Elucidating the acid-base mechanisms underlying otolith overgrowth in fish exposed to ocean acidification

Impacts of ash-induced environmental alkalinization on fish physiology, and their implications to wildfire-scarred watersheds

Changes in land use, a warming climate and increased drought have amplified wildfire frequency and magnitude globally. Subsequent rainfall in wildfire-scarred watersheds washes ash into aquatic systems, increasing water pH and exposing organisms to environmental alkalinization. In this study, 15 or 20 °C-acclimated Chinook salmon (Oncorhynchus tshawytscha) yearlings were exposed to an environmentally-relevant ash concentration (0.25 % w/v), increasing water pH from ∼8.1 to ∼9.2. Salmon experienced significant disturbance to blood plasma pH (pHe) and red blood cell intracellular pH (RBC pHi) within 1 h, but recovered within 24 h. Impacts on plasma ion concentrations were relatively mild, and plasma glucose increased by 2- to 4-fold at both temperatures. Temperature-specific differences were observed: 20 °C salmon recovered their pHe more rapidly, perhaps facilitated by higher basal metabolism and anaerobic metabolic H+ production. Additionally, 20 °C salmon experienced dramatically greater spikes in plasma total ammonia, [NH3] and [NH4+] after 1 h of exposure that decreased over time, whereas 15 °C salmon experienced a gradual nitrogenous waste accumulation. Despite pHe and RBC pHi recovery and non-lethal nitrogenous waste levels, we observed 20 % and 33 % mortality in 15 and 20 °C treatments within 12 h of exposure, respectively. The mortalities cannot be explained by high water pH alone, nor was it likely to be singularly attributable to a heavy metal or organic compound released from ash input. This demonstrates post-wildfire ash input can induce lethal yet previously unexplored physiological disturbances in fish, and further highlights the complex interaction with warmer temperatures typical of wildfire-scarred landscapes.

Mutligenerational chronic exposure to near future ocean acidification in European sea bass (Dicentrarchus labrax): Insights into the regulation of the transcriptome in a sensory organ involved in feed intake, the tongue

In this study, we examined the effect of near future ocean acidification (OA) on the transcriptome of a sensory organ in contact with surrounding water, the tongue in adult European sea bass (Dicentrarchus labrax) by mean of RNAseq experiment. We acquired a total of 14.1 Mb quality-trimmed reads covering 18,703 expressed genes from the tongue of fish reared from two generations at actual (pH 8.0 condition) and predicted near-future seawater pH (pH 7.6 condition). Gene ontologies analyses of expressed genes support the evidence that the tongue exhibits biological processes related to the sensory system, tooth mineralization and immune defences among others. Our data revealed only 295 OA-induced regulated genes with 114 up- and 181 down-regulated by OA. Functions over-represented encompass processes involved in organic substance metabolic process, RNA metabolism and especially RNA methylation which, combined with the regulation of some hsp genes expression, suggest a molecular response to stress which might contribute to lingual cell homeostasis under OA. The immune system process is also found enriched within OA-induced regulated genes. With the exception of one fatty acid receptor, known taste perception effectors were not impacted by OA in the tongue. However, a complementary droplet digital PCR approach dedicated to genes involved in gustatory signal transduction revealed the down regulation by OA of pyrimidinergic receptor (p2ry4) transcript expression in the gills of the fish. Combined with scanning electron microscopy analysis, our RNAseq data revealed that OA has no impact on processes related to teeth development and mineralization. Altogether, our data reveal that multigenerational exposure to OA has not a substantially effect on the tongue transcriptome but emphasis should be placed on investigating the potential physiological consequences related to the regulation of genes related to cell stress, immune system and fatty acid sensitivity to conclude on species resilience in face of OA.

Gill ionocyte remodeling mediates blood pH regulation in rockfish (Sebastes diploproa) exposed to environmentally relevant hypercapnia

Marine fishes excrete excess H+ using basolateral Na+-K+-ATPase (NKA) and apical Na+/H+ exchanger 3 (NHE3) in gill ionocytes. However, the mechanisms that regulate H+ excretion during exposure to environmentally relevant hypercapnia (ERH) remain poorly understood. Here, we explored transcriptomic, proteomic, and cellular responses in gills of juvenile splitnose rockfish (Sebastes diploproa) exposed to 3 days of ERH conditions (pH ∼7.5, ∼1,600 μatm Pco2). Blood pH was fully regulated at ∼7.75 despite a lack of significant changes in gill 1) mRNAs coding for proteins involved in blood acid-base regulation, 2) total NKA and NHE3 protein abundance, and 3) ionocyte density. However, ERH-exposed rockfish demonstrated increased NKA and NHE3 abundance on the ionocyte plasma membrane coupled with wider apical membranes and greater extension of apical microvilli. The observed gill ionocyte remodeling is consistent with enhanced H+ excretion that maintains blood pH homeostasis during exposure to ERH and does not necessitate changes at the expression or translation levels. These mechanisms of phenotypic plasticity may allow fishes to regulate blood pH during environmentally relevant acid-base challenges and thus have important implications for both understanding how organisms respond to climate change and for selecting appropriate metrics to evaluate its impact on marine ecosystems.NEW & NOTEWORTHY Splitnose rockfish exposed to environmentally relevant hypercapnia utilize existing proteins (rather than generate additional machinery) to maintain homeostasis.

