Heat hardening in Antarctic notothenioid fishes

Intergenerational effects of ocean temperature variation: Early life benefits are short-lived in threespine stickleback

Current climate change models predict an increase in temperature variability and extreme events such as heatwaves, and organisms need to cope with consequent changes to environmental variation. Non-genetic inheritance mechanisms can enable parental generations to prime their offspring's abilities to acclimate to environmental change-but they may also be deleterious. When parents are exposed to predictable environments, intergenerational plasticity can lead to better offspring trait performance in matching environments. Alternatively, parents exposed to variable or unpredictable environments may use plastic bet-hedging strategies to adjust the phenotypic variance among offspring. Here, we used a model species, the threespine stickleback (Gasterosteus aculeatus), to test whether putatively adaptive intergenerational effects can occur in response to shifts in environmental variation as well as to shifts in environmental mean, and whether parents employ plastic bet-hedging strategies in response to increasing environmental variation. We used a full-factorial, split-clutch experiment with parents and offspring exposed to three temperature regimes: constant, natural variation, and increased variation. We show that within-generation exposure to increased temperature variation reduces growth of offspring, but having parents that were exposed to natural temperature variation during gametogenesis may offset some early-life negative growth effects. However, these mitigating intergenerational effects do not appear to persist later in life. We found no indication that stickleback mothers plastically altered offspring phenotypic variance (egg size or clutch size) in response to temperature variation. However, lower inter-individual variance of juvenile fish morphology in offspring of increased variation parents may imply the presence of conservative bet-hedging strategies in natural populations. Overall, in our experiment, parental exposure to temperature variation had limited effects on offspring fitness-related traits. Natural levels of environmental variation promoted a potentially adaptive intergenerational response in early life development, but under more challenging conditions associated with increased environmental variation, the effect was lost.

Plasticity of cold and heat stress tolerance induced by hardening and acclimation in the melon thrips

Extreme temperatures threaten species under climate change and can limit range expansions. Many species cope with changing environments through plastic changes. This study tested phenotypic changes in heat and cold tolerance under hardening and acclimation in the melon thrips, Thrips palmi Karny (Thysanoptera: Thripidae), an agricultural pest of many vegetables. We first measured the critical thermal maximum (CTmax) of the species by the knockdown time under static temperatures and found support for an injury accumulation model of heat stress. The inferred knockdown time at 39 °C was 82.22 min. Rapid heat hardening for 1 h at 35 °C slightly increased CTmax by 1.04 min but decreased it following exposure to 31 °C by 3.46 min and 39 °C by 6.78 min. Heat acclimation for 2 and 4 days significantly increased CTmax at 35 °C by 1.83, and 6.83 min, respectively. Rapid cold hardening at 0 °C and 4 °C for 2 h, and cold acclimation at 10 °C for 3 days also significantly increased cold tolerance by 6.09, 5.82, and 2.00 min, respectively, while cold hardening at 8 °C for 2 h and acclimation at 4 °C and 10 °C for 5 days did not change cold stress tolerance. Mortality at 4 °C for 3 and 5 days reached 24.07 % and 43.22 % respectively. Our study showed plasticity for heat and cold stress tolerance in T. palmi, but the thermal and temporal space for heat stress induction is narrower than for cold stress induction.

Transcriptome profiling reveals the strategy of thermal tolerance enhancement caused by heat-hardening in Mytilus coruscus

The thick-shell mussel Mytilus coruscus serves as a common sessile intertidal species and holds economic significance as an aquatic organism. M. coruscus often endure higher temperatures than their ideal range during consecutive low tides in the spring. This exposure to elevated temperatures provides them with a thermal tolerance boost, enabling them to adapt to high-temperature events caused by extreme low tides and adverse weather conditions. This phenomenon is referred to as heat-hardening. Some related studies showed the phenomenon of heat-hardening in sessile intertidal species but not reported at the mechanism level based on transcriptome so far. In this study, physiological experiments, gene family identification and transcriptome sequencing were performed to confirm the thermotolerance enhancement based on heat-hardening and explore the mechanism in M. coruscus. A total of 2935 DEGs were identified and the results of the KEGG enrichment showed that seven heat-hardening relative pathways were enriched, including Toll-like receptor signal pathway, Arachidonic acid metabolism, and others. Then, 24 HSP70 members and 36 CYP2 members, were identified, and the up-regulated members are correlated with increasing thermotolerance. Finally, we concluded that the heat-hardening M. coruscus have a better thermotolerance because of the capability of maintaining the integrity and the phenomenon of vasodilation of the gill under thermal stress. Further, the physiological experiments yielded the same conclusions. Overall, this study confirms the thermotolerance enhancement caused by heat-hardening and reveals the survival strategy in M. coruscus. In addition, the conclusion provides a new reference for studying the intertidal species' heat resistance mechanisms to combat extreme heat events and the strategies for dealing with extreme weather in aquaculture under the global warming trend.

