Metabolic shifts in the Antarctic fish Notothenia rossii in response to rising temperature and PCO2

Warming-induced "plastic floors" improve hypoxia vulnerability, not aerobic scope, in red drum (Sciaenops ocellatus)

Ocean warming is a prevailing threat to marine ectotherms. Recently the "plastic floors, concrete ceilings" hypothesis was proposed, which suggests that a warmed fish will acclimate to higher temperatures by reducing standard metabolic rate (SMR) while keeping maximum metabolic rate (MMR) stable, therefore improving aerobic scope (AS). Here we evaluated this hypothesis on red drum (Sciaenops ocellatus) while incorporating measures of hypoxia vulnerability (critical oxygen threshold; Pcrit) and mitochondrial performance. Fish were subjected to a 12-week acclimation to 20 °C or 28 °C. Respirometry was performed every 4 weeks to obtain metabolic rate and Pcrit; mitochondrial respirometry was performed on liver and heart samples at the end of the acclimation. 28 °C fish had a significantly higher SMR, MMR, and Pcrit than 20 °C controls at time 0, but SMR declined by 36.2 % over the 12-week acclimation. No change in SMR was observed in the control treatment. Contrary to expectations, SMR suppression did not improve AS relative to time 0 owing to a progressive decline in MMR over acclimation time. Pcrit decreased by 27.2 % in the warm-acclimated fishes, which resulted in temperature treatments having statistically similar values by 12-weeks. No differences in mitochondrial traits were observed in the heart - despite a Δ8 °C assay temperature - while liver respiratory and coupling control ratios were significantly improved, suggesting that mitochondrial plasticity may contribute to the reduced SMR with warming. Overall, this work suggests that warming induced metabolic suppression offsets the deleterious consequences of high oxygen demand on hypoxia vulnerability, and in so doing greatly expands the theoretical range of metabolically available habitats for red drum.

Testing the Resilience, Physiological Plasticity and Mechanisms Underlying Upper Temperature Limits of Antarctic Marine Ectotherms

Antarctic marine ectotherms live in the constant cold and are characterised by limited resilience to elevated temperature. Here we tested three of the central paradigms underlying this resilience. Firstly, we assessed the ability of eight species, from seven classes representing a range of functional groups, to survive, for 100 to 303 days, at temperatures 0 to 4 °C above previously calculated long-term temperature limits. Survivors were then tested for acclimation responses to acute warming and acclimatisation, in the field, was tested in the seastar Odontaster validus collected in different years, seasons and locations within Antarctica. Finally, we tested the importance of oxygen limitation in controlling upper thermal limits. We found that four of 11 species studied were able to survive for more than 245 days (245-303 days) at higher than previously recorded temperatures, between 6 and 10 °C. Only survivors of the anemone Urticinopsis antarctica did not acclimate CTmax and there was no evidence of acclimatisation in O. validus. We found species-specific effects of mild hyperoxia (30% oxygen) on survival duration, which was extended (two species), not changed (four species) or reduced (one species), re-enforcing that oxygen limitation is not universal in dictating thermal survival thresholds. Thermal sensitivity is clearly the product of multiple ecological and physiological capacities, and this diversity of response needs further investigation and interpretation to improve our ability to predict future patterns of biodiversity.

A simulated heat wave-but not herbicide exposure-alters resource investment strategy in an insect

Animals are increasingly exposed to potential stressors related to environmental change, and multiple stressors may alter the dynamics by which animals acquire resources and invest those resources into important life-history traits. Stress may lead to the prioritization of current reproduction to maximize lifetime reproduction (i.e., terminal investment [TI]) or, in contrast, prioritize somatic investment over current reproduction to facilitate future reproductive opportunities (i.e., reproductive restraint [RR]). Tests of the TI and RR hypotheses typically use immune challenges as stressors, and have not been explicitly tested in the context of environmental change even though warming influences resource allocation patterns across taxa. Further, the multiple-stressor framework has been a useful construct to clarify the costs of complex environmental shifts to animals, but it has not been leveraged to understand such effects on investment strategy. Thus, we tested the TI and RR hypotheses by manipulating widespread features of environmental change-glyphosate-based herbicide (GBH; Roundup®) exposure and a simulated heat wave-in the variable field cricket (Gryllus lineaticeps). A simulated heat wave affected the life-history tradeoff between investment into reproduction and soma. Specifically, heat wave prioritized investment into ovary mass over non-reproductive tissue, even after accounting for food consumption, in support of the TI hypothesis. In contrast, GBH exposure did not affect any measured trait, and crickets did not discriminate between tap water and GBH solution during drinking. Therefore, some-but not all-aspects of environmental change may alter resource investment strategies in animals. We encourage continued integration of the multiple-stressor framework and life-history theory to better understand how animals respond to their rapidly changing environments.

Ocean acidification and seasonal temperature extremes combine to impair the thermal physiology of a sub-Antarctic fish

To predict the potential impacts of climate change on marine organisms, it is critical to understand how multiple stressors constrain the physiology and distribution of species. We evaluated the effects of seasonal changes in seawater temperature and near-future ocean acidification (OA) on organismal and sub-organismal traits associated with the thermal performance of Eleginops maclovinus, a sub-Antarctic notothenioid species with economic importance to sport and artisanal fisheries in southern South America. Juveniles were exposed to mean winter and summer sea surface temperatures (4 and 10 °C) at present-day and near-future pCO₂ levels (~500 and 1800 μatm). After a month, the Critical Thermal maximum and minimum (CTmax, CTmin) of fish were measured using the Critical Thermal Methodology and the aerobic scope of fish was measured based on the difference between their maximal and standard rates determined from intermittent flow respirometry. Lipid peroxidation and the antioxidant capacity were also quantified to estimate the oxidative damage potentially caused to gill and liver tissue. Although CTmax and CTmin were higher in individuals acclimated to summer versus winter temperatures, the increase in CTmax was minimal in juveniles exposed to the near-future compared to present-day pCO₂ levels (there was a significant interaction between temperature and pCO₂ on CTmax). The reduction in the thermal tolerance range under summer temperatures and near-future OA conditions was associated with a reduction in the aerobic scope observed at the elevated pCO₂ level. Moreover, an oxidative stress condition was detected in the gill and liver tissues. Thus, chronic exposure to OA and the current summer temperatures pose limits to the thermal performance of juvenile E. maclovinus at the organismal and sub-organismal levels, making this species vulnerable to projected climate-driven warming.

A comparative and ontogenetic examination of mitochondrial function in Antarctic notothenioid species

Notothenioidei fishes have evolved under stable cold temperatures; however, ocean conditions are changing globally, with polar regions poised to experience the greatest changes in environmental factors, such as warming. These stressors have the potential to dramatically affect energetic demands, and the persistence of the notothenioids will be dependent on metabolic capacity, or the ability to match energy supply with energy demand, to restore homeostasis in the face of changing climate conditions. In this study we examined aerobic metabolic capacity in three species, Trematomus bernacchii, T. pennellii and T. newnesi, and between two life stages, juvenile and adult, by assessing mitochondrial function of permeabilized cardiac fibers. Respiratory capacity differed among the adult notothenioids in this study, with greater oxidative phosphorylation (OXPHOS) respiration in the pelagic T. newnesi than the benthic T. bernacchii and T. pennellii. The variation in mitochondrial respiratory capacity was likely driven by differences in the mitochondrial content, as measured by citrate synthase activity, which was the highest in T. newnesi. In addition to high OXPHOS, T. newnesi exhibited lower LEAK respiration, resulting in greater mitochondrial efficiency than either T. bernacchii or T. pennellii. Life stage largely had an effect on mitochondrial efficiency and excess complex IV capacity, but there were little differences in OXPHOS respiration and electron transfer capacity, pointing to a lack of significant differences in the metabolic capacity between juveniles and adults. Overall, these results demonstrate species-specific differences in cardiac metabolic capacity, which may influence the acclimation potential of notothenioid fishes to changing environmental conditions.

Warm acclimation alters antioxidant defences but not metabolic capacities in the Antarctic fish, Notothenia coriiceps

The Southern Ocean surrounding the Western Antarctic Peninsula region is rapidly warming. Survival of members of the dominant suborder of Antarctic fishes, the Notothenioidei, will likely require thermal plasticity and adaptive capacity in key traits delimiting thermal tolerance. Herein, we have assessed the thermal plasticity of several cellular and biochemical pathways, many of which are known to be associated with thermal tolerance in notothenioids, including mitochondrial function, activities of aerobic and anaerobic enzymes, antioxidant defences, protein ubiquitination and degradation in cardiac, oxidative skeletal muscles and gill of Notothenia coriiceps warm acclimated to 4°C for 22 days or 5°C for 42 days. Levels of triacylglycerol (TAG) were measured in liver and oxidative and glycolytic skeletal muscles, and glycogen in liver and glycolytic muscle to assess changes in energy stores. Metabolic pathways displayed minimal thermal plasticity, yet antioxidant defences were lower in heart and oxidative skeletal muscles of warm-acclimated animals compared with animals held at ambient temperature. Despite higher metabolic rates at elevated temperature, energy storage depots of TAG and glycogen increase in liver and remain unchanged in muscle with warm acclimation. Overall, our studies reveal that N. coriiceps displays thermal plasticity in some key traits that may contribute to their survival as the Southern Ocean continues to warm.

Effects of exposure to elevated temperature and different food levels on the escape response and metabolism of early life stages of white seabream, Diplodus sargus

Recent literature suggests that anthropogenic stressors can disrupt ecologically relevant behaviours in fish, such as the ability to escape from predators. Disruption of these behaviours at critical life history transitions, such as the transition from the pelagic environment to the juvenile/adult habitat, may have even greater repercussions. The literature suggests that an increase in temperature can affect fish escape response, as well as metabolism; however, few studies have focused on the acute sensitivity responses and the potential for acclimation through developmental plasticity. Here, we aimed at evaluating the acute and long-term effects of exposure to warming conditions on the escape response and routine metabolic rate (RMR) of early life stages of the white seabream, Diplodus sargus. Additionally, as food availability may modulate the response to warming, we further tested the effects of long-term exposure to high temperature and food shortage, as individual and interacting drivers, on escape response and RMR. Temperature treatments were adjusted to ambient temperature (19°C) and a high temperature (22°C). Feeding treatments were established as high ration and low ration (50% of high ration). Escape response and RMR were measured after the high temperature was reached (acute exposure) and after 4 weeks (prolonged exposure). Acute warming had a significant effect on escape response and generated an upward trend in RMR. In the long term, however, there seems to be an acclimation of the escape response and RMR. Food shortage, interacting with high temperature, led to an increase in latency response and a significant reduction in RMR. The current study provides relevant experimental data on fishes' behavioural and physiological responses to the combined effects of multiple stressors. This knowledge can be incorporated in recruitment models, thereby contributing to fine-tuning of models required for fisheries management and species conservation.

Gradual increase of temperature trigger metabolic and oxidative responses in plasma and body tissues in the Antarctic fish Notothenia rossii

Antarctica is considered a thermally stable ecosystem; however, climate studies point to increases in water temperatures in this region. These thermal changes may affect the biological processes and promote metabolic changes in the adapted organisms that live in this region, rendering the animals more vulnerable to oxidative damage. This study assessed the effect of acclimation temperature on the levels of stress response markers in plasma, kidney, gill, liver, and brain tissues of Notothenia rossii subjected to gradual temperature changes of 0.5 °C/day until reaching temperatures of 2, 4, 6, and 8 °C. Under the effect of the 0.5 °C/day acclimation rate, gill tissue showed increased glutathione-S-transferase (GST) activity; kidney tissue showed increased H⁺-ATPase activity. In the liver, there was also an increase in GSH. In plasma, gradual decreases in the concentrations of total proteins and globulins were observed. These responses indicate a higher production of reactive oxygen species ROS, an imbalance in energy demand, and a lack in protein synthesis. Gradual increase in temperature may cause opposite responses to the thermal shock model in N. rossii.

Combined effects of ocean warming and acidification on marine fish and shellfish: A molecule to ecosystem perspective

It is expected that by 2050 human population will exceed nine billion leading to increased pressure on marine ecosystems. Therefore, it is conjectured various levels of ecosystem functioning starting from individual to population-level, species distribution, food webs and trophic interaction dynamics will be severely jeopardized in coming decades. Ocean warming and acidification are two prime threats to marine biota, yet studies about their cumulative effect on marine fish and shellfishes are still in its infancy. This review assesses existing information regarding the interactive effects of global environmental factors like warming and acidification in the perspective of marine capture fisheries and aquaculture industry. As climate change continues, distribution pattern of species is likely to be altered which will impact fisheries and fishing patterns. Our work is an attempt to compile the existing literatures in the biological perspective of the above-mentioned stressors and accentuate a clear outline of knowledge in this subject. We reviewed studies deciphering the biological consequences of warming and acidification on fish and shellfishes in the light of a molecule to ecosystem perspective. Here, for the first time impacts of these two global environmental drivers are discussed in a holistic manner taking into account growth, survival, behavioural response, prey predator dynamics, calcification, biomineralization, reproduction, physiology, thermal tolerance, molecular level responses as well as immune system and disease susceptibility. We suggest urgent focus on more robust, long term, comprehensive and ecologically realistic studies that will significantly contribute to the understanding of organism's response to climate change for sustainable capture fisheries and aquaculture.

Antarctic teleosts with and without hemoglobin behaviorally mitigate deleterious effects of acute environmental warming

Recent studies forecast that many ectothermic animals, especially aquatic stenotherms, may not be able to thrive or even survive predicted climate change. These projections, however, generally do not call much attention to the role of behavior, an essential thermoregulatory mechanism of many ectotherms. Here we characterize species-specific locomotor and respiratory responses to acute ambient warming in two highly stenothermic Antarctic Notothenioid fishes, one of which (Chaenocephalus aceratus) lacks hemoglobin and appears to be less tolerant to thermal stress as compared to the other (Notothenia coriiceps), which expresses hemoglobin. At the onset of ambient warming, both species perform distinct locomotor maneuvers that appear to include avoidance reactions. In response to unavoidable progressive hyperthermia, fishes demonstrate a range of species-specific maneuvers, all of which appear to provide some mitigation of the deleterious effects of obligatory thermoconformation and to compensate for increasing metabolic demand by enhancing the efficacy of branchial respiration. As temperature continues to rise, Chaenocephalus aceratus supplements these behaviors with intensive pectoral fin fanning which may facilitate cutaneous respiration through its scaleless integument, and Notothenia coriiceps manifests respiratory-locomotor coupling during repetitive startle-like maneuvers which may further augment gill ventilation. The latter behaviors, found only in Notothenia coriiceps, have highly stereotyped appearance resembling Fixed Action Pattern sequences. Altogether, this behavioral flexibility could contribute to the reduction of the detrimental effects of acute thermal stress within a limited thermal range. In an ecologically relevant setting, this may enable efficient thermoregulation of fishes by habitat selection, thus facilitating their resilience in persistent environmental change.

Warming, not CO2-acidified seawater, alters otolith development of juvenile Antarctic emerald rockcod (Trematomus bernacchii)

The combustion of fossil fuels is currently causing rapid rates of ocean warming and acidification worldwide. Projected changes in these parameters have been repeatedly observed to stress the physiological limits and plasticity of many marine species from the molecular to organismal levels. High latitude oceans are among the fastest changing ecosystems; therefore, polar species are projected to be some of the most vulnerable to climate change. Antarctic species are particularly sensitive to environmental change, having evolved for millions of years under stable ocean conditions. Otoliths, calcified structures found in a fish’s inner ear used to sense movement and direction, have been shown to be affected by both warming and CO2-acidified seawater in temperate and tropical fishes but there is no work to date on Antarctic fishes. In this study, juvenile emerald rockcod (Trematomus bernacchii) were exposed to projected seawater warming and CO2-acidification for the year 2100 over 28 days. Sagittal otoliths were analyzed for changes in area, perimeter, length, width and shape. We found ocean warming increased the growth rate of otoliths, while CO2-acidified seawater and the interaction of warming and acidification did not have an effect on otolith development. Elevated temperature also altered the shape of otoliths. If otolith development is altered under future warming scenarios, sensory functions such as hearing, orientation, and movement may potentially be impaired. Changes in these basic somatic abilities could have broad implications for the general capabilities and ecology of early life stages of Antarctic fishes.

Productivity and Change in Fish and Squid in the Southern Ocean

Southern Ocean ecosystems are globally important and vulnerable to global drivers of change, yet they remain challenging to study. Fish and squid make up a significant portion of the biomass within the Southern Ocean, filling key roles in food webs from forage to mid-trophic species and top predators. They comprise a diverse array of species uniquely adapted to the extreme habitats of the region. Adaptations such as antifreeze glycoproteins, lipid-retention, extended larval phases, delayed senescence, and energy-conserving life strategies equip Antarctic fish and squid to withstand the dark winters and yearlong subzero temperatures experienced in much of the Southern Ocean. In addition to krill exploitation, the comparatively high commercial value of Antarctic fish, particularly the lucrative toothfish, drives fisheries interests, which has included illegal fishing. Uncertainty about the population dynamics of target species and ecosystem structure and function more broadly has necessitated a precautionary, ecosystem approach to managing these stocks and enabling the recovery of depleted species. Fisheries currently remain the major local driver of change in Southern Ocean fish productivity, but global climate change presents an even greater challenge to assessing future changes. Parts of the Southern Ocean are experiencing ocean-warming, such as the West Antarctic Peninsula, while other areas, such as the Ross Sea shelf, have undergone cooling in recent years. These trends are expected to result in a redistribution of species based on their tolerances to different temperature regimes. Climate variability may impair the migratory response of these species to environmental change, while imposing increased pressures on recruitment. Fisheries and climate change, coupled with related local and global drivers such as pollution and sea ice change, have the potential to produce synergistic impacts that compound the risks to Antarctic fish and squid species. The uncertainty surrounding how different species will respond to these challenges, given their varying life histories, environmental dependencies, and resiliencies, necessitates regular assessment to inform conservation and management decisions. Urgent attention is needed to determine whether the current management strategies are suitably precautionary to achieve conservation objectives in light of the impending changes to the ecosystem.

Transcriptional changes revealed water acidification leads to the immune response and ovary maturation delay in the Chinese mitten crab Eriocheir sinensis

Nowadays, due to increasing carbon dioxide released, water acidification poses a series of serious impacts on aquatic organisms. To evaluate the effects of water acidification on crustaceans, we focused on the Chinese mitten crab Eriocheir sinensis, which is a spawning migration and farmed species in China. Based on histological and oocyte transparent liquid observation, we found that the acidified environment significantly delayed the ovarian maturation of E. sinensis. Moreover, RNA-seq was applied to obtain gene expression profile from the crab's gills and ovaries in response to acidified environment. Compared with control groups, a total of 5471 differentially expressed genes (DEGs) were identified in acidified gills and 485 DEGs were identified in acidified ovaries. Enrichment analysis indicated that some pathways also responded to the acidified environment, such as PI3K-Akt signaling pathway, Chemokine signaling pathway, apoptosis, and toll-like receptor signaling pathway. Subsequently, some DEGs involved in immune response (ALF, Cathepsin A, HSP70, HSP90, and catalase) and ovarian maturation (Cyclin B, Fem-1a, Fem-1b, and Fem-1c) were selected to further validate the influence of water acidification on gene expression by qRT-PCR. The results showed that the expression level of immune-related genes was significantly increased to response to the water acidification, while the ovarian maturation-related genes were significantly decreased. Overall, our data suggested that E. sinensis was sensitive to the reduced pH. This comparative transcriptome also provides valuable molecular information on the mechanisms of the crustaceans responding to acidified environment.

Resilience of cardiac performance in Antarctic notothenioid fishes in a warming climate

Warming in the region of the Western Antarctic Peninsula is occurring at an unprecedented rate, which may threaten the survival of Antarctic notothenioid fishes. Herein, we review studies characterizing thermal tolerance and cardiac performance in notothenioids - a group that includes both red-blooded species and the white-blooded, haemoglobinless icefishes - as well as the relevant biochemistry associated with cardiac failure during an acute temperature ramp. Because icefishes do not feed in captivity, making long-term acclimation studies unfeasible, we focus only on the responses of red-blooded notothenioids to warm acclimation. With acute warming, hearts of the white-blooded icefish Chaenocephalus aceratus display persistent arrhythmia at a lower temperature (8°C) compared with those of the red-blooded Notothenia coriiceps (14°C). When compared with the icefish, the enhanced cardiac performance of N. coriiceps during warming is associated with greater aerobic capacity, higher ATP levels, less oxidative damage and enhanced membrane integrity. Cardiac performance can be improved in N. coriiceps with warm acclimation to 5°C for 6-9 weeks, accompanied by an increase in the temperature at which cardiac failure occurs. Also, both cardiac mitochondrial and microsomal membranes are remodelled in response to warm acclimation in N. coriiceps, displaying homeoviscous adaptation. Overall, cardiac performance in N. coriiceps is malleable and resilient to warming, yet thermal tolerance and plasticity vary among different species of notothenioid fishes; disruptions to the Antarctic ecosystem driven by climate warming and other anthropogenic activities endanger the survival of notothenioids, warranting greater protection afforded by an expansion of marine protected areas.

When it's hot and dry: life-history strategy influences the effects of heat waves and water limitation

The frequency, duration and co-occurrence of several environmental stressors, such as heat waves and droughts, are increasing globally. Such multiple stressors may have compounding or interactive effects on animals, resulting in either additive or non-additive costs, but animals may mitigate these costs through various strategies of resource conservation or shifts in resource allocation. Through a factorial experiment, we investigated the independent and interactive effects of a simulated heat wave and water limitation on life-history, physiological and behavioral traits. We used the variable field cricket, Gryllus lineaticeps, which exhibits a wing dimorphism that mediates two distinct life-history strategies during early adulthood. Long-winged individuals invest in flight musculature and are typically flight capable, whereas short-winged individuals lack flight musculature and capacity. A comprehensive and integrative approach with G. lineaticeps allowed us to examine whether life-history strategy influenced the costs of multiple stressors as well as the resulting cost-limiting strategies. Concurrent heat wave and water limitation resulted in largely non-additive and single-stressor costs to important traits (e.g. survival and water balance), extensive shifts in resource allocation priorities (e.g. reduced prioritization of body mass) and a limited capacity to conserve resources (e.g. heat wave reduced energy use only when water was available). Life-history strategy influenced the emergency life-history stage because wing morphology and stressor(s) interacted to influence body mass, boldness behavior and immunocompetence. Our results demonstrate that water availability and life-history strategy should be incorporated into future studies integrating important conceptual frameworks of stress across a suite of traits - from survival and life history to behavior and physiology.

Transcriptional and Catalytic Responsiveness of the Antarctic Fish Trematomus bernacchii Antioxidant System toward Multiple Stressors

Ocean-warming and acidification jeopardize Antarctic marine species, adapted to cold and constant conditions and naturally exposed to high pro-oxidant pressures and cadmium (Cd) bioavailability. The aim of this study was to investigate if projected temperature increase and pH reduction may affect the accumulation and the effects of Cd in the rockcod Trematomus bernacchii. Organisms were exposed for 14 days to six scenarios, combining environmental or increased temperature (−1 °C, +1 °C) and control or reduced pH (8.05, 7.60), either with or without Cd (40 µg/L). Responses in liver and gills were analyzed at different levels, including mRNA and functional measurements of metallothioneins and of a wide battery of antioxidants, integrated with the evaluation of the total antioxidant capacity and onset of oxidative damages. In the gills, metallothioneins and mRNA of antioxidant genes (nrf2, keap1, cat, gpx1) increased after Cd exposure, but such effects were softened by warming and acidification. Antioxidants showed slighter variations at the enzymatic level, while Cd caused glutathione increase under warming and acidified scenarios. In the liver, due to higher basal antioxidant protection, limited effects were observed. Genotoxic damage increased under the combined stressors scenario. Overall results highlighted the modulation of the oxidative stress response to Cd by multiple stressors, suggesting the vulnerability of T. bernacchii under predicted ocean change scenarios.

Ocean acidification compromises energy management in Sparus aurata (Pisces: Teleostei)

The effects of ocean acidification mediated by an increase in water pCO₂ levels on marine organisms are currently under debate. Elevated CO₂ concentrations in the seawater induce several physiological responses in teleost fish, including acid-base imbalances and osmoregulatory changes. However, the consequences of CO₂ levels enhancement on energy metabolism are mostly unknown. Here we show that 5 weeks of exposure to hypercapnia (950 and 1800 μatm CO₂) altered intermediary metabolism of gilthead seabream (Sparus aurata) compared to fish acclimated to current ocean values (440 μatm CO₂). We found that seabream compromises its physiological acid-base balance with increasing water CO₂ levels and the subsequent acidification. Intestinal regions (anterior, mid, and rectum) engaged in maintaining this balance are thus altered, as seen for Na⁺/K⁺-ATPase and the vacuolar-type H⁺-ATPase activities. Moreover, liver and muscle counteracted these effects by increasing catabolic routes e.g., glycogenolysis, glycolysis, amino acid turnover, and lipid catabolism, and plasma energy metabolites were altered. Our results demonstrate how a relatively short period of 5 weeks of water hypercapnia is likely to disrupt the acid-base balance, osmoregulatory capacity and intermediary metabolism in S. aurata. However, long-term studies are necessary to fully understand the consequences of ocean acidification on growth and other energy-demanding activities, such as reproduction.

Thermal acclimation of tropical coral reef fishes to global heat waves

As climate-driven heat waves become more frequent and intense, there is increasing urgency to understand how thermally sensitive species are responding. Acute heating events lasting days to months may elicit acclimation responses to improve performance and survival. However, the coordination of acclimation responses remains largely unknown for most stenothermal species. We documented the chronology of 18 metabolic and cardiorespiratory changes that occur in the gills, blood, spleen, and muscles when tropical coral reef fishes are thermally stressed (+3.0°C above ambient). Using representative coral reef fishes (Caesio cuning and Cheilodipterus quinquelineatus) separated by >100 million years of evolution and with stark differences in major life-history characteristics (i.e. lifespan, habitat use, mobility, etc.), we show that exposure duration illicited coordinated responses in 13 tissue and organ systems over 5 weeks. The onset and duration of biomarker responses differed between species, with C. cuning – an active, mobile species – initiating acclimation responses to unavoidable thermal stress within the first week of heat exposure; conversely, C. quinquelineatus – a sessile, territorial species – exhibited comparatively reduced acclimation responses that were delayed through time. Seven biomarkers, including red muscle citrate synthase and lactate dehydrogenase activities, blood glucose and hemoglobin concentrations, spleen somatic index, and gill lamellar perimeter and width, proved critical in evaluating acclimation progression and completion, as these provided consistent evaluation of thermal responses across species.

Acidification and hypoxia interactively affect metabolism in embryos, but not larvae, of the coastal forage fish Menidia menidia

Ocean acidification is occurring in conjunction with warming and deoxygenation as a result of anthropogenic greenhouse gas emissions. Multistressor experiments are critically needed to better understand the sensitivity of marine organisms to these concurrent changes. Growth and survival responses to acidification have been documented for many marine species, but studies that explore underlying physiological mechanisms of carbon dioxide (CO2) sensitivity are less common. We investigated oxygen consumption rates as proxies for metabolic responses in embryos and newly hatched larvae of an estuarine forage fish (Atlantic silverside, Menidia menidia) to factorial combinations of CO2×temperature or CO2×oxygen. Metabolic rates of embryos and larvae significantly increased with temperature, but partial pressure of CO2 (PCO2 ) alone did not affect metabolic rates in any experiment. However, there was a significant interaction between PCO2  and partial pressure of oxygen (PO2 ) in embryos, because metabolic rates were unaffected by PO2  level at ambient PCO2 , but decreased with declining PO2  under elevated PCO2 For larvae, however, PCO2  and PO2  had no significant effect on metabolic rates. Our findings suggest high individual variability in metabolic responses to high PCO2 , perhaps owing to parental effects and time of spawning. We conclude that early life metabolism is largely resilient to elevated PCO2  in this species, but that acidification likely influences energetic responses and thus vulnerability to hypoxia.

Understanding the Metabolic Capacity of Antarctic Fishes to Acclimate to Future Ocean Conditions

Antarctic fishes have evolved under stable, extreme cold temperatures for millions of years. Adapted to thrive in the cold environment, their specialized phenotypes will likely render them particularly susceptible to future ocean warming and acidification as a result of climate change. Moving from a period of stability to one of environmental change, species persistence will depend on maintaining energetic equilibrium, or sustaining the increased energy demand without compromising important biological functions such as growth and reproduction. Metabolic capacity to acclimate, marked by a return to metabolic equilibrium through physiological compensation of routine metabolic rate (RMR), will likely determine which species will be better poised to cope with shifts in environmental conditions. Focusing on the suborder Notothenioidei, a dominant group of Antarctic fishes, and in particular four well-studied species, Trematomus bernacchii, Pagothenia borchgrevinki, Notothenia rossii, and N. coriiceps, we discuss metabolic acclimation potential to warming and CO2-acidification using an integrative and comparative framework. There are species-specific differences in the physiological compensation of RMR during warming and the duration of acclimation time required to achieve compensation; for some species, RMR fully recovered within 3.5 weeks of exposure, such as P. borchgrevinki, while for other species, such as N. coriiceps, RMR remained significantly elevated past 9 weeks of exposure. In all instances, added exposure to increased PCO2, further compromised the ability of species to return RMR to pre-exposure levels. The period of metabolic imbalance, marked by elevated RMR, was underlined by energetic disturbance and elevated energetic costs, which shifted energy away from fitness-related functions, such as growth. In T. bernacchii and N. coriiceps, long duration of elevated RMR impacted condition factor and/or growth rate. Low growth rate can affect development and ultimately the timing of reproduction, severely compromising the species' survival potential and the biodiversity of the notothenioid lineage. Therefore, the ability to achieve full compensation of RMR, and in a short-time frame, in order to avoid long term consequences of metabolic imbalance, will likely be an important determinant in a species' capacity to persist in a changing environment. Much work is still required to develop our understanding of the bioenergetics of Antarctic fishes in the face of environmental change, and a targeted approach of nesting a mechanistic focus in an ecological and comparative framework will better aid our predictions on the effect of global climate change on species persistence in the polar regions.

Mechanisms of acid-base regulation following respiratory alkalosis in red drum (Sciaenops ocellatus)

Respiratory acidosis and subsequent metabolic compensation are well-studied processes in fish exposed to elevated CO₂ (hypercapnia). Yet, such exposures in the marine environment are invariably accompanied by a return of environmental CO₂ to atmospheric baselines. This understudied phenomenon has the potential to cause a respiratory alkalosis that would necessitate base excretion. Here we sought to explore this question and the associated physiological mechanisms that may accompany base excretions using the red drum (Sciaenops ocellatus). As expected, when high pCO₂ (15,000 μatm CO₂) acclimated red drum were transferred to normal pCO₂, their net H⁺ excretion shifted from positive (0.157 ± 0.044 μmol g^-1 h^-1) to negative (-0.606 ± 0.116 μmol g^-1 h^-1) in the 2 h post-transfer period. Net H⁺ excretion returned to control rates during the 3 to 24 h flux period. Gene expression and enzyme activity assays demonstrated that while the acidosis resulted in significant changes in several relevant transporters, no significant changes accompanied the alkalosis phase. Confocal microscopy was used to assess alkalosis-stimulated translocation of V-type H⁺ ATPase to the basolateral membrane previously seen in other marine species; however, no apparent translocation was observed. Overall, these data demonstrate that fluctuations in environmental CO₂ result in both acidic and alkalotic respiratory disturbances; however, red drum maintain sufficient regulatory capacity to accommodate base excretion. Furthermore, this work does not support a role for basolateral VHA translocation in metabolic compensation from a systemic alkalosis in teleosts.

Transgenerational regulation of cbln11 gene expression in the olfactory rosette of the European sea bass (Dicentrarchus labrax) exposed to ocean acidification

Elevated amounts of atmospheric CO₂ are causing ocean acidification (OA) that may affect marine organisms including fish species. While several studies carried out in fish revealed that OA induces short term dysfunction in sensory systems including regulation of neurons activity in olfactory epithelium, information on the effects of OA on other physiological processes and actors is scarcer. In the present study we focused our attention on a European sea bass (Dicentrarchus labrax) sghC1q gene, a member of the C1q-domain-containing (C1qDC) protein family. In vertebrates, C1qDC family includes actors involved in different physiological processes including immune response and synaptic organization. Our microsynteny analysis revealed that this sghC1q gene is the orthologous gene in European sea bass to zebrafish (Danio rerio) cbln11 gene. We cloned the full length cbln11 mRNA and identified the different domains (the signal peptide, the coiled coil region and the globular C1q domain) of the deduced protein sequence. Investigation of mRNA expression by qPCR and in situ hybridization revealed that cbln11gene is especially expressed in the non-sensory epithelium of the olfactory rosette at larval and adult stages. The expression of cbln11 mRNA was analysed by qPCR in the first generation (F0) of European sea bass broodstock exposed since larval stages to water pH of 8.0 (control) or 7.6 (predicted for year 2100) and in their offspring (F1) maintained in the environmental conditions of their parents. Our results showed that cbln11 mRNA expression level was lower in larvae exposed to OA then up-regulated at adult stage in the olfactory rosette of F0 and that this up-regulation is maintained under OA at larval and juvenile stages in F1. Overall, this work provides evidence of a transgenerational inheritance of OA-induced up-regulation of cbln11 gene expression in European sea bass. Further studies will investigate the potential immune function of cbln11 gene and the consequences of these regulations, as well as the possible implications in terms of fitness and adaptation to OA in European sea bass.

Effects of ocean acidification on Antarctic marine organisms: A meta-analysis

Southern Ocean waters are among the most vulnerable to ocean acidification. The projected increase in the CO2 level will cause changes in carbonate chemistry that are likely to be damaging to organisms inhabiting these waters. A meta-analysis was undertaken to examine the vulnerability of Antarctic marine biota occupying waters south of 60°S to ocean acidification. This meta-analysis showed that ocean acidification negatively affects autotrophic organisms, mainly phytoplankton, at CO2 levels above 1,000 μatm and invertebrates above 1,500 μatm, but positively affects bacterial abundance. The sensitivity of phytoplankton to ocean acidification was influenced by the experimental procedure used. Natural, mixed communities were more sensitive than single species in culture and showed a decline in chlorophyll a concentration, productivity, and photosynthetic health, as well as a shift in community composition at CO2 levels above 1,000 μatm. Invertebrates showed reduced fertilization rates and increased occurrence of larval abnormalities, as well as decreased calcification rates and increased shell dissolution with any increase in CO2 level above 1,500 μatm. Assessment of the vulnerability of fish and macroalgae to ocean acidification was limited by the number of studies available. Overall, this analysis indicates that many marine organisms in the Southern Ocean are likely to be susceptible to ocean acidification and thereby likely to change their contribution to ecosystem services in the future. Further studies are required to address the poor spatial coverage, lack of community or ecosystem-level studies, and the largely unknown potential for organisms to acclimate and/or adapt to the changing conditions.

Food availability modulates the combined effects of ocean acidification and warming on fish growth

When organisms are unable to feed ad libitum they may be more susceptible to negative effects of environmental stressors such as ocean acidification and warming (OAW). We reared sea bass (Dicentrarchus labrax) at 15 or 20 °C and at ambient or high PCO2 (650 versus 1750 µatm PCO2; pH = 8.1 or 7.6) at ad libitum feeding and observed no discernible effect of PCO2 on the size-at-age of juveniles after 277 (20 °C) and 367 (15 °C) days. Feeding trials were then conducted including a restricted ration (25% ad libitum). At 15 °C, growth rate increased with ration but was unaffected by PCO2. At 20 °C, acidification and warming acted antagonistically and low feeding level enhanced PCO2 effects. Differences in growth were not merely a consequence of lower food intake but also linked to changes in digestive efficiency. The specific activity of digestive enzymes (amylase, trypsin, phosphatase alkaline and aminopeptidase N) at 20 °C was lower at the higher PCO2 level. Our study highlights the importance of incorporating restricted feeding into experimental designs examining OAW and suggests that ad libitum feeding used in the majority of the studies to date may not have been suitable to detect impacts of ecological significance.

Ocean Acidification and Coastal Marine Invertebrates: Tracking CO2 Effects from Seawater to the Cell

In the last few decades, numerous studies have investigated the impacts of simulated ocean acidification on marine species and communities, particularly those inhabiting dynamic coastal systems. Despite these research efforts, there are many gaps in our understanding, particularly with respect to physiological mechanisms that lead to pathologies. In this review, we trace how carbonate system disturbances propagate from the coastal environment into marine invertebrates and highlight mechanistic links between these disturbances and organism function. We also point toward several processes related to basic invertebrate biology that are severely understudied and prevent an accurate understanding of how carbonate system dynamics influence organismic homeostasis and fitness-related traits. We recommend that significant research effort be directed to studying cellular phenotypes of invertebrates acclimated or adapted to elevated seawater pCO₂ using biochemical and physiological methods.

Seabream Larval Physiology under Ocean Warming and Acidification

The vulnerability of early fish stages represents a critical bottleneck for fish recruitment; therefore, it is essential to understand how climate change affects their physiology for more sustainable management of fisheries. Here, we investigated the effects of warming (OW; +4 °C) and acidification (OA; ΔpH = 0.5) on the heart and oxygen consumption rates, metabolic enzymatic machinery—namely citrate synthase (CS), lactate dehydrogenase (LDH), and ß-hydroxyacyl CoA dehydrogenase (HOAD), of seabream (Sparus aurata) larvae (fifteen days after hatch). Oxygen consumption and heart rates showed a significant increase with rising temperature, but decreased with pCO2. Results revealed a significant increase of LDH activity with OW and a significant decrease of the aerobic potential (CS and HOAD activity) of larvae with OA. In contrast, under OA, the activity levels of the enzyme LDH and the LDH:CS ratio indicated an enhancement of anaerobic pathways. Although such a short-term metabolic strategy may eventually sustain the basic costs of maintenance, it might not be adequate under the future chronic ocean conditions. Given that the potential for adaptation to new forthcoming conditions is yet experimentally unaccounted for this species, future research is essential to accurately predict the physiological performance of this commercially important species under future ocean conditions.

Future ocean warming may prove beneficial for the northern population of European seabass, but ocean acidification will not

The world's oceans are acidifying and warming as a result of increasing atmospheric CO₂ concentrations. The thermal tolerance of fish greatly depends on the cardiovascular ability to supply the tissues with oxygen. The highly oxygen-dependent heart mitochondria thus might play a key role in shaping an organism's tolerance to temperature. The present study aimed to investigate the effects of acute and chronic warming on the respiratory capacity of European sea bass (Dicentrarchus labrax L.) heart mitochondria. We hypothesized that acute warming would impair mitochondrial respiratory capacity, but be compensated for by life-time conditioning. Increasing P _CO2  may additionally cause shifts in metabolic pathways by inhibiting several enzymes of the cellular energy metabolism. Among other shifts in metabolic pathways, acute warming of heart mitochondria of cold life-conditioned fish increased leak respiration rate, suggesting a lower aerobic capacity to synthesize ATP with acute warming. However, thermal conditioning increased mitochondrial functionality, e.g. higher respiratory control ratios in heart mitochondria of warm life-conditioned compared with cold life-conditioned fish. Exposure to high P _CO2  synergistically amplified the effects of acute and long-term warming, but did not result in changes by itself. This high ability to maintain mitochondrial function under ocean acidification can be explained by the fact that seabass are generally able to acclimate to a variety of environmental conditions. Improved mitochondrial energy metabolism after warm conditioning could be due to the origin of this species in the warm waters of the Mediterranean. Our results also indicate that seabass are not yet fully adapted to the colder temperatures in their northern distribution range and might benefit from warmer temperatures in these latitudes.

Antarctic environmental change and biological responses

Antarctica and the surrounding Southern Ocean are facing complex environmental change. Their native biota has adapted to the region's extreme conditions over many millions of years. This unique biota is now challenged by environmental change and the direct impacts of human activity. The terrestrial biota is characterized by considerable physiological and ecological flexibility and is expected to show increases in productivity, population sizes and ranges of individual species, and community complexity. However, the establishment of non-native organisms in both terrestrial and marine ecosystems may present an even greater threat than climate change itself. In the marine environment, much more limited response flexibility means that even small levels of warming are threatening. Changing sea ice has large impacts on ecosystem processes, while ocean acidification and coastal freshening are expected to have major impacts.

From Africa to Antarctica: Exploring the Metabolism of Fish Heart Mitochondria Across a Wide Thermal Range

The thermal sensitivity of ectotherms is largely dictated by the impact of temperature on cellular bioenergetics, particularly on mitochondrial functions. As the thermal sensitivity of bioenergetic pathways depends on the structural and kinetic properties of its component enzymes, optimization of their collective function to different thermal niches is expected to have occurred through selection. In the present study, we sought to characterize mitochondrial phenotypic adjustments to thermal niches in eight ray-finned fish species occupying a wide range of thermal habitats by comparing the activities of key mitochondrial enzymes in their hearts. We measured the activity of four enzymes that control substrate entrance into the tricarboxylic acid (TCA) cycle: pyruvate kinase (PK), pyruvate dehydrogenase complex (PDHc), carnitine palmitoyltransferase (CPT), and hydroxyacyl-CoA dehydrogenase (HOAD). We also assayed enzymes of the electron transport system (ETS): complexes I, II, I + III, and IV. Enzymes were assayed at five temperatures (5, 10, 15, 20, and 25°C). Our results showed that the activity of CPT, a gatekeeper of the fatty acid pathway, was higher in the cold-water fish than in the warmer-adapted fish relative to the ETS (complexes I and III) when measured close to the species optimal temperatures. The activity of HOAD showed a similar pattern relative to CI + III and thermal environment. By contrast, PDHc and PK did not show the similar patterns with respect to CI + III and temperature. Cold-adapted species had high CIV activities compared to those of upstream complexes (I, II, I + III) whereas the converse was true for warm-adapted species. Our findings reveal a significant variability of heart mitochondrial organization among species that can be linked to temperature adaptation. Cold-adapted fish do not appear to compensate for PDHc activity but likely adjust fatty acids oxidation through higher activities of CPT and HOAD relative to complexes I + III.

Ocean acidification affects acid-base physiology and behaviour in a model invertebrate, the California sea hare (Aplysia californica)

Behavioural impairment following exposure to ocean acidification-relevant CO2 levels has been noted in a broad array of taxa. The underlying cause of these disruptions is thought to stem from alterations of ion gradients ( HC O 3 - / C l - ) across neuronal cell membranes that occur as a consequence of maintaining pH homeostasis via the accumulation of HC O 3 - . While behavioural impacts are widely documented, few studies have measured acid-base parameters in species showing behavioural disruptions. In addition, current studies examining mechanisms lack resolution in targeting specific neural pathways corresponding to a given behaviour. With these considerations in mind, acid-base parameters and behaviour were measured in a model organism used for decades as a research model to study learning, the California sea hare (Aplysia californica). Aplysia exposed to elevated CO2 increased haemolymph HC O 3 - , achieving full and partial pH compensation at 1200 and 3000 µatm CO2, respectively. Increased CO2 did not affect self-righting behaviour. In contrast, both levels of elevated CO2 reduced the time of the tail-withdrawal reflex, suggesting a reduction in antipredator response. Overall, these results confirm that Aplysia are promising models to examine mechanisms underlying CO2-induced behavioural disruptions since they regulate HC O 3 - and have behaviours linked to neural networks amenable to electrophysiological testing.

Effect of gradual temperature increase on the carbohydrate energy metabolism responses of the Antarctic fish Notothenia rossii

The warming of the Southern Ocean waters may affect the biological processes and the performance of the fish inhabiting it. The notothenioid group is metabolically specialized to low-temperature environments and may be vulnerable to the climatic changes imposed on the Antarctic continent. However, gradual temperature changes potentially allow an opportunity for plasticity adjustments. The present study evaluated the effect of gradual increase of temperature on the enzymatic and nonenzymatic parameters of energy metabolism in renal, branchial, hepatic, and encephalic tissue of Notothenia rossii subjected to a gradual temperature change of 0.5 °C/day until reaching 2 °C, 4 °C, 6 °C, and 8 °C. Under the effect of an acclimation rate of 0.5 °C/day, the gill tissue showed increased phosphofructokinase (PFK) enzyme activity. In the kidney, there was increased activity of the malate dehydrogenase (MDH), glucose-6-phosphatase (G6PDH), and glycogen phosphorylase (GP) enzymes. There was an increase in lactate concentration in the liver and an increase in GP enzyme activity in the brain. The specific tissue responses indicate the presence of thermal plasticity and an attempt to regulate energy metabolism to mitigate thermal stress in this species under these experimental conditions, possibly through the activation of glycolysis, gluconeogenesis, and glycogenolysis.

Effect of long-term thermal challenge on the Antarctic notothenioid Notothenia rossii

The thermal stability of the Antarctic Ocean raises questions concerning the metabolic plasticity of Antarctic notothenioids to changes in the environmental temperature. In this study, Notothenia rossii survived 90 days at 8 °C, and their condition factor level was maintained. However, their hepatosomatic (0.29×) index decreased, indicating a decrease in nutrient storage as a result of changes in the energy demands to support survival. At 8 °C, the plasma calcium, magnesium, cholesterol, and triglyceride concentrations decreased, whereas the glucose (1.91×) and albumin (1.26×) concentrations increased. The main energy substrate of the fish changed from lipids to glucose due to a marked increase in lactate dehydrogenase activity, as demonstrated by an increase in anaerobic metabolism. Moreover, malate dehydrogenase activity increased in all tissues, suggesting that fish acclimated at 8 °C exhibit enhanced gluconeogenesis. The aerobic demand increased only in the liver due to an increase (2.23×) in citrate synthase activity. Decreases in the activities of superoxide dismutase, catalase, and glutathione-S-transferase to levels that are most likely sufficient at 8 °C were observed, establishing a new physiological activity range for antioxidant defense. Our findings indicate that N. rossii has some compensatory mechanisms that enabled its long-term survival at 8 °C.

Combined Effects of Acute Temperature Change and Elevated pCO2 on the Metabolic Rates and Hypoxia Tolerances of Clearnose Skate (Rostaraja eglanteria), Summer Flounder (Paralichthys dentatus), and Thorny Skate (Amblyraja radiata)

Understanding how rising temperatures, ocean acidification, and hypoxia affect the performance of coastal fishes is essential to predicting species-specific responses to climate change. Although a population's habitat influences physiological performance, little work has explicitly examined the multi-stressor responses of species from habitats differing in natural variability. Here, clearnose skate (Rostaraja eglanteria) and summer flounder (Paralichthys dentatus) from mid-Atlantic estuaries, and thorny skate (Amblyraja radiata) from the Gulf of Maine, were acutely exposed to current and projected temperatures (20, 24, or 28 °C; 22 or 30 °C; and 9, 13, or 15 °C, respectively) and acidification conditions (pH 7.8 or 7.4). We tested metabolic rates and hypoxia tolerance using intermittent-flow respirometry. All three species exhibited increases in standard metabolic rate under an 8 °C temperature increase (Q₁₀ of 1.71, 1.07, and 2.56, respectively), although this was most pronounced in the thorny skate. At the lowest test temperature and under the low pH treatment, all three species exhibited significant increases in standard metabolic rate (44-105%; p < 0.05) and decreases in hypoxia tolerance (60-84% increases in critical oxygen pressure; p < 0.05). This study demonstrates the interactive effects of increasing temperature and changing ocean carbonate chemistry are species-specific, the implications of which should be considered within the context of habitat.

Physiological trade-offs, acid-base balance and ion-osmoregulatory plasticity in European sea bass (Dicentrarchus labrax) juveniles under complex scenarios of salinity variation, ocean acidification and high ammonia challenge

In this era of global climate change, ocean acidification is becoming a serious threat to the marine ecosystem. Despite this, it remains almost unknown how fish will respond to the co-occurrence of ocean acidification with other conventional environmental perturbations typically salinity fluctuation and high ammonia threat. Therefore, the present work evaluated the interactive effects of elevated pCO₂, salinity reduction and high environmental ammonia (HEA) on the ecophysiological performance of European sea bass (Dicentrarchus labrax). Fish were progressively acclimated to seawater (32 ppt), to brackish water (10 ppt) and to hyposaline water (2.5 ppt). Following acclimation to different salinities for at least two weeks, fish were exposed to CO₂-induced water acidification representing present-day (control pCO₂, 400 μatm, LoCO₂) and future (high pCO₂, 1000 μatm, HiCO₂) sea-surface CO₂ level for 3, 7 and 21 days. At the end of each exposure period, fish were challenged with HEA for 6 h (1.18 mM representing 50% of 96 h LC₅₀). Results show that, in response to the individual HiCO₂ exposure, fish within each salinity compensated for blood acidosis. Fish subjected to HiCO₂ were able to maintain ammonia excretion rate (J_amm) within control levels, suggesting that HiCO₂ exposure alone had no impact on J_amm at any of the salinities. For 32 and 10 ppt fish, up-regulated expression of Na⁺/K⁺-ATPase was evident in all exposure groups (HEA, HiCO₂ and HEA/HiCO₂ co-exposed), whereas Na⁺/K⁺/2Cl^- co-transporter was up-regulated mainly in HiCO₂ group. Plasma glucose and lactate content were augmented in all exposure conditions for all salinity regimes. During HEA and HEA/HiCO₂, J_amm was inhibited at different time points for all salinities, which resulted in a significant build-up of ammonia in plasma and muscle. Branchial expressions of Rhesus glycoproteins (Rhcg isoforms and Rhbg) were upregulated in response to HiCO₂ as well as HEA at 10 ppt, with a more moderate response in 32 ppt groups. Overall, our findings denote that the adverse effect of single exposures of ocean acidification or HEA is exacerbated when present together, and suggests that fish are more vulnerable to these environmental threats at low salinities.

Morpho-functional effects of heat stress on the gills of Antarctic T. bernacchii and C. hamatus

The effect of increasing ocean water temperature on morpho-functional traits of Antarctic marine species is under intense attention. In this work, we evaluated the effects of acute heat stress on the gills of the Antarctic haemoglobinless Chionodraco hamatus and the red blooded Trematomus bernacchii in terms of morphology, heat shock response, antioxidant defense and NOS/NO system. We showed in both species that the exposure to high temperature (4 °C) induced structural alterations, such as epithelial lifting and oedema of secondary lamellae. By immunolocalization we also observed that HSP-90, HSP-70, Xantine Oxidase, Heme Oxigenase and NOS are expressed in both species under control conditions. After heat stress the signals increase in C. hamatus being absent/or reduced in T. bernacchii. Our preliminary results suggest a specie-specific morpho-functional response of the gills of the two Antarctic teleosts to heat stress.

Metabolic responses in Antarctic Nototheniidae brains subjected to thermal stress

Antarctic Nototheniidae is an attractive group for studying metabolic and physiological responses at high temperatures. The present work investigated the metabolic responses of the carbohydrate metabolism and antioxidant system to thermal stress at 8 °C (for 2-144 h) in the brains of Notothenia rossii and Notothenia coriiceps. In N. coriiceps, glycogenolysis was essential in the first hours of exposure (2 h) at 8 °C and, in addition to inhibiting glucose-6-phosphatase activity, was important for activating the pentose phosphate pathway. In N. rossii, anaerobic metabolism was reduced in the first hours of exposure (2 and 6 h) at 8 °C, followed by reduced hexokinase activity, suggesting energy regulation between neurons and astrocytes. The antioxidant system results indicated the importance of the actions of the glutathione-dependent antioxidant enzymes glutathione-S-transferase and glutathione peroxidase as well as those of catalase in N. coriiceps and the action of glutathione-S-transferase, glutathione peroxidase and glutathione reductase in N. rossii, especially during the first 12 h of thermal stress exposure. These results indicate tissue-specific patterns and species-specific responses to this stress.

Aerobic capacities and swimming performance of polar cod (Boreogadus saida) under ocean acidification and warming conditions

Polar cod (Boreogadus saida) is an important prey species in the Arctic ecosystem, yet its habitat is changing rapidly: climate change, through rising seawater temperatures and CO2 concentrations, is projected to be most pronounced in Arctic waters. This study aimed to investigate the influence of ocean acidification and warming on maximum performance parameters of B. saida as indicators for the species' acclimation capacities under environmental conditions projected for the end of this century. After 4 months at four acclimation temperatures (0, 3, 6, 8°C) each combined with two PCO2  levels (390 and 1170 µatm), aerobic capacities and swimming performance of B. saida were recorded following a Ucrit protocol. At both CO2 levels, standard metabolic rate (SMR) was elevated at the highest acclimation temperature indicating thermal limitations. Maximum metabolic rate (MMR) increased continuously with temperature, suggesting an optimum temperature for aerobic scope for exercise (ASex) at 6°C. Aerobic swimming performance (Ugait) increased with acclimation temperature irrespective of CO2 levels, while critical swimming speed (Ucrit) did not reveal any clear trend with temperature. Hypercapnia evoked an increase in MMR (and thereby ASex). However, swimming performance (both Ugait and Ucrit) was impaired under elevated near-future PCO2  conditions, indicating reduced efficiencies of oxygen turnover. The contribution of anaerobic metabolism to swimming performance was very low overall, and further reduced under hypercapnia. Our results revealed high sensitivities of maximum performance parameters (MMR, Ugait, Ucrit) of B. saida to ocean acidification. Impaired swimming capacity under ocean acidification may reflect reduced future competitive strength of B. saida.

You Better Repeat It: Complex CO2 × Temperature Effects in Atlantic Silverside Offspring Revealed by Serial Experimentation

Concurrent ocean warming and acidification demand experimental approaches that assess biological sensitivities to combined effects of these potential stressors. Here, we summarize five CO2 × temperature experiments on wild Atlantic silverside, Menidia menidia, offspring that were reared under factorial combinations of CO2 (nominal: 400, 2200, 4000, and 6000 µatm) and temperature (17, 20, 24, and 28 °C) to quantify the temperature-dependence of CO2 effects in early life growth and survival. Across experiments and temperature treatments, we found few significant CO2 effects on response traits. Survival effects were limited to a single experiment, where elevated CO2 exposure reduced embryo survival at 17 and 24 °C. Hatch length displayed CO2 × temperature interactions due largely to reduced hatch size at 24 °C in one experiment but increased length at 28 °C in another. We found no overall influence of CO2 on larval growth or survival to 9, 10, 15 and 13–22 days post-hatch, at 28, 24, 20, and 17 °C, respectively. Importantly, exposure to cooler (17 °C) and warmer (28 °C) than optimal rearing temperatures (24 °C) in this species did not appear to increase CO2 sensitivity. Repeated experimentation documented substantial inter- and intra-experiment variability, highlighting the need for experimental replication to more robustly constrain inherently variable responses. Taken together, these results demonstrate that the early life stages of this ecologically important forage fish appear largely tolerate to even extreme levels of CO2 across a broad thermal regime.

Acid-base physiology, neurobiology and behaviour in relation to CO2-induced ocean acidification

Experimental exposure to ocean and freshwater acidification affects the behaviour of multiple aquatic organisms in laboratory tests. One proposed cause involves an imbalance in plasma chloride and bicarbonate ion concentrations as a result of acid-base regulation, causing the reversal of ionic fluxes through GABA_A receptors, which leads to altered neuronal function. This model is exclusively based on differential effects of the GABA_A receptor antagonist gabazine on control animals and those exposed to elevated CO₂ However, direct measurements of actual chloride and bicarbonate concentrations in neurons and their extracellular fluids and of GABA_A receptor properties in aquatic organisms are largely lacking. Similarly, very little is known about potential compensatory mechanisms, and about alternative mechanisms that might lead to ocean acidification-induced behavioural changes. This article reviews the current knowledge on acid-base physiology, neurobiology, pharmacology and behaviour in relation to marine CO₂-induced acidification, and identifies important topics for future research that will help us to understand the potential effects of predicted levels of aquatic acidification on organisms.

Microplastics have a more profound impact than elevated temperatures on the predatory performance, digestion and energy metabolism of an Amazonian cichlid

Knowledge on the impacts of microplastics (MPs) pollution on freshwater environments and biota remains limited. Meanwhile, freshwater ecosystems have been threatened by elevated temperatures caused by climate change. To date, no information exists on how MPs-especially under elevated temperature conditions-affect predatory performance, digestive processes and metabolic pathways in freshwater organisms. Here, we examined MPs, elevated temperature and their combined effects on juveniles (0+ group) of an Amazonian cichlid, the discus fish (Symphysodon aequifasciatus). For 30 days, fish were exposed to ambient or elevated temperatures (i.e., 28 or 31 °C) in the absence or presence of MPs (i.e., 0 or 200 μg/L). The following metrics were quantified: MPs accumulation; predatory performance; and biomarkers involved in neurotransmission, digestion and energy production. The results showed that survival rate and body length were not affected by MPs, elevated temperatures or their combination. Elevated temperatures resulted in an increase in MP concentrations in fish bodies. Exposure to MPs decreased the post-exposure predatory performance (PEPP) at ambient temperatures but not at elevated temperatures. Elevated temperatures, however, had no effect on the PEPP but antagonistically interacted with MPs, leading to similar predatory performances under present and future conditions. Acetylcholinesterase (AChE) activity was only affected by MPs and decreased in the presence of MPs, indicating adverse effects in nervous and neuromuscular function and, thus, potentially in predatory performance. Trypsin activity was only influenced by MPs and decreased during exposure to MPs. Elevated temperatures or MPs alone increased the amylase activity but interacted antagonistically. Lipase activity was not influenced by either of the two stressors. In contrast, alkaline phosphatase (ALP) activity was affected by MPs or elevated temperatures alone and decreased with both stressors. Such results indicate deficits in the digestive capabilities of early-stage S. aequifasciatus under elevated temperature conditions and especially during exposure to MPs. Electron transport system (ETS) activity was not influenced by either of the two stressors. Both elevated temperatures and MPs alone increased LDH activity; however, the interaction between the two stressors cancelled activity but was still higher than activity in present conditions. Citrate synthase (CS) activity decreased with elevated temperature but increased during exposure to MPs. Cytochrome c oxidase (COX) activity was only influenced by MPs and increased in the presence of MPs. Thus, S. aequifasciatus juveniles exposed to elevated temperatures and MPs not only relied on anaerobic glycolysis for energy production but also depended on aerobic metabolism in the presence of MPs. Overall, these findings suggested that MPs showed a greater impact than elevated temperatures on the predatory performance, digestion and energy production of S. aequifasciatus. Nevertheless, juvenile survival and growth were minimally impacted, and thus, S. aequifasciatus could cope with near-future temperature increases and MP exposure.

Persistent organic pollutants in red- and white-blooded High-Antarctic notothenioid fish from the remote Weddell Sea

It has been suggested that High-Antarctic waters, despite their remoteness from human activities, are impacted by anthropogenic pollution, and that the local biota are accumulating the contaminants. At present, no data exist on persistent organic pollutant (POP) body burdens for notothenioid fish inhabiting the High-Antarctic Weddell Sea. We determined the pollutant load in white muscle tissue of red- and white-blooded notothenoids from the Weddell Sea (Trematomus loennbergii and Chionodraco hamatus, respectively), and compared them to our previous measurements of POPs in Low-Antarctic notothenioids. Analytes included various organochlorine pesticides (OCPs), polychlorinated biphenyls (indicator (i) PCBs, dioxine-like (dl) PCBs) and polybrominated diphenyl ethers (PBDEs). The analytical concentrations were converted into 2,3,7,8-TCDD toxic equivalents (TEQs). Compared to T. loennbergii, C. hamatus had lower levels of β-HCH (0.45 vs. 4.5 ng g^-1 lipid weight), and Σ iPCBs (30 vs. 39 ng g^-1 lipid weight), as well as lower levels of Σ PBDEs (131 vs. 261 ng g^-1 fresh weight). POP body burdens and TEQs were mostly similar to those of Low-Antarctic notothenioids analysed previously, and not related to the trophic positions of the species. The variations in POP levels between and within High- and Low-Antarctic notothenioids only marginally corresponded to sampling site, ecological differences or trophic levels of the species, and might rather be related to metabolism or age effects. The present findings suggest that fishes of High-Antarctic waters, although this area is more remote and less influenced by local human activities, do not show lower POP body burdens than fishes from Low-Antarctic waters.

Antarctic emerald rockcod have the capacity to compensate for warming when uncoupled from CO2 -acidification

Increases in atmospheric CO₂ levels and associated ocean changes are expected to have dramatic impacts on marine ecosystems. Although the Southern Ocean is experiencing some of the fastest rates of change, few studies have explored how Antarctic fishes may be affected by co-occurring ocean changes, and even fewer have examined early life stages. To date, no studies have characterized potential trade-offs in physiology and behavior in response to projected multiple climate change stressors (ocean acidification and warming) on Antarctic fishes. We exposed juvenile emerald rockcod Trematomus bernacchii to three PCO₂ treatments (~450, ~850, and ~1,200 μatm PCO₂ ) at two temperatures (-1 or 2°C). After 2, 7, 14, and 28 days, metrics of physiological performance including cardiorespiratory function (heart rate [f_H ] and ventilation rate [f_V ]), metabolic rate (M˙O2), and cellular enzyme activity were measured. Behavioral responses, including scototaxis, activity, exploration, and escape response were assessed after 7 and 14 days. Elevated PCO₂ independently had little impact on either physiology or behavior in juvenile rockcod, whereas warming resulted in significant changes across acclimation time. After 14 days, f_H , f_V and M˙O2 significantly increased with warming, but not with elevated PCO₂ . Increased physiological costs were accompanied by behavioral alterations including increased dark zone preference up to 14%, reduced activity by 12%, as well as reduced escape time suggesting potential trade-offs in energetics. After 28 days, juvenile rockcod demonstrated a degree of temperature compensation as f_V , M˙O2, and cellular metabolism significantly decreased following the peak at 14 days; however, temperature compensation was only evident in the absence of elevated PCO₂ . Sustained increases in f_V and M˙O2 after 28 days exposure to elevated PCO₂ indicate additive (f_V ) and synergistic (M˙O2) interactions occurred in combination with warming. Stressor-induced energetic trade-offs in physiology and behavior may be an important mechanism leading to vulnerability of Antarctic fishes to future ocean change.

Ocean acidification does not limit squid metabolism via blood oxygen supply

Ocean acidification is hypothesized to limit the performance of squids due to their exceptional oxygen demand and pH-sensitivity of blood-oxygen binding, which may reduce oxygen supply in acidified waters. The critical oxygen partial pressure (Pcrit), the PO2 below which oxygen supply cannot match basal demand, is a commonly reported index of hypoxia tolerance. Any CO2-induced reduction in oxygen supply should be apparent as an increase in Pcrit. In this study, we assessed the effects of CO2 (46-143 Pa; 455-1410 μatm) on the metabolic rate and Pcrit of two squid species - Dosidicus gigas and Doryteuthis pealeii - through manipulative experiments. We also developed a model, with inputs for hemocyanin pH-sensitivity, blood PCO2, and buffering capacity that simulates blood oxygen supply under varying seawater CO2 partial pressures. We compare model outputs to measured Pcrit in squids. Using blood-O2 parameters from the literature for model inputs, we estimated that, in the absence of blood acid-base regulation, an increase in seawater PCO2 to 100 Pa (≈ 1000 μatm) would result in a maximum drop in arterial hemocyanin-O2 saturation by 1.6% at normoxia and a Pcrit increase of ≈0.5 kPa. Our live-animal experiments support this supposition, as CO2 had no effect on measured metabolic rate or Pcrit in either squid species.

The effects of thermal acclimation on cardio-respiratory performance in an Antarctic fish (Notothenia coriiceps)

The Southern Ocean has experienced stable, cold temperatures for over 10 million years, yet particular regions are currently undergoing rapid warming. To investigate the impacts of warming on cardiovascular oxygen transport, we compared the cardio-respiratory performance in an Antarctic notothenioid (Notothenia coriiceps) that was maintained at 0 or 5°C for 6.0-9.5 weeks. When compared at the fish's respective acclimation temperature, the oxygen consumption rate and cardiac output were significantly higher in 5°C-acclimated than 0°C-acclimated fish. The 2.7-fold elevation in cardiac output in 5°C-acclimated fish (17.4 vs. 6.5 ml min^-1 kg^-1) was predominantly due to a doubling of stroke volume, likely in response to increased cardiac preload, as measured by higher central venous pressure (0.15 vs. 0.08 kPa); tachycardia was minor (29.5 vs. 25.2 beats min^-1). When fish were acutely warmed, oxygen consumption rate increased by similar amounts in 0°C- and 5°C-acclimated fish at equivalent test temperatures. In both acclimation groups, the increases in oxygen consumption rate during acute heating were supported by increased cardiac output achieved by elevating heart rate, while stroke volume changed relatively little. Cardiac output was similar between both acclimation groups until 12°C when cardiac output became significantly higher in 5°C-acclimated fish, driven largely by their higher stroke volume. Although cardiac arrhythmias developed at a similar temperature (~14.5°C) in both acclimation groups, the hearts of 5°C-acclimated fish continued to pump until significantly higher temperatures (CT_max for cardiac function 17.7 vs. 15.0°C for 0°C-acclimated fish). These results demonstrate that N. coriiceps is capable of increasing routine cardiac output during both acute and chronic warming, although the mechanisms are different (heart rate-dependent versus primarily stroke volume-dependent regulation, respectively). Cardiac performance was enhanced at higher temperatures following 5°C acclimation, suggesting cardiovascular function may not constrain the capacity of N. coriiceps to withstand a warming climate.

Untargeted metabolic profiling reveals distinct patterns of thermal sensitivity in two related notothenioids

Antarctic marine ectothermal animals may be affected more than temperate species by rising temperatures due to ongoing climate change. Their specialisation on stable cold temperatures makes them vulnerable to even small degrees of warming. Thus, addressing the impacts of warming on Antarctic organisms and identifying their potentially limited capacities to respond is of interest. The objective of the study was to determine changes in metabolite profiles related to temperature acclimation. In a long-term experiment adult fish of two Antarctic sister species Notothenia rossii and Notothenia coriiceps were acclimated to 0°C and 5°C for three months. Impacts and indicators of acclimation at the cellular level were determined from metabolite profiles quantified in gill tissue extracts using nuclear magnetic resonance (NMR) spectroscopy. Furthermore, the metabolite profiles of the two con-generic species were compared. NMR spectroscopy identified 37 metabolites that were present in each sample, but varied in their absolute concentration between species and between treatments. A decrease in amino acid levels indicated an increased amino acid catabolism after incubation to 5°C. In addition, long term warming initiated shifts in organic osmolyte concentrations and modified membrane structure observed by altered levels of phospholipid compounds. Differences in the metabolite profile between the two notothenioid species can be related to their divergent lifestyles, especially their different rates of motor activity. Increased levels of the Krebs cycle intermediate succinate and a higher reduction of amino acid concentrations in warm-acclimated N. rossii showed that N. rossii is more affected by warming than N. coriiceps.