22, and 30 °C) for comparison. We measured enzymatic antioxidants (catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx)), total antioxidant capacity, lipid peroxidation (LPO) damage, and citrate synthase (CS) activity in white epaxial muscle. Of particular note, we found significant increases in log CAT activity and SOD concentration in the warm cycling temperatures, and significant increases in GPx activity in the cold cycling temperatures. We found no significant accumulation of LPO damage in any of our thermal acclimation treatments. Thus, sheepshead minnows demonstrate two particularly different mechanisms towards dealing with thermal variation in low and high temperatures. The enzymatic differences between low and high cycling temperatures may define pathways of eurytolerant organisms and how they may survive predicted variability in thermal regimes.

Changing ROS, NAD and AMP: A path to longevity via mitochondrial therapeutics

Lifespan of many organisms, from unicellular yeast to extremely complex human organism, strongly depends on the genetic background and environmental factors. Being among most influential target energy metabolism is affected by macronutrients, their caloric values, and peculiarities of catabolism. Mitochondria are central organelles that respond for energy metabolism in eukaryotic cells. Mitochondria generate reactive oxygen species (ROS), which are lifespan modifying metabolites and a kind of biological clock. Oxidized nicotinamide adenine dinucleotide (NAD⁺) and adenosine monophosphate (AMP) are important metabolic intermediates and molecules that trigger or inhibit several signaling pathways involved in gene silencing, nutrient allocation, and cell regeneration and programmed death. A part of NAD⁺ and AMP metabolism is tied to mitochondria. Using substances that able to target mitochondria, as well as allotopic expression of specific enzymes, are envisioned to be innovative approaches to prolong lifespan by modulation of ROS, NAD⁺, and AMP levels. Among substances, an anti-diabetic drug metformin is believed to increase NAD⁺ and AMP levels, indirectly influencing histone deacetylases, involved in gene silencing, and AMP-activated protein kinase, an energy sensor of cells. Mitochondrially targeted derivatives of ubiquinone were found to interact with ROS. A mitochondrially targeted non-proton-pumping NADH dehydrogenase may influence both ROS and NAD⁺ levels. Chapter describes putative how mitochondria-targeted drugs and NADH dehydrogenase extend lifespan, perspectives of creating drugs with similar properties and their usage as senotherapeutic pills are discussed in the chapter.

Blood biomarkers as diagnostic tools: An overview of climate-driven stress responses in fish

Global climate change due to anthropogenic activities affects the dynamics of aquatic communities by altering the adaptive capacities of their inhabitants. Analysis of blood provides valuable insights in the form of a comprehensive representation of the physiological and functional status of fish under various environmental and treatment conditions. This review synthesizes currently available information about blood biomarkers used in climate change induced stress responses in fish. Alterations in informative blood-based indicators are used to monitor the physiological fitness of individual fishes or entire populations. Specific characteristics of fish blood, such as serum and plasma metabolites, cell composition, cellular abnormalities, cellular and antioxidant enzymes necessitate adapted protocols, as well as careful attention to experimental designs and meticulous interpretation of patterns of data. Moreover, the sampling technique, transportation, type of culture system, acclimation procedure, and water quality must all be considered for valid interpretation of hemato-biochemical parameters. Besides, blood collection, handling, and storage time of blood samples can all have significant impacts on the results of a hematological analysis, so it is optimal to perform hemato-biochemical evaluations immediately after blood collection because long-term storage can alter the results of the analyses, at least in part as a result of storage-related degenerative changes that may occur. However, the scarcity of high-throughput sophisticated approaches makes fish blood examination studies promising for climate-driven stress responses in fish.

Intestinal metabolomics of juvenile lenok (Brachymystax lenok) in response to heat stress

Changes in the metabolic profile within the intestine of lenok (Brachymystax lenok) when challenged to acute and lethal heat stress (HS) are studied using no-target HPLC-MS/MS metabonomic analysis. A total of 51 differentially expressed metabolites (VIP > 1, P < 0.05) were identified in response to HS, and 34 occurred in the positive ion mode and 17 in negative ion mode, respectively. After heat stress, changes in metabolites related to glycolysis (i.e., alpha-D-glucose, stachyose, and L-lactate) were identified. The metabolites (acetyl carnitine, palmitoylcarnitine, carnitine, and erucic acid) related to fatty acid β-oxidation accumulated significantly, and many amino acids (L-tryptophan, D-proline, L-leucine, L-phenylalanine, L-aspartate, L-tyrosine, L-methionine, L-histidine, and L-glutamine) were significantly decreased in HS-treated lenok. The mitochondrial β-oxidation pathway might be inhibited, while severe heat stress might activate the anaerobic glycolysis and catabolism of amino acid for energy expenditure. Oxidative damage in HS-treated lenok was indicated by the decreased glycerophospholipid metabolites (i.e., glycerophosphocholine, 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphoethanolamine, 1-palmitoyl-sn-glycero-3-phosphocholine, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine, and 1, 2-dioleoyl-sn-glycero-3-phosphatidylcholine) and the increased oxylipin production (12-HETE and 9R, 10S-EpOME). The minor oxidative pathways (omega-oxidation and peroxisomal beta-oxidation) were likely to be induced in HS-treated lenok.

Absence of mitochondrial responses in muscles of zebrafish exposed to several heat waves

Heat waves are extreme thermal events whose frequency and intensity will increase with global warming. As metabolic responses to temperature are time-dependent, we explored the effects of an exposure to several heat waves on the mitochondrial metabolism of zebrafish Danio rerio. For this purpose, zebrafish were acclimated at 26 °C or 31 °C for 4 weeks and some fish acclimated at 26 °C underwent 2 types of heat waves: 2 periods of 5 days at 31 °C or 10 days at 31 °C. After this acclimation period, mitochondrial respiration of red muscle fibres was measured at 26 °C and 31 °C for each fish, with the phosphorylation (OXPHOS) and basal (LEAK) respirations obtained with activation of complex I, complex II or complexes I and II. The respiratory control ratio (RCR) and the mitochondrial aerobic scope (CAS) were also calculated at both temperatures after the activation of complexes I and II. Under our conditions, heat waves did not result in variations in any mitochondrial parameters, suggesting a high tolerance of zebrafish to environmental temperature fluctuations. However, an acute in vitro warming led to an increase in the LEAK respiration together with a higher temperature effect on complex II than complex I, inducing a decrease of mitochondrial efficiency to produce energy at high temperatures. Increased interindividual variability for some parameters at 26 °C or 31 °C also suggests that each individual has its own ability to cope with temperature fluctuations.

