Combined effects of salinity and intermittent hypoxia on mitochondrial capacity and reactive oxygen species efflux in the Pacific oyster, Crassostrea gigas

Lasting impacts of rapid salinity change on physiological energetics of estuarine oysters (Crassostrea hongkongensis)

The duration of rapid salinity change (RSC) prevailing in estuarine and coastal regions is increasing due to extreme climate and weather events, posing significant challenges to marine bivalves. The Hong Kong oyster (Crassostrea hongkongensis), an ecologically and economically important species in tropical estuarine ecosystems, has experienced increasing mass mortality during prolonged periods of RSC, yet little is known about underlying physiological processes. Here, we investigated how physiological energetics of C. hongkongensis were affected by longer-lasting scenarios and four-week episodes of RSC. Compared with ambient conditions with seawater salinity ranging from 15 to 20, rapid salinity change by ± 10 units significantly decreased the survival of oysters, with RSC-induced hyposaline stress (-10) resulting in more serious consequences than that of hypersaline regime (+10). Continuing exposure of oysters to both RSC scenarios significantly affected their feeding activities, but the food absorption efficiency were still virtually unchanged. Significantly depressed respiration and increased excretion activities were observed in RSC-stressed oysters, resulting in significantly lowered O:N ratio. Overall, when exposed to RSC, oysters showed significantly decreased scope for growth, due to shifts in energy budget toward maintenance of essential physiological processes. Our results demonstrate the vulnerability of estuarine oysters to prolonged RSC events, and underscore the pressing need to develop strategies to enhance oyster tolerance under intensifying RSC conditions and safeguard oyster aquaculture in this era of unprecedented climate change.

Effects of hypoxia-reoxygenation on the bioenergetics and oxidative stress in the isolated mitochondria of the king scallop, Pecten maximus

The king scallop (Pecten maximus) is a highly aerobic subtidal bivalve species vulnerable to fluctuations in oxygen availability. This study investigated the effects of short-term (15 min) and long-term (90 min) hypoxia-reoxygenation (H/R) stress on substrate-specific mitochondrial functions in the gill and digestive gland tissues of P. maximus, oxidizing substrates that engage mitochondrial Complex I (pyruvate, palmitate) and Complex II (succinate). Under normoxic conditions, scallop mitochondria preferentially oxidized pyruvate. H/R stress induced a significant decline in Complex I-driven ATP synthesis, increased proton leak and dysregulated fatty acid oxidation, indicating mitochondrial vulnerability to H/R stress. Following H/R, both tissues demonstrated a greater capacity for succinate oxidation than for Complex I substrates; however, long-term H/R exposure led to a reduction in respiratory coupling efficiency across all substrates. Notably, gill mitochondria exhibited more effective regulation of reactive oxygen species efflux and electron leak compared with digestive gland mitochondria under H/R stress. Despite these physiological changes, no evidence of oxidative damage was detected, suggesting the presence of a robust mitochondrial antioxidant defense. Collectively, these findings suggest that succinate oxidation plays an important role in stress recovery in P. maximus, providing insights into mitochondrial resilience and the management of oxidative stress during intermittent hypoxia.

Osmotolerance reflected in mitochondrial respiration of Mytilus populations from three different habitat salinities

Mussels from the Mytilus edulis species complex experience a salinity gradient from the North Sea into the Baltic Proper ranging from 32 to 5. As osmoconformers, they adjust their internal osmolarity to match that of their surroundings, which presents a significant challenge to the metabolic machinery, including their mitochondria. We hypothesized that the osmotic optima for the mitochondrial function of mussels matches the prevailing habitat salinity and is accompanied by a population specific metabolite profile. To test this hypothesis, mussels from three populations along the salinity gradient were assessed. We found a population specific shift in the optimal osmolarities for maximal mitochondrial respiration capacity that mirrored the populations' habitat salinity. So, mitochondria from North Sea mussels reached their highest capacity at higher osmotic concentrations than their Baltic Sea congeners. Additionally, Baltic Sea populations appear to have traded off an adaptation to low salinities for a narrower mitochondrial tolerance range resulting in a more specialized mitochondrial phenotype, while North Sea populations have mitochondria with a more general functioning phenotype. The local adaptation to a low salinity habitat was supported by the analysis of gill tissue metabolites via LC-MS/MS. Abundances of metabolites involved in energy generation, osmotic homeostasis or the urea cycle were similar between North Sea and southern Baltic Sea mussels, while northern Baltic Sea mussels seem to follow a different metabolic strategy, which may allow them to inhabit very low salinities. Thus, northern Baltic Sea mussels have adapted to low salinities on a mitochondrial and metabolic level.

Effects of single or combined exposure to tralopyril and ocean acidification on energy metabolism response and sex development in Pacific oysters (Crassostrea gigas)

The combined effects of the novel antifouling biocide tralopyril (TP) nitrile and ocean acidification (OA) on marine organisms are still not well understood, despite the increasing attention given to the toxic effects of emerging pollutants and OA on marine organisms in recent years. In this study, Crassostrea gigas (C. gigas) was exposed to TP, OA, and a combination of TP and OA for 21 days with a 14-day depuration. This study investigated the inter-tissue variability in energy metabolism responses and the impacts on gonadal development in C. gigas under both single and combined exposures to TP and OA. The results indicate that TP exposure and OA resulted in up-regulation of energy metabolism genes in the C. gigas, with tissues exhibiting enhanced aerobic metabolism. Furthermore, OA influences the sex determination of C. gigas, promoting the development of female individuals. Moreover, following depuration, C. gigas is able to restore normal energy metabolism and sexual development through the accumulation of suitable energy reserves. This study provides a valuable reference for the environmental and ecological risk assessment of TP, addressing the research gap in understanding the combined toxicity of TP and OA on aquatic organisms.

An evolving roadmap: using mitochondrial physiology to help guide conservation efforts

The crucial role of aerobic energy production in sustaining eukaryotic life positions mitochondrial processes as key determinants of an animal's ability to withstand unpredictable environments. The advent of new techniques facilitating the measurement of mitochondrial function offers an increasingly promising tool for conservation approaches. Herein, we synthesize the current knowledge on the links between mitochondrial bioenergetics, ecophysiology and local adaptation, expanding them to the wider conservation physiology field. We discuss recent findings linking cellular bioenergetics to whole-animal fitness, in the current context of climate change. We summarize topics, questions, methods, pitfalls and caveats to help provide a comprehensive roadmap for studying mitochondria from a conservation perspective. Our overall aim is to help guide conservation in natural populations, outlining the methods and techniques that could be most useful to assess mitochondrial function in the field.

Mitochondrial responses to constant and cyclic hypoxia depend on the oxidized fuel in a hypoxia-tolerant marine bivalve Crassostrea gigas

Sessile benthic organisms like oysters inhabit the intertidal zone, subject to alternating hypoxia and reoxygenation (H/R) episodes during tidal movements, impacting respiratory chain activities and metabolome compositions. We investigated the effects of constant severe hypoxia (90 min at ~ 0% O2 ) followed by 10 min reoxygenation, and cyclic hypoxia (5 cycles of 15 min at ~ 0% O2 and 10 min reoxygenation) on isolated mitochondria from the gill and the digestive gland of Crassostrea gigas respiring on pyruvate, palmitate, or succinate. Constant hypoxia suppressed oxidative phosphorylation (OXPHOS), particularly during Complex I-linked substrates oxidation. It had no effect on mitochondrial reactive oxygen species (ROS) efflux but increased fractional electron leak (FEL). In mitochondria oxidizing Complex I substrates, exposure to cyclic hypoxia prompted a significant drop after the first H/R cycle. In contrast, succinate-driven respiration only showed significant decline after the third to fifth H/R cycle. ROS efflux saw little change during cyclic hypoxia regardless of the oxidized substrate, but Complex I-driven FEL tended to increase with each subsequent H/R cycle. These observations suggest that succinate may serve as a beneficial stress fuel under H/R conditions, aiding in the post-hypoxic recovery of oysters by reducing oxidative stress and facilitating rapid ATP re-synthesis. The impacts of constant and cyclic hypoxia of similar duration on mitochondrial respiration and oxidative lesions in the proteins were comparable indicating that the mitochondrial damage is mostly determined by the lack of oxygen and mitochondrial depolarization. The ROS efflux in the mitochondria of oysters was minimally affected by oxygen fluctuations indicating that tight regulation of ROS production may contribute to robust mitochondrial phenotype of oysters and protect against H/R induced stress.

Spawning acts as a metabolic stressor enhanced by hypoxia and independent of sex in a broadcast marine spawner

Broadcast spawners, like the blue mussel Mytilus edulis, experience substantial energy expenditure during spawning due to extensive gamete release that can divert energy from other functions. This energetic cost might be intensified by environmental stressors, including hypoxia that suppress aerobic metabolism. However, the energy implications of spawning in marine broadcast spawners have not been well studied. We examined the effects of short-term hypoxia (7 days) and spawning on mitochondrial activity, reactive oxygen species (ROS) production, and cellular energy allocation (ratio of tissue energy reserves to energy demand) in somatic tissues of M. edulis. Under normoxic conditions, post-spawning (72 h) recovery correlated with increased phosphorylation (OXPHOS) rate in mitochondria from the digestive gland, while hypoxia inhibited this response. Regardless of oxygen levels, mitochondrial ROS production decreased after spawning, indicating M. edulis' ability to prevent oxidative stress. Spawning led to reduced energy reserves in somatic tissues (the gills and the digestive gland), highlighting significant energy cost of spawning primarily fueled by lipid and protein breakdown. Additionally, cellular energy allocation dropped 3 h post-spawning, indicating a shift in energy demand and supply. Normoxic conditions allowed recovery in 72 h, but hypoxia hindered recuperation. These findings underscore spawning's bioenergetic challenge for broadcast spawners like M. edulis, potentially elevating post-spawning mortality risk, especially in hypoxic coastal habitats.

Redox control of antioxidants, metabolism, immunity, and development at the core of stress adaptation of the oyster Crassostrea gigas to the dynamic intertidal environment

This review uses the marine bivalve Crassostrea gigas to highlight redox reactions and control systems in species living in dynamic intertidal environments. Intertidal species face daily and seasonal environmental variability, including temperature, oxygen, salinity, and nutritional changes. Increasing anthropogenic pressure can bring pollutants and pathogens as additional stressors. Surprisingly, C. gigas demonstrates impressive adaptability to most of these challenges. We explore how ROS production, antioxidant protection, redox signaling, and metabolic adjustments can shed light on how redox biology supports oyster survival in harsh conditions. The review provides (i) a brief summary of shared redox sensing processes in metazoan; (ii) an overview of unique characteristics of the C. gigas intertidal habitat and the suitability of this species as a model organism; (iii) insights into the redox biology of C. gigas, including ROS sources, signaling pathways, ROS-scavenging systems, and thiol-containing proteins; and examples of (iv) hot topics that are underdeveloped in bivalve research linking redox biology with immunometabolism, physioxia, and development. Given its plasticity to environmental changes, C. gigas is a valuable model for studying the role of redox biology in the adaptation to harsh habitats, potentially providing novel insights for basic and applied studies in marine and comparative biochemistry and physiology.