Otolith reliability is context-dependent for estimating warming and CO2 acidification impacts on fish growth

Otoliths are frequently used as proxies to examine the impacts of climate change on fish growth in marine and freshwater ecosystems worldwide. However, the large sensitivity differences in otolith growth responses to typical changing environmental factors (i.e., temperature and CO2 concentration), coupled with unclear drivers and potential inconsistencies with fish body growth, fundamentally challenge the reliability of such otolith applications. Here, we performed a global meta-analysis of experiments investigating the direct effects of warming (297 cases) and CO2 acidification (293 cases) on fish otolith growth and compared them with fish body growth responses. Hierarchical models were used to assess the overall effect and quantify the influence of nine explanatory factors (e.g., fish feeding habit, life history stage, habitat type, and experimental amplitude and duration). The overall effects of warming and acidification on otolith growth were positive and significant, and the effect size of warming (effect size = 0.4003, otolith size of the treatment group increased by 49.23% compared to that of the control group) was larger than that of acidification (0.0724, 7.51%). All factors examined contributed to the heterogeneity of effect sizes, with larger responses commonly observed in carnivorous fish, marine species, and young individuals. Warming amplitudes and durations and acidification amplitudes increased the effect sizes, while acidification durations decreased the effect sizes. Otolith growth responses were consistent with, but greater than, fish body growth responses under warming. In contrast, fish body growth responses were not significant under acidification (effect size = -0.0051, p = .6185) and thus cannot be estimated using otoliths. Therefore, our study highlights that the reliability of applying otoliths to examine climate change impacts is likely varied, as the sensitivity of otolith growth responses and the consistency between the growth responses of otoliths and fish bodies are context-dependent.

Immunohistochemical and ultrastructural characterization of the inner ear epithelial cells of splitnose rockfish (Sebastes diploproa)

The inner ear of teleost fish regulates the ionic and acid-base chemistry and secretes protein matrix into the endolymph to facilitate otolith biomineralization, which is used to maintain vestibular and auditory functions. The otolith is biomineralized in a concentric ring pattern corresponding to seasonal growth, and this calcium carbonate (CaCO3) polycrystal has become a vital aging and life-history tool for fishery managers, ecologists, and conservation biologists. Moreover, biomineralization patterns are sensitive to environmental variability including climate change, thereby threatening the accuracy and relevance of otolith-reliant toolkits. However, the cellular biology of the inner ear is poorly characterized, which is a hurdle for a mechanistic understanding of the underlying processes. This study provides a systematic characterization of the cell types in the inner ear of splitnose rockfish (Sebastes diploproa). Scanning electron microscopy revealed the apical morphologies of six inner ear cell types. In addition, immunostaining and confocal microscopy characterized the expression and subcellular localization of the proteins Na+-K+-ATPase, carbonic anhydrase, V-type H+-ATPase, Na+-K+-2Cl--cotransporter, otolith matrix protein 1, and otolin-1 in six inner ear cell types bordering the endolymph. This fundamental cytological characterization of the rockfish inner ear epithelium illustrates the intricate physiological processes involved in otolith biomineralization and highlights how greater mechanistic understanding is necessary to predict their multistressor responses to future climate change.

Evolving views of ionic, osmotic and acid-base regulation in aquatic animals

The regulation of ionic, osmotic and acid-base (IOAB) conditions in biological fluids is among the most fundamental functions in all organisms; being surrounded by water uniquely shapes the IOAB regulatory strategies of water-breathing animals. Throughout its centennial history, Journal of Experimental Biology has established itself as a premier venue for publication of comparative, environmental and evolutionary studies on IOAB regulation. This Review provides a synopsis of IOAB regulation in aquatic animals, some of the most significant research milestones in the field, and evolving views about the underlying cellular mechanisms and their evolutionary implications. It also identifies promising areas for future research and proposes ideas for enhancing the impact of aquatic IOAB research.