Impacts on antioxidative enzymes and transcripts in darter (Etheostoma spp.) brains in the Grand River exposed to wastewater effluent

The Grand River watershed is the largest in southern Ontario and assimilates thirty wastewater treatment plants (WWTP) with varied degrees of treatment. Many WWTPs are unable to effectively eliminate several contaminants of emerging concern (CECs) from final effluent, leading to measurable concentrations in surface waters. Exposures to CECs have reported impacts on oxidative stress measured through antioxidative enzymes (SOD, CAT, GPX). This study focuses on the effects of WWTP effluent on four Etheostoma (Darter) species endemic to the Grand River, by investigating if increased antioxidative response markers are present in darter brains downstream from the effluent outfall compared to an upstream reference site relative to the Waterloo, ON WWTP across two separate years (Oct 2020 and Oct 2021). This was assessed using transcriptional and enzyme analysis of antioxidant enzymes and an enzyme involved in serotonin synthesis, tryptophan hydroxylase (tph). In fall 2020, significant differences in transcript markers were found between sites and sexes in GSD with SOD and CAT showing increased expression downstream, in JD with both sexes showing increased SOD downstream, and an interactive effect for tph in RBD. Changes in transcripts aligned with enzyme activity where interactive effects with sex-related differences were observed in fish collected fall 2020. In contrast, transcripts measured in fall 2021 were increased upstream compared to downstream species in RBD and GSD. This study additionally displayed yearly, species and sex differences in antioxidant responses. Continued investigation on the impacts of CECs in effluent in non-target species is required to better understand WWTP effluent impacts.

Rates of warming impact oxidative stress in zebrafish (Danio rerio)

Potentially negative effects of thermal variation on physiological functions may be modulated by compensatory responses, but their efficacy depends on the timescale of phenotypic adjustment relative to the rate of temperature change. Increasing temperatures in particular can affect mitochondrial bioenergetics and rates of reactive oxygen species (ROS) production. Our aim was to test whether different rates of temperature increase impact mitochondrial bioenergetics and modulate oxidative stress. We exposed zebrafish (Danio rerio) to warming from 20 to 28°C over 3, 6, 24, or 48 h, and compared these to a control group that was kept at constant 20°C. Fish exposed to the fastest (3 h) and slowest (48 h) rates of warming had significantly higher rates of H2O2 production relative to the control treatment, and the proportion of O2 converted to H2O2 (H2O2/O2 ratio) was significantly greater in these groups. However, ROS production was not paralleled by differences in mitochondrial substrate oxidation rates, leak respiration rates, or coupling (respiratory control ratios). Increased rates of ROS production did not lead to damage of proteins or membranes, which may be explained by a moderate increase in catalase activity at the fastest, but not the slowest rate of warming. The increase in ROS production at the slowest rate of heating indicates that even seemingly benign environments may be stressful. Understanding how animals respond to different rates of temperature change is important, because the rate determines the time period for phenotypic adjustments and it also alters the environmental thermal signal that triggers compensatory pathways.

Mitochondrial connections with immune system in Zebrafish

Mitochondria are organelles commonly associated with adenosine triphosphate (ATP) formation through the oxidative phosphorylation (OXPHOS) process. However, mitochondria are also responsible for functions such as calcium homeostasis, apoptosis, autophagy, and production of reactive oxygen species (ROS) that, in conjunction, can lead to different cell fate decisions. Mitochondrial morphology changes rely on nutrients' availability and the bioenergetics demands of the cells, in a process known as mitochondrial dynamics, which includes both fusion and fission. This organelle senses the microenvironment and can modify the cells to either a pro or anti-inflammatory profile. The zebrafish has been increasingly used to research mitochondrial dynamics and its connection with the immune system since the pathways and molecules involved in these processes are conserved on this fish. Several genetic tools and technologies are currently available to analyze the behavior of mitochondria in zebrafish. However, even though zebrafish presents several similar processes known in mammals, the effect of the mitochondria in the immune system has not been so broadly studied in this model. In this review, we summarize the current knowledge in zebrafish studies regarding mitochondrial function and immuno metabolism.

Heavy metals bioaccumulation and subsequent multiple biomarkers based appraisal of toxicity in the critically endangered Tor putitora

The construction of hydropower projects discharges effluents to aquatic bodies. The effluents consist of different chemicals including heavy metals. The current study assessed the effects of effluents discharged from an under-construction hydropower project on the bioaccumulation of heavy metals in the tissues of critically endangered Tor putitora (Hamilton, 1822) in the river Panjkora. The subsequent toxic impacts of higher bioaccumulation of heavy metals on different biochemical, hematological, and serum biochemical profiles were also studied. Different biochemical changes were observed in the tissues of T. putitora including stress biomarkers such as reactive oxygen species, lipid peroxidation, total protein contents, antioxidant enzymes (peroxidase, superoxide dismutase, catalase, reduced glutathione, glutathione reductase, and glutathione-s-transferase), acetylcholinesterase, and whole-body cortisol. The hematotoxic effects were also observed as the count of red blood cells, hematocrit, and hemoglobin decreased whereas the count of white blood cells increased. Serum biochemical analysis revealed that cholesterol, urea, total bilirubin, and glucose concentration increased, whereas total proteins and albumin decreased with an increase in the concentration of heavy metals across the sampling sites. The fish from the river was found to be under severe stress as compared to the fish from the reference site. To mitigate the current scenario, stocking fish in an appropriate amount is suggested. The fish diversity and water quality should be assessed at regular intervals to avoid further deterioration and diversity loss. The safety and conservation of wild fisheries should be ensured by implementing strict environmental protection and fishing laws.

A higher mitochondrial content is associated with greater oxidative damage, oxidative defenses, protein synthesis and ATP turnover in resting skeletal muscle

An unavoidable consequence of aerobic metabolism is the production of reactive oxygen species (ROS). Mitochondria have historically been considered the primary source of ROS; however, recent literature has highlighted the uncertainty in primary ROS production sites and it is unclear how variation in mitochondrial density influences ROS-induced damage and protein turnover. Fish skeletal muscle is composed of distinct, highly aerobic red muscle and anaerobic white muscle, offering an excellent model system in which to evaluate the relationship of tissue aerobic capacity and ROS-induced damage under baseline conditions. The present study used a suite of indices to better understand potential consequences of aerobic tissue capacity in red and white muscle of the pinfish, Lagodon rhomboides. Red muscle had a 7-fold greater mitochondrial volume density than white muscle, and more oxidative damage despite also having higher activity of the antioxidant enzymes superoxide dismutase and catalase. The dominant protein degradation system appears to be tissue dependent. Lysosomal degradation markers and autophagosome volume density were greater in white muscle, while ubiquitin expression and 20S proteasome activity were significantly greater in red muscle. However, ubiquitin ligase expression was significantly higher in white muscle. Red muscle had a more than 2-fold greater rate of translation and total ATP turnover than white muscle, results that may be due in part to the higher mitochondrial density and the associated increase in oxidative damage. Together, these results support the concept that an elevated aerobic capacity is associated with greater oxidative damage and higher costs of protein turnover.

Effects of temperature and salinity on antioxidant responses in livers of temperate (Dicentrarchus labrax) and tropical (Chanos Chanos) marine euryhaline fish

Temperature and salinity are abiotic factors that affect physiological responses in aquaculture species. The European sea bass (Dicentrarchus labrax) is a temperate species that is generally farmed at 18 °C in seawater (SW). In the wild, its incursions in shallow habitats such as lagoons may result in hyperthermal damage despite its high thermal tolerance. Meanwhile, the milkfish (Chanos chanos), a tropical species, is generally reared at 28 °C, and in winter, high mortality usually occurs under hypothermal stress such as cold snaps. This study compared changes in hepatic antioxidant enzymes (superoxide dismutase, SOD; and catalase, CAT) in these two important marine euryhaline aquaculture species in Europe and Southeast Asia, respectively, under temperature challenge combined with hypo-osmotic (fresh water, FW) stress. After a four-week hyper- or hypo-thermal treatment, hepatic SOD activity was upregulated in both species reared in SW and FW, indicating enhanced oxidative stress in European sea bass and milkfish. The expression profiles of sod isoforms suggested that in milkfish, the increase in reactive oxygen species (ROS) was mainly at the cytosol level, leading to increased sod1 expression. In European sea bass, however, no obvious difference was found between the expression of sod isoforms at different temperatures. A lower expression of sod2 was observed in FW compared to SW in the latter species. Moreover, no significant change was observed in the mRNA expression and activity of CAT in the livers of these two species under the different temperature treatments, with the exception of the lower CAT activity in milkfish challenged with SW at 18 °C. Taken together, our results indicated that the antioxidant responses were not changed under long-term hypoosmotic challenge but were enhanced during the four-week temperature treatments in livers of both the temperate and tropical euryhaline species.

The Atlantic salmon's stress- and immune-related transcriptional responses to moderate hypoxia, an incremental temperature increase, and these challenges combined

The marine environment is predicted to become warmer, and more hypoxic, and these conditions may negatively impact the health and survival of coastal fish species, including wild and farmed Atlantic salmon (Salmo salar). Thus, we examined how: (1) moderate hypoxia (∼70% air saturation) at 12°C for 3 weeks; (2) an incremental temperature increase from 12°C to 20°C (at 1°C week-1) followed by 4 weeks at 20°C; and (3) treatment "2" combined with moderate hypoxia affected transcript expression in the liver of post-smolts as compared to control conditions (normoxia, 12°C). Specifically, we assessed the expression of 45 genes related to the heat shock response, oxidative stress, apoptosis, metabolism and immunity using a high-throughput qPCR approach (Fluidigm Biomark™ HD). The expression profiles of 27 "stress"-related genes indicated that: (i) moderate hypoxia affected the expression of several stress genes at 12°C; (ii) their expression was impacted by 16°C under normoxic conditions, and this effect increased until 20°C; (iii) the effects of moderate hypoxia were not additive to those at temperatures above 16°C; and (iv) long-term (4 weeks) exposure to 20°C, with or without hypoxia, resulted in a limited acclimatory response. In contrast, the expression of 15 immune-related genes was not greatly affected until temperatures reached 20°C, and this effect was particularly evident in fish exposed to the added challenge of hypoxia. These results provide valuable information on how these two important environmental factors affect the "stress" physiology and immunology of Atlantic salmon, and we identify genes that may be useful as hypoxia and/or temperature biomarkers in salmonids and other fishes.

Ecology of oxidative stress in the Danube barbel (Barbus balcanicus) from a winegrowing district: Effects of water parameters, trace and rare earth elements on biochemical biomarkers

Fish can be highly vulnerable to environmental pressures because they are exposed to oxidative stressors in the aquatic environment. Such stressors can affect the levels of antioxidant biomarkers against reactive oxygen species (ROS). With this study we investigated the oxidative stress ecology in Danube barbel (Barbus balcanicus) from the Barbucina creek (northeast Italy), a watercourse in the Collio winegrowing district. To do this, superoxide dismutase (SOD), catalase (CAT), and glutathione S-transferase (GST) activity was measured in gills, liver, and muscle, while metallothioneins (MT) and trace and rare earth elements (REEs) levels were determined in muscle. The effect of environmental factors (physicochemical parameters of water, trace elements and REEs) on oxidative stress biomarkers was thus assessed. High concentrations were determined for cerium (Ce), scandium (Sc), neodymium (Nd), lanthanum (La), yttrium (Y), and praseodymium (Pr) among the REEs. Among the trace elements, arsenic (As), copper (Cu), and mercury (Hg) levels were higher compared to published data, suggesting their role as stressors. The multiple linear regression (MLR) model showed a statistically significant association (R² = 0.858; F = 10.07; p = 0.015) between As, Cu, Hg, and Pr and SOD activity in the gills, indicating a functional relationship between them. Differently, CAT activity was significantly higher in the liver, probably in response to long-term Cu contamination of the watercourse. This was confirmed by the MLR model that showed a significant association (R² = 0.638; F = 8.152; p = 0.02) between the concentration of MT and of Cu. Our data show a biochemical defensive response by Danube barbel to the disturbances in the aquatic ecosystem of the Barbucina creek. These insights advance our understanding of the role and the effects of environmental factors as trace elements and REEs on oxidative stress in fish.

Review: Using isolated mitochondria to investigate mitochondrial hydrogen peroxide metabolism

Mitochondria are recognized as centrally important to cellular reactive oxygen species (ROS), both as a potential source and due to their substantial antioxidant capacity. While much of the initial ROS formed by mitochondria is superoxide, this is rapidly converted to hydrogen peroxide (H₂O₂) which more readily crosses membranes making H₂O₂ important in both redox signalling mechanisms and conditions of oxidative stress. Here I outline our studies on mitochondrial H₂O₂ metabolism with a focus on some of the challenges and strategies involved with developing an integrated model of mitochondria being intrinsic regulators of H₂O₂. This view of mitochondria as regulators of H₂O₂ goes beyond the simpler contention of them being net producers or consumers. Moreover, the integration of both consumption and production can then be tied to a putative mechanism linking energy sensing at the level of the mitochondrial protonmotive force. This mechanism would provide a means of mitochondria communicating their energetic status the extramitochondrial compartment via local H₂O₂ concentrations. I conclude by explaining how these concepts developed using rodent muscle as a model have high relevance and applicability to comparative studies.

The transcriptomic responses of Atlantic salmon (Salmo salar) to high temperature stress alone, and in combination with moderate hypoxia

BACKGROUND

Increases in ocean temperatures and in the frequency and severity of hypoxic events are expected with climate change, and may become a challenge for cultured Atlantic salmon and negatively affect their growth, immunology and welfare. Thus, we examined how an incremental temperature increase alone (Warm & Normoxic-WN: 12 → 20 °C; 1 °C week- 1), and in combination with moderate hypoxia (Warm & Hypoxic-WH: ~ 70% air saturation), impacted the salmon's hepatic transcriptome expr\ession compared to control fish (CT: 12 °C, normoxic) using 44 K microarrays and qPCR.

RESULTS

Overall, we identified 2894 differentially expressed probes (DEPs, FDR < 5%), that included 1111 shared DEPs, while 789 and 994 DEPs were specific to WN and WH fish, respectively. Pathway analysis indicated that the cellular mechanisms affected by the two experimental conditions were quite similar, with up-regulated genes functionally associated with the heat shock response, ER-stress, apoptosis and immune defence, while genes connected with general metabolic processes, proteolysis and oxidation-reduction were largely suppressed. The qPCR assessment of 41 microarray-identified genes validated that the heat shock response (hsp90aa1, serpinh1), apoptosis (casp8, jund, jak2) and immune responses (apod, c1ql2, epx) were up-regulated in WN and WH fish, while oxidative stress and hypoxia sensitive genes were down-regulated (cirbp, cyp1a1, egln2, gstt1, hif1α, prdx6, rraga, ucp2). However, the additional challenge of hypoxia resulted in more pronounced effects on heat shock and immune-related processes, including a stronger influence on the expression of 14 immune-related genes. Finally, robust correlations between the transcription of 19 genes and several phenotypic traits in WH fish suggest that changes in gene expression were related to impaired physiological and growth performance.

CONCLUSION

Increasing temperature to 20 °C alone, and in combination with hypoxia, resulted in the differential expression of genes involved in similar pathways in Atlantic salmon. However, the expression responses of heat shock and immune-relevant genes in fish exposed to 20 °C and hypoxia were more affected, and strongly related to phenotypic characteristics (e.g., growth). This study provides valuable information on how these two environmental challenges affect the expression of stress-, metabolic- and immune-related genes and pathways, and identifies potential biomarker genes for improving our understanding of fish health and welfare.

Water temperature modulates mercury accumulation and oxidative stress status of common goby (Pomatoschistus microps)

Mercury (Hg) is a widespread pollutant across estuarine and coastal areas, raising concern on its potential impact on aquatic organisms. Hg may origin from natural and anthropogenic sources, being persistent and potentially toxic to biota, ultimately representing a serious risk to human health. Hg accumulation and toxicity may also induce reactive oxygen species (ROS) production in marine organisms, responsible for cell and tissue damage. Additionally, the temperature is undoubtedly an important environmental factor to consider regarding accumulation, due to its marked influence on the physiology and ecology of aquatic organisms. This study aimed to investigate the effect of different temperature scenarios (15, 20 and 25 °C) on the Hg accumulation in Pomatoschistus microps (Krøyer, 1838) liver and muscle, as well as on oxidative stress responses and energy metabolism, after short-term exposure to a naturally contaminated sediment with an environmentally relevant [Hg] (1.2 μg g^-1). The results showed that Hg accumulation tends to increase along the temperature gradient with higher values of Hg accumulated in liver than in muscle tissue. The action of antioxidant enzymes and stress proteins seems to be effective in combating oxidative stress in the liver. Despite the action of antioxidant defences in the muscle, oxidative damage was observed at the protein level concomitantly with a decrease in aerobic energy production after exposure to Hg at higher temperatures. These findings are ecologically relevant and highlight the importance of further investigation of combined effects of Hg and other stressors, especially in a scenario of a changing climate where events leading to rapid alterations on water parameters are more frequent.

Acclimation to warm temperatures has important implications for mitochondrial function in Atlantic salmon (Salmo salar)

In fish, the capacity of thermal acclimation to preserve cardiac mitochondrial function under future warming scenarios is important to understand given the central roles that cardiac energy metabolism and performance play in this taxa's thermal tolerance. We acclimated Atlantic salmon to 12 and 20°C (for >2 months), and investigated the effects of acute and chronic warming on cardiac mitochondrial respiration and reactive oxygen species (ROS) production (release rate) using high-resolution fluorespirometry. Further, we compared the sensitivity of mitochondrial respiration to nitric oxide (i.e. the NO IC50), and assessed the mitochondrial response to anoxia-reoxygenation (AR). Acute exposure to 20°C increased maximal mitochondrial respiration by ∼55%; however, the mitochondria's complex I respiratory control ratio was 17% lower and ROS production was increased by ≥60%. Acclimation to 20°C: (1) preserved mitochondrial coupling and aerobic capacity; (2) decreased the mitochondria's ROS production by ∼30%; (3) increased the mitochondria's NO IC50 by ∼23%; and (4) improved mitochondrial membrane integrity at 20°C. AR did not affect mitochondrial function at 12°C, but acute exposure to 20°C and AR depressed maximal mitochondrial respiration (by ∼9%) and coupling (by ∼16%) without impacting ROS production. Finally, warm acclimation did not improve the capacity of mitochondria to recover from AR, indicating that there was no 'cross-tolerance' between these challenges. Our findings provide compelling evidence that thermal plasticity of cardiac mitochondrial function contributes to the Atlantic salmon's capability to survive at ≥20°C for prolonged periods, but call into question whether this plasticity may allow them to withstand high temperatures when combined with other stressors.

Mitochondria and the thermal limits of ectotherms

Temperature is a critical abiotic factor shaping the distribution and abundance of species, but the mechanisms that underpin organismal thermal limits remain poorly understood. One possible mechanism underlying these limits is the failure of mitochondrial processes, as mitochondria play a crucial role in animals as the primary site of ATP production. Conventional measures of mitochondrial performance suggest that these organelles can function at temperatures much higher than those that limit whole-organism function, suggesting that they are unlikely to set organismal thermal limits. However, this conclusion is challenged by recent data connecting sequence variation in mitochondrial genes to whole-organism thermal tolerance. Here, we review the current state of knowledge of mitochondrial responses to thermal extremes and ask whether they are consistent with a role for mitochondrial function in shaping whole-organism thermal limits. The available data are fragmentary, but it is possible to draw some conclusions. There is little evidence that failure of maximal mitochondrial oxidative capacity as assessed in vitro sets thermal limits, but there is some evidence to suggest that temperature effects on ATP synthetic capacity may be important. Several studies suggest that loss of mitochondrial coupling is associated with the thermal limits for organismal growth, although this needs to be rigorously tested. Most studies have utilized isolated mitochondrial preparations to assess the effects of temperature on these organelles, and there remain many untapped opportunities to address these questions using preparations that retain more of their biological context to better connect these subcellular processes with whole-organism thermal limits.

Adjustments of cardiac mitochondrial phenotype in a warmer thermal habitat is associated with oxidative stress in European perch, Perca fluviatilis

Mitochondria are playing key roles in setting the thermal limits of fish, but how these organelles participate in selection mechanisms during extreme thermal events associated with climate warming in natural populations is unclear. Here, we investigated the thermal effects on mitochondrial metabolism, oxidative stress, and mitochondrial gene expression in cardiac tissues of European perch (Perca fluviatilis) collected from an artificially heated ecosystem, the "Biotest enclosure", and an adjacent reference area in the Baltic sea with normal temperatures (~ 23 °C and ~ 16 °C, respectively, at the time of capture in summer). Fish were sampled one month after a heat wave that caused the Biotest temperatures to peak at ~ 31.5 °C, causing significant mortality. When assayed at 23 °C, Biotest perch maintained high mitochondrial capacities, while reference perch displayed depressed mitochondrial functions relative to measurements at 16 °C. Moreover, mitochondrial gene expression of nd4 (mitochondrial subunit of complex I) was higher in Biotest fish, likely explaining the increased respiration rates observed in this population. Nonetheless, cardiac tissue from Biotest perch displayed higher levels of oxidative damage, which may have resulted from their chronically warm habitat, as well as the extreme temperatures encountered during the preceding summer heat wave. We conclude that eurythermal fish such as perch are able to adjust and maintain mitochondrial capacities of highly aerobic organs such as the heart when exposed to a warming environment as predicted with climate change. However, this might come at the expense of exacerbated oxidative stress, potentially threatening performance in nature.

Climate change and ageing in ectotherms

Human activity is changing climatic conditions at an unprecedented rate. The impact of these changes may be especially acute on ectotherms since they have limited capacities to use metabolic heat to maintain their body temperature. An increase in temperature is likely to increase the growth rate of ectothermic animals, and may also induce thermal stress via increased exposure to heat waves. Fast growth and thermal stress are metabolically demanding, and both factors can increase oxidative damage to essential biomolecules, accelerating the rate of ageing. Here, we explore the potential impact of global warming on ectotherm ageing through its effects on reactive oxygen species production, oxidative damage, and telomere shortening, at the individual and intergenerational levels. Most evidence derives primarily from vertebrates, although the concepts are broadly applicable to invertebrates. We also discuss candidate mechanisms that could buffer ectotherms from the potentially negative consequences of climate change on ageing. Finally, we suggest some potential applications of the study of ageing mechanisms for the implementation of conservation actions. We find a clear need for more ecological, biogeographical, and evolutionary studies on the impact of global climate change on patterns of ageing rates in wild populations of ectotherms facing warming conditions. Understanding the impact of warming on animal life histories, and on ageing in particular, needs to be incorporated into the design of measures to preserve biodiversity to improve their effectiveness.

Fluctuating temperatures extend median lifespan, improve reproduction and reduce growth in turquoise killifish

In natural populations, individuals experience daily fluctuations in environmental conditions that synchronise endogenous biorhythms. Artificial alterations of environmental fluctuations can have negative consequences for life history traits, including lifespan. In laboratory studies of aging, the role of fluctuating temperature is usually overlooked and we know little of how thermal fluctuation modulates senescence in vertebrates. In this longitudinal study we followed individually-housed turquoise killifish, Nothobranchius furzeri, from two thermal regimes; ecologically relevant diel fluctuations (20 °C - 35 °C) and stable temperature (27.5 °C), and compared their survival, growth and reproduction. Fish experiencing fluctuating temperatures had a longer median lifespan but reached smaller asymptotic body size. Within-treatment variation indicated that extended lifespan in fluctuating temperatures was not causally linked to decreased growth rate or smaller body size, but occurred solely due to the effect of thermal fluctuations. Male body size was positively associated with lifespan in stable temperatures but this relationship was disrupted in fluctuating thermal regimes. Females exposed to fluctuating temperatures effectively compensated egg production for their smaller size. Thus, there was no difference in absolute fecundity between thermal regimes and body-size corrected fecundity was higher in females in fluctuating temperatures. Overall, despite a brief exposure to sub-optimal thermal conditions during fluctuations, fluctuating temperature had a positive effect on survival and reproduction. These results suggest that the expression of life history traits and their associations under stable temperatures are a poor representation of the relationships obtained from ecologically relevant thermal fluctuations.

Mediation of oxidative stress toxicity induced by pyrethroid pesticides in fish

Organophosphate and organochlorine pesticides are banned in most countries because they cause high toxicity and bioaccumulation in non-target organisms. Pyrethroid pesticides have been applied to agriculture and aquaculture since the 1970s to replace traditional pesticides. However, pyrethroids are approximately 1000 times more toxic to fish than to mammals and birds. Fish-specific organs such as the gills and their late metabolic action against this type of pesticide make fish highly susceptible to the toxicity of pyrethroid pesticides. Oxidative stress plays an important role in the neurological, reproductive, and developmental toxicity caused by pyrethroids. Deltamethrin, cypermethrin, and lambda-cyhalothrin are representative pyrethroid pesticides that induce oxidative stress in tissues such as the gills, liver, and muscles of fish and cause histopathological changes. Although they are observed in low concentrations in aquatic environments such as rivers, lakes, and surface water they induce DNA damage and apoptosis in fish. Pyrethroid pesticides cause ROS-mediated oxidative stress in fish species including carp, tilapia, and trout. They also cause lipid peroxidation and alter the state of DNA, proteins, and lipids in the cells of fish. Moreover, changes in antioxidant enzyme activity following pyrethroid pesticide exposure make fish more susceptible to oxidative stress caused by environmental pollutants. In this review, we examine the occurrence of pyrethroid pesticides in the aquatic environment and oxidative stress-induced toxicity in fish exposed to pyrethroids.

Linking multiple biomarker responses in Daphnia magna under thermal stress

Temperature is an important abiotic variable that greatly influences the performance of aquatic ectotherms, especially under current anthropogenic global warming and thermal discharges. The aim of the present study was to evaluate thermal stress (20 °C vs 28 °C) in Daphnia magna over 21 d, focusing on the linkage among molecular and biochemical biomarker responses. Thermal stress significantly increased the levels of reactive oxygen species (ROS) and lipid peroxidation, especially in the 3-d short-term exposure treatment. This change in the ROS level was also correlated with mitochondrial membrane damage. These findings suggest that oxidative stress is the major pathway for thermally-induced toxicity of D. magna. Additionally, the expression levels of genes related to hypoxia (Hb), development (Vtg1), and sex determination (Dsx1-α, Dsx1-β, and Dsx2) were greatly increased by elevated temperature in a time-dependent manner. The cellular energy allocation was markedly decreased at the elevated temperature in the 3-d exposure treatment, mainly due to carbohydrates consumption for survival (oxidative stress defense). The present study showed that linking multiples biomarker responses are crucial for understanding the underlying mechanism of thermal stress on D. magna.

Thermal stability vs. variability: Insights in oxidative stress from a eurytolerant fish

Climate change will increase the frequency, intensity, and duration of heatwaves. This thermal volatility will challenge to the oxidative homeostasis of aquatic ectotherms through many temperature-dependent environmental factors. In this study, we examined the effects of chronic exposure of sheepshead minnows (Cyprinodon variegatus) to multiple thermal regimes on the oxidative physiology of white muscle in these eurytolerant fish. The thermal treatments included stable (15 °C and 30 °C) and cycling regimes (between 21 and 29 °C at 6, 8 and 10-h intervals). The effect of these thermal treatments on oxidative stress during an acute thermal challenge (12 h at 32 °C) was also examined. Enzymatic activity of catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx), scavenging capacities of hydroxyl and peroxyl radicals, and lipid peroxidation (LPO) damage were quantified. We found no differences between or across treatments in any of the enzymatic antioxidants or LPO damage. We found that peroxyl radical scavenging was greatest at the peak of the 8- and 10-h thermal cycles. Peroxyl scavenging after an acute thermal challenge was greater than before the challenge for the steady 15 °C and 8-h cycle treatments, greater before the acute challenge for the steady 30 °C and 6-h cycle, and equivalent in the 10-h cycle. These findings demonstrate that even the most tolerant of marine ectotherms must engage oxidative defenses when presented with thermal variability and heighten concerns about the impact of climate change on less tolerant species.

Lactic acid accumulation under heat stress related to accelerated glycolysis and mitochondrial dysfunction inhibits the mycelial growth of Pleurotus ostreatus

High temperature is a major threat to Pleurotus ostreatus cultivation. In this study, a potential mechanism by which P. ostreatus mycelia growth is inhibited under heat stress was explored. Lactate, as a microbial fermentation product, was found unexpectedly in the mycelia of P. ostreatus under heat stress, and the time-dependent accumulation and corresponding inhibitory effect of lactate on mycelial growth was further confirmed. The addition of a glycolysis inhibitor, 2-deoxy-D-glucose (2DG), reduced the lactate content in mycelia and slightly restored mycelial growth under high-temperature conditions, which indicated the accumulation of lactate can be inhibited by glycolysis inhibition. Further data revealed mitochondrial dysfunction under high-temperature conditions, with evidence of decreased oxygen consumption and adenosine triphosphate (ATP) synthesis and increased reactive oxygen species (ROS). The removal of ROS with ascorbic acid decreased the lactate content, and mycelial growth recovered to a certain extent, indicating lactate accumulation could be affected by the mitochondrial ROS. Moreover, metabolic data showed that glycolysis and the tricarboxylic acid cycle were enhanced. This study reported the accumulation of lactate in P. ostreatus mycelia under heat stress and the inhibitory effect of lactate on the growth of mycelia, which might provide further insights into the stress response mechanism of edible fungi. Key Points • Lactate can accumulate in Pleurotus ostreatus mycelia under heat stress and inhibit its growth. • The accumulation of lactate may be due to the acceleration of glycolysis and the dysfunction of mitochondria of P. ostreatus mycelia under high-temperature stress. • The glycolysis and tricarboxylic acid cycle of P. ostreatus mycelia were accelerated under high-temperature stress.

Effects of copper and temperature on heart mitochondrial hydrogen peroxide production

High energy demand for continuous mechanical work and large number of mitochondria predispose the heart to excessive reactive oxygen species (ROS) production that may precipitate oxidative stress and heart failure. While mitochondria have been proposed as a unifying cellular target and driver of adverse effects induced by diverse stressful states, there is limited understanding of how heart mitochondrial ROS homeostasis is affected by combinations of stress factors. Thus, we probed the effect of copper (Cu) and thermal stress on ROS (as hydrogen peroxide, H₂O₂) emission and elucidated the effects of Cu on ROS production sites in rainbow trout heart mitochondria using the Amplex UltraRed-horseradish peroxidase detection system optimized for our model. Mitochondria oxidizing malate-glutamate or succinate were incubated at 4, 11 (control) and 23 °C and exposed to a range (1-100 μM) of Cu concentrations. We found that the rates and patterns of H₂O₂ emission depended on substrate type, Cu concentration and temperature. In mitochondria oxidizing malate-glutamate, Cu increased the rate of H₂O₂ emission with a spike at 1 μM while temperature had no effect. In contrast, both temperature and Cu increased the rate of H₂O₂ emission in mitochondria oxidizing succinate with a prominent spike at 25 μM Cu. The rates of H₂O₂ emission at the three temperatures during the spike imposed by 25 μM Cu were of the order 11 > 23 > 4 °C. Interestingly, 5 μM Cu supressed H₂O₂ emission in mitochondria oxidizing succinate or malate-glutamate suggesting a common mechanism of action independent of substrate type. In the absence of Cu, the site-specific capacities of H₂O₂ emission were: complex III outer ubiquinone binding site (site III_Qo) > complex II flavin site (site II_F) ≥ complex I flavin site (site I_F) > complex I ubiquinone-binding site (site I_Q). Rotenone marginally increased succinate-driven H₂O₂ emission suggesting either the absence of reverse electron transport (RET)-driven ROS production at site I_Q or masking of the expected rotenone response (reduction) by H₂O₂ produced from other sites. Cu acted at multiple sites in the electron transport system resulting in different site-specific H₂O₂ emission responses depending on the concentration. Specifically, site I_F H₂O₂ emission was suppressed by Cu concentration-dependently while H₂O₂ emission by site II_F was inhibited and stimulated by low and high concentrations of Cu, respectively. Additionally, emission from site III_Qo was stimulated by low and inhibited by high Cu concentrations. Overall, our study unveiled distinctive effects and sites of modulation of mitochondrial ROS production by Cu with implications for cardiac redox signaling networks and development of mitochondria-targeted Cu-based drugs.

Thermal tolerance and fish heart integrity: fatty acids profiles as predictors of species resilience

The cardiovascular system is a major limiting system in thermal adaptation, but the exact physiological mechanisms underlying responses to thermal stress are still not completely understood. Recent studies have uncovered the possible role of reactive oxygen species production rates of heart mitochondria in determining species' upper thermal limits. The present study examines the relationship between individual response to a thermal challenge test (CT_max), susceptibility to peroxidation of membrane lipids, heart fatty acid profiles and cardiac antioxidant enzyme activities in two salmonid species from different thermal habitats (Salvelinus alpinus, Salvelinus fontinalis) and their hybrids. The susceptibility to peroxidation of membranes in the heart was negatively correlated with individual thermal tolerance. The same relationship was found for arachidonic and eicosapentaenoic acid. Total H₂O₂ buffering activity of the heart muscle was higher for the group with high thermal resistance. These findings underline a potential general causative relationship between sensitivity to oxidative stress, specific fatty acids, antioxidant activity in the cardiac muscle and thermal tolerance in fish and likely other ectotherms. Heart fatty acid profile could be indicative of species resilience to global change, and more importantly the plasticity of this trait could predict the adaptability of fish species or populations to changes in environmental temperature.

Effects of bioenergetics, temperature and cadmium on liver mitochondria reactive oxygen species production and consumption

A by-product of mitochondrial substrate oxidation and electron transfer to generate cellular energy (ATP) is reactive oxygen species (ROS). Superoxide anion radical and hydrogen peroxide (H₂O₂) are the proximal ROS produced by the mitochondria. Because low levels of ROS serve critical regulatory roles in cell physiology while excessive levels or inappropriately localized ROS result in aberrant physiological states, mitochondrial ROS need to be tightly regulated. While it is known that regulation of mitochondrial ROS involves balancing the rates of production and removal, the effects of stressors on these processes remain largely unknown. To illuminate how stressors modulate mitochondrial ROS homeostasis, we investigated the effects of temperature and cadmium (Cd) on H₂O₂ emission and consumption in rainbow trout liver mitochondria. We show that H₂O₂ emission rates increase with temperature and Cd exposure. Energizing mitochondria with malate-glutamate or succinate increased the rate of H₂O₂ emission; however, Cd exposure imposed different patterns of H₂O₂ emission depending on the concentration and substrate. Specifically, mitochondria respiring on malate-glutamate exhibited a saturable graded concentration-response curve that plateaued at 5 μM while mitochondria respiring on succinate had a biphasic concentration-response curve characterized by a spike in the emission rate at 1 μM Cd followed by gradual diminution at higher Cd concentrations. To explain the observed substrate- and concentration-dependent effects of Cd, we sequestered specific mitochondrial ROS-emitting sites using blockers of electron transfer and then tested the effect of the metal. The results indicate that the biphasic H₂O₂ emission response imposed by succinate is due to site II_F but is further modified at sites I_Q and III_Qo. Moreover, the saturable graded H₂O₂ emission response in mitochondria energized with malate-glutamate is consistent with effect of Cd on site I_F. Additionally, Cd and temperature acted cooperatively to increase mitochondrial H₂O₂ emission suggesting that increased toxicity of Cd at high temperature may be due to increased oxidative insult. Surprisingly, despite their clear stimulatory effect on H₂O₂ emission, Cd, temperature and bioenergetic status did not affect the kinetics of mitochondrial H₂O₂ consumption; the rate constants and half-lives for all the conditions tested were similar. Overall, our study indicates that the production processes of rainbow trout liver mitochondrial H₂O₂ metabolism are highly responsive to stressors and bioenergetics while the consumption processes are recalcitrant. The latter denotes the presence of a robust H₂O₂ scavenging system in liver mitochondria that would maintain H₂O₂ homeostasis in the face of increased production and reduced scavenging capacity.

Increased reactive oxygen species production and maintenance of membrane potential in VDAC-less Neurospora crassa mitochondria

The highly abundant voltage-dependent anion-selective channel (VDAC) allows transit of metabolites across the mitochondrial outer membrane. Previous studies in Neurospora crassa showed that the LoPo strain, expressing 50% of normal VDAC levels, is indistinguishable from wild-type (WT). In contrast, the absence of VDAC (ΔPor-1), or the expression of an N-terminally truncated variant VDAC (ΔN2-12porin), is associated with deficiencies in cytochromes b and aa₃ of complexes III and IV and concomitantly increased alternative oxidase (AOX) activity. These observations led us to investigate complex I and complex II activities in these strains, and to explore their mitochondrial bioenergetics. The current study reveals that the total NADH dehydrogenase activity is similar in mitochondria from WT, LoPo, ΔPor-1 and ΔN2-12porin strains; however, in ΔPor-1 most of this activity is the product of rotenone-insensitive alternative NADH dehydrogenases. Unexpectedly, LoPo mitochondria have increased complex II activity. In all mitochondrial types analyzed, oxygen consumption is higher in the presence of the complex II substrate succinate, than with the NADH-linked (complex I) substrates glutamate and malate. When driven by a combination of complex I and II substrates, membrane potentials (Δψ) and oxygen consumption rates (OCR) under non-phosphorylating conditions are similar in all mitochondria. However, as expected, the induction of state 3 (phosphorylating) conditions in ΔPor-1 mitochondria is associated with smaller but significant increases in OCR and smaller decreases in Δψ than those seen in wild-type mitochondria. High ROS production, particularly in the presence of rotenone, was observed under non-phosphorylating conditions in the ΔPor-1 mitochondria. Thus, the absence of VDAC is associated with increased ROS production, in spite of AOX activity and wild-type OCR in ΔPor-1 mitochondria.

Brown seaweed (AquaArom) supplementation increases food intake and improves growth, antioxidant status and resistance to temperature stress in Atlantic salmon, Salmo salar

Seaweeds represent a vast resource that remains underutilized as an ingredient in aquafeeds. Here we probed the effect of addition of AquaArom, a seaweed meal derived from brown seaweeds (Laminaria sp., kelp), to fish feed on growth performance, antioxidant capacity and temperature responsiveness of mitochondrial respiration. A commercial salmonid feed was mixed with 0 (control), 3, 6 and 10% seaweed and fed to Atlantic salmon (Salmo salar) smolts for 30 days. The smolts consumed more of the seaweed-supplemented food relative to the control and there were no mortalities. Compared with the control, the final fish weight, standard length, weight gain and SGR were higher in fish fed diets supplemented with the 3 and 10% seaweed, while growth performance for fish maintained on 6% seaweed remained neutral. Importantly, seaweed supplementation increased protein efficiency ratio (PER) and tended to improve food conversion ratio (FCR). Although the hepatosomatic and visceral indices did not change, whole gut and intestinal weights and lengths were higher in fish maintained on seaweed-supplemented diets suggesting increased retention time and a larger surface area for food digestion and nutrient absorption. Measurement of antioxidant status revealed that seaweed supplementation dose-dependently increased plasma total antioxidant capacity as well as the level of glutathione, and activities of catalase and superoxide dismutase in liver mitochondria. Moreover, seaweed supplementation reduced the effect of acute temperature rise on mitochondrial respiration and proton leak. Overall, these data suggest that AquaArom can be mixed with fish food up to 10% to increase food consumption and enhance growth performance, as well as to improve antioxidant capacity and alleviate adverse effects of stressors such as temperature in fish.

Exposure to acute ammonia stress influences survival, immune response and antioxidant status of pacific white shrimp (Litopenaeus vannamei) pretreated with diverse levels of inositol

The effect of acute ammonia challenge on survival, immune response and antioxidant status of Litopenaeus vannamei pretreated with diets containing different inositol levels was investigated. Shrimp (initial mean weight 0.40 ± 0.00 g) were randomly allocated in 18 tanks (30 shrimp per tank) and triplicate tanks were fed with a control diet without myo-inositol (MI) supplementation (242.6 mg inositol kg^-1 diet) or diets containing diverse levels of inositol (368.8, 459.7, 673.1, 993.8 and 1674.4  mg kg^-1 diet) as treatment groups for 8-week. Randomly selected 10 shrimp per tank (final mean weight approximately 11.1-13.8g) were exposed to ammonia stress (total ammonia-nitrogen, 60.21  mg L^-1) for 24 h after feeding trial. The results showed that after exposed to ammonia stress, survival rates of MI-supplemented groups were enhanced by 31-77% when compared with the control group. MI supplementation increased activities of alkaline phosphatase (AKP) and acid phosphatase (ACP) in plasma, and reduced its activities in hepatopancreas. It also enhanced activities of total antioxidant capacity (T-AOC), glutathione S-transferase (GST) and glutathione peroxidase (GPX) and content of reduced glutathione (GSH), and lowered malondialdehyde (MDA) and protein carbonyl (PC) content in plasma or hepatopancreas. In addition, mRNA expression levels of ferritin (FT), arginine kinase (AK), thioredoxin (Trx), heat shock protein 70 (Hsp70), catalase (CAT) and peroxiredoxin (Prx) were significantly differentially regulated in hepatopancreas owing to MI supplementation. Therefore, it suggested that L. vannamei pretreated with higher dietary inositol content may have better ammonia stress tolerance and antioxidant status after ammonia stress, and the optimum levels ranged from 459.7 to 993.8 mg inositol kg^-1 when total ammonia-nitrogen concentration was 60.21  mg L^-1.

Modeling cardiac complexity: Advancements in myocardial models and analytical techniques for physiological investigation and therapeutic development in vitro

Cardiomyopathies, heart failure, and arrhythmias or conduction blockages impact millions of patients worldwide and are associated with marked increases in sudden cardiac death, decline in the quality of life, and the induction of secondary pathologies. These pathologies stem from dysfunction in the contractile or conductive properties of the cardiomyocyte, which as a result is a focus of fundamental investigation, drug discovery and therapeutic development, and tissue engineering. All of these foci require in vitro myocardial models and experimental techniques to probe the physiological functions of the cardiomyocyte. In this review, we provide a detailed exploration of different cell models, disease modeling strategies, and tissue constructs used from basic to translational research. Furthermore, we highlight recent advancements in imaging, electrophysiology, metabolic measurements, and mechanical and contractile characterization modalities that are advancing our understanding of cardiomyocyte physiology. With this review, we aim to both provide a biological framework for engineers contributing to the field and demonstrate the technical basis and limitations underlying physiological measurement modalities for biologists attempting to take advantage of these state-of-the-art techniques.

Transcriptome and Proteome of Fish-Pathogenic Streptococcus agalactiae Are Modulated by Temperature

Streptococcus agalactiae is one of the most important pathogens associated with streptococcosis outbreaks in Nile tilapia farms worldwide. High water temperature (above 27°C) has been described as a predisposing factor for the disease in fish. At low temperatures (below 25°C), fish mortalities are not usually observed in farms. Temperature variation can modulate the expression of genes and proteins involved in metabolism, adaptation, and bacterial pathogenicity, thus increasing or decreasing the ability to infect the host. This study aimed to evaluate the transcriptome and proteome of a fish-pathogenic S. agalactiae strain SA53 subjected to in vitro growth at different temperatures using a microarray and label-free shotgun LC-HDMS^E approach. Biological triplicates of isolates were cultured in BHIT broth at 22 or 32°C for RNA and protein isolation and submitted for transcriptomic and proteomic analyses. In total, 1,730 transcripts were identified in SA53, with 107 genes being differentially expressed between the temperatures evaluated. A higher number of genes related to metabolism, mainly from the phosphotransferase system (PTS) and ATP-binding cassette (ABC) transport system, were upregulated at 32°C. In the proteome analysis, 1,046 proteins were identified in SA53, of which 81 were differentially regulated between 22 and 32°C. Proteins involved in defense mechanisms, lipid transport and metabolism, and nucleotide transport and metabolism were upregulated at 32°C. A higher number of interactions were observed in proteins involved in nucleotide transport and metabolism. We observed a low correlation between the transcriptome and proteome datasets. Our study indicates that the transcriptome and proteome of a fish-adapted S. agalactiae strain are modulated by temperature, particularly showing differential expression of genes/proteins involved in metabolism, virulence factors, and adaptation.

H2O2 metabolism in liver and heart mitochondria: Low emitting-high scavenging and high emitting-low scavenging systems

Although mitochondria are presumed to emit and consume reactive oxygen species (ROS), the quantitative interplay between the two processes in ROS regulation is not well understood. Here, we probed the role of mitochondrial bioenergetics in H₂O₂ metabolism using rainbow trout liver and heart mitochondria. Both liver and heart mitochondria emitted H₂O₂ at rates that depended on their metabolic state, with the emission rates (free radical leak) constituting 0.8-2.9% and 0.2-2.5% of the respiration rate in liver and heart mitochondria, respectively. When presented with exogenous H₂O₂, liver and heart mitochondria consumed it by first order reactions with half-lives (s) of 117 and 210, and rate constants of 5.96 and 3.37 (× 10^-3 s^-1), respectively. The mitochondrial bioenergetic status greatly affected the rate of H₂O₂ consumption in heart but not liver mitochondria. Moreover, the activities and contribution of H₂O₂ scavenging systems varied between liver and heart mitochondria. The significance of the scavenging systems ranked by the magnitude (%) of inhibition of H₂O₂ removal after correcting for emission were, liver (un-energized and energized): catalase > glutathione (GSH) ≥ thioredoxin reductase (TrxR); un-energized heart mitochondria: catalase > TrxR > GSH and energized heart mitochondria: GSH > TrxR > catalase. Notably, depletion of GSH evoked a massive surge in H₂O₂ emission that grossly masked the contribution of this pathway to H₂O₂ scavenging in heart mitochondria. Irrespective of the organ of their origin, mitochondria behaved as H₂O₂ regulators that emitted or consumed it depending on the ambient H₂O₂ concentration, mitochondrial bioenergetic state and activity of the scavenging enzyme systems. Indeed, manipulation of mitochondrial bioenergetics and H₂O₂ scavenging systems caused mitochondria to switch from being net consumers to net emitters of H₂O₂. Overall, our data suggest that the low levels of H₂O₂ typically present in cells would favor emission of this metabolite but the scavenging systems would prevent its accumulation.