Heat hardening enhances metabolite-driven thermoprotection in the Mediterranean mussel Mytilus galloprovincialis

Introduction: Temperature affects organisms' metabolism and ecological performance. Owing to climate change, sea warming constituting a severe source of environmental stress for marine organisms, since it increases at alarming rates. Rapid warming can exceed resilience of marine organisms leading to fitness loss and mortality. However, organisms can improve their thermal tolerance when briefly exposed to sublethal thermal stress (heat hardening), thus generating heat tolerant phenotypes. Methods: We investigated the "stress memory" effect caused by heat hardening on M. galloprovincialis metabolite profile of in order to identify the underlying biochemical mechanisms, which enhance mussels' thermal tolerance. Results: The heat hardening led to accumulation of amino acids (e.g., leucine, isoleucine and valine), including osmolytes and cytoprotective agents with antioxidant and anti-inflammatory properties that can contribute to thermal protection of the mussels. Moreover, proteolysis was inhibited and protein turnover regulated by the heat hardening. Heat stress alters the metabolic profile of heat stressed mussels, benefiting the heat-hardened individuals in increasing their heat tolerance compared to the non-heat-hardened ones. Discussion: These findings provide new insights in the metabolic mechanisms that may reinforce mussels' tolerance against thermal stress providing both natural protection and potential manipulative tools (e.g., in aquaculture) against the devastating climate change effects on marine organisms.

Rapid heat hardening in embryos of the lizard Anolis sagrei

Adaptive thermal tolerance plasticity can dampen the negative effects of warming. However, our knowledge of tolerance plasticity is lacking for embryonic stages that are relatively immobile and may benefit the most from an adaptive plastic response. We tested for heat hardening capacity (a rapid increase in thermal tolerance that manifests in minutes to hours) in embryos of the lizard Anolis sagrei. We compared the survival of a lethal temperature exposure between embryos that either did (hardened) or did not (not hardened) receive a high but non-lethal temperature pre-treatment. We also measured heart rates (HRs) at common garden temperatures before and after heat exposures to assess metabolic consequences. 'Hardened' embryos had significantly greater survival after lethal heat exposure relative to 'not hardened' embryos. That said, heat pre-treatment led to a subsequent increase in embryo HR that did not occur in embryos that did not receive pre-treatment, indicative of an energetic cost of mounting the heat hardening response. Our results are not only consistent with adaptive thermal tolerance plasticity in these embryos (greater heat survival after heat exposure), but also highlight associated costs. Thermal tolerance plasticity may be an important mechanism by which embryos respond to warming that warrants greater consideration.

Metabolomic analysis provides new insights into the heat-hardening response of Manila clam (Ruditapes philippinarum) to high temperature stress

The temperature has always been a key environmental factor in Manila clam (Ruditapes philippinarum) culture. In this study, the Manila clam was treated to different temperature pre-heat (28 °C, 30 °C) and gained heat tolerance after recover of 12 h, and a survival rate (14.7 %-49.1 %) advantage after high temperature challenge (30 and 32 °C). To further investigate the physiological and metabolism changes in Manila clam that had experienced a heat stress, non-targeted metabolomics (LC-MS/MS) was used to analyze the metabolic responses of gills in three group Manila clams during the heat challenge. Metabolic profiles revealed that high temperature caused changes in fatty acid composition, energy metabolism, antioxidant metabolites, hydroxyl compounds, and amino acids in heat-hardened clams compared to non-hardened clams. We found a number of significantly enriched pathways, including cAMP signaling pathway, serotonergic synapse, and biosynthesis of unsaturated fatty acids in heat-hardened Manila clam compared with non-hardened and untreated Manila clam. After a brief high temperature treatment, the physiological maintenance ability of Manila clam was improved. Combined with metabolomics analysis, heat hardening treatment may improve the energy metabolism and antioxidant ability of Manila clam. These results provide new insights into the cellular and metabolic responses of Manila clams following high temperature stress.

Metabolic remodeling caused by heat hardening in the Mediterranean mussel Mytilus galloprovincialis

Organisms can modify and increase their thermal tolerance faster and more efficiently after a brief exposure to sublethal thermal stress. This response is called 'heat hardening' as it leads to the generation of phenotypes with increased heat tolerance. The aim of this study was to investigate the impact of heat hardening on the metabolomic profile of Mytilus galloprovincialis in order to identify the associated adjustments of biochemical pathways that might benefit the mussels' thermal tolerance. Thus, mussels were exposed sequentially to two different phases (heat hardening and acclimation phases). To gain further insight into the possible mechanisms underlying the metabolic response of the heat-hardened M. galloprovincialis, metabolomics analysis was complemented by the estimation of mRNA expression of phosphoenolpyruvate carboxykinase (PEPCK), pyruvate kinase (PK) and alternative oxidase (AOX) implicated in the metabolic pathways of gluconeogenesis, glycolysis and redox homeostasis, respectively. Heat-hardened mussels showed evidence of higher activity of the tricarboxylic acid (TCA) cycle and diversification of upregulated metabolic pathways, possibly as a mechanism to increase ATP production and extend survival under heat stress. Moreover, formate and taurine accumulation provide an antioxidant and cytoprotective role in mussels during hypoxia and thermal stress. Overall, the metabolic responses in non-heat-hardened and heat-hardened mussels underline the upper thermal limits of M. galloprovincialis, set at 26°C, and are in accordance with the OCLTT concept. The ability of heat-hardened mussels to undergo a rapid gain and slow loss of heat tolerance may be an advantageous strategy for coping with intermittent and often extreme temperatures.

Plastic responses of survival and fertility following heat stress in pupal and adult Drosophila virilis

The impact of rising global temperatures on survival and reproduction is putting many species at risk of extinction. In particular, it has recently been shown that thermal effects on reproduction, especially limits to male fertility, can underpin species distributions in insects. However, the physiological factors influencing fertility at high temperatures are poorly understood. Key factors that affect somatic thermal tolerance such as hardening, the ability to phenotypically increase thermal tolerance after a mild heat shock, and the differential impact of temperature on different life stages are largely unexplored for thermal fertility tolerance. Here, we examine the impact of high temperatures on male fertility in the cosmopolitan fruit fly Drosophila virilis. We first determined whether temperature stress at either the pupal or adult life history stage impacts fertility. We then tested the capacity for heat-hardening to mitigate heat-induced sterility. We found that thermal stress reduces fertility in different ways in pupae and adults. Pupal heat stress delays sexual maturity, whereas males heated as adults can reproduce initially following heat stress, but become sterile within seven days. We also found evidence that while heat-hardening in D. virilis can improve high temperature survival, there is no significant protective impact of this same hardening treatment on fertility. These results suggest that males may be unable to prevent the costs of high temperature stress on fertility through heat-hardening, which limits a species' ability to quickly and effectively reduce fertility loss in the face of short-term high temperature events.

Membrane lipid metabolism, heat shock response and energy costs mediate the interaction between acclimatization and heat-hardening response in the razor clam Sinonovacula constricta

Thermal plasticity on different time scales, including acclimation/acclimatization and heat-hardening response - a rapid adjustment for thermal tolerance after non-lethal thermal stress, can interact to improve the resilience of organisms to thermal stress. However, little is known about physiological mechanisms mediating this interaction. To investigate the underpinnings of heat-hardening responses after acclimatization in warm seasons, we measured thermal tolerance plasticity, and compared transcriptomic and metabolomic changes after heat hardening at 33 or 37°C followed by recovery of 3 or 24 h in an intertidal bivalve Sinonovacula constricta. Clams showed explicit heat-hardening responses after acclimatization in a warm season. The higher inducing temperature (37°C) caused less effective heat-hardening effects than the inducing temperature that was closer to the seasonal maximum temperature (33°C). Metabolomic analysis highlighted the elevated content of glycerophospholipids in all heat-hardened clams, which may help to maintain the structure and function of the membrane. Heat shock proteins (HSPs) tended to be upregulated after heat hardening at 37°C but not at 33°C, indicating that there was no complete dependency of heat-hardening effects on upregulated HSPs. Enhanced energy metabolism and decreased energy reserves were observed after heat hardening at 37°C, suggesting more energy costs during exposure to a higher inducing temperature, which may restrict heat-hardening effects. These results highlight the mediating role of membrane lipid metabolism, heat shock responses and energy costs in the interaction between heat-hardening response and seasonal acclimatization, and contribute to the mechanistic understanding of evolutionary change and thermal plasticity during global climate change.

Heat hardening enhances mitochondrial potential for respiration and oxidative defence capacity in the mantle of thermally stressed Mytilus galloprovincialis

Ectotherms are exposed to a range of environmental temperatures and may face extremes beyond their upper thermal limits. Such temperature extremes can stimulate aerobic metabolism toward its maximum, a decline in aerobic substrate oxidation, and a parallel increase of anaerobic metabolism, combined with ROS generation and oxidative stress. Under these stressful conditions, marine organisms recruit several defensive strategies for their maintenance and survival. However, thermal tolerance of ectothermic organisms may be increased after a brief exposure to sub-lethal temperatures, a process known as "hardening". In our study, we examined the