Soluble adenylyl cyclase coordinates intracellular pH homeostasis and biomineralization in calcifying cells of a marine animal

Biomineralizing cells concentrate dissolved inorganic carbon (DIC) and remove protons from the site of mineral precipitation. However, the molecular regulatory mechanisms that orchestrate pH homeostasis and biomineralization of calcifying cells are poorly understood. Here, we report that the acid-base sensing enzyme soluble adenylyl cyclase (sAC) coordinates intracellular pH (pH_i) regulation in the calcifying primary mesenchyme cells (PMCs) of sea urchin larvae. Single-cell transcriptomics, in situ hybridization, and immunocytochemistry elucidated the spatiotemporal expression of sAC during skeletogenesis. Live pH_i imaging of PMCs revealed that the downregulation of sAC activity with two structurally unrelated small molecules inhibited pH_i regulation of PMCs, an effect that was rescued by the addition of cell-permeable cAMP. Pharmacological sAC inhibition also significantly reduced normal spicule growth and spicule regeneration, establishing a link between PMC pH_i regulation and biomineralization. Finally, increased expression of sAC mRNA was detected during skeleton remineralization and exposure to CO₂-induced acidification. These findings suggest that transcriptional regulation of sAC is required to promote remineralization and to compensate for acidic stress. This work highlights the central role of sAC in coordinating acid-base regulation and biomineralization in calcifying cells of a marine animal.

Dining on the dead in the deep: Active NH4 + excretion via Na+ /H+ (NH4 + ) exchange in the highly ammonia tolerant Pacific hagfish, Eptatretus stoutii

AIM

Pacific hagfish are exceptionally tolerant to high environmental ammonia (HEA). Here, we elucidated a cellular mechanism that enables hagfish to actively excrete ammonia against steep ammonia gradients expected to be found inside a decomposing whale carcass.

METHODS

Hagfish were exposed to varying concentrations of HEA in the presence or absence of environmental Na⁺ , while plasma ammonia levels were tracked. ¹⁴ C-methylammonium was used as a proxy for NH₄ ⁺ to measure efflux in whole animals and in isolated gill pouches; the latter allowed us to assess the effects of amiloride specifically on Na⁺ /H⁺ exchangers (NHEs) in gill cells. Western blotting and immunohistochemistry were utilized to evaluate the abundance and sub-cellular localization of Rhesus glycoprotein (Rh) channels in the response to HEA.

RESULTS

Hagfish actively excreted NH₄ ⁺ against steep inwardly directed E_NH4 ⁺ (ΔE_NH4 ⁺  ~ 35 mV) and pNH₃ (ΔpNH₃  ~ 2000 μtorr) gradients. Active NH₄ ⁺ excretion and plasma ammonia hypo-regulation were contingent on the presence of environmental Na⁺ , indicating a Na⁺ /NH₄ ⁺ exchange mechanism. Active NH₄ ⁺ excretion across isolated gill pouches was amiloride-sensitive. Exposure to HEA resulted in decreased abundance of Rh channels in the apical membrane of gill ionocytes.

CONCLUSIONS

During HEA exposure, hagfish can actively excrete ammonia against a steep concentration gradient using apical NHEs energized by Na⁺ -K⁺ -ATPase in gill ionocytes. Additionally, apical Rh channels are removed from the apical membrane, presumably to reduce ammonia loading from the environment. We suggest that this mechanism allows hagfish to maintain tolerable ammonia levels while feeding inside decomposing carrion, allowing them to exploit nutrient-rich food-falls.

Optimizing immunostaining of archival fish samples to enhance museum collection potential

Immunohistochemistry (IHC) is a powerful biochemical technique that uses antibodies to specifically label and visualize proteins of interests within biological samples. However, fluid-preserved specimens within natural history collection often use fixatives and protocols that induce high background signal (autofluorescence), which hampers IHC as it produces low signal-to-noise ratio. Here, we explored techniques to reduce autofluorescence using sodium borohydride (SBH), citrate buffer, and their combination on fish tissue preserved with paraformaldehyde, formaldehyde, ethanol, and glutaraldehyde. We found SBH was the most effective quenching technique, and applied this pretreatment to the gill or skin of 10 different archival fishes - including specimens that had been preserved in formaldehyde or ethanol for up to 65 and 37 years, respectively. The enzyme Na⁺/K⁺-ATPase (NKA) was successfully immunostained and imaged using confocal fluorescence microscopy, allowing for the identification and characterization of NKA-rich ionocytes essential for fish ionic and acid-base homeostasis. Altogether, our SBH-based method facilitates the use of IHC on archival samples, and unlocks the historical record on fish biological responses to environmental factors (such as climate change) using specimens from natural history collections that were preserved decades to centuries ago.

EcoPhysioMechanics: Integrating energetics and biomechanics to understand fish locomotion under climate change

Ecological physiologists and biomechanists have been broadly investigating swimming performance in a diversity of fishes, however the connection between form, function and energetics of locomotion has been rarely evaluated in the same system and under climate change scenarios. In this perspective I argue that working within the framework of 'EcoPhysioMechanics', i.e., integrating energetics and biomechanics tools, to measure locomotor performance and behavior under different abiotic factors, improves our understanding of the mechanisms, limits and costs of movement. To demonstrate how ecophysiomechanics can be applied to locomotor studies, I outline how linking biomechanics and physiology allows us to understand how fishes may modulate their movement to achieve high speeds or reduce the costs of locomotion. I also discuss how the framework is necessary to quantify swimming capacity under climate change scenarios. Finally, I discuss current dearth of integrative studies and gaps in empirical datasets that are necessary to understand fish swimming under changing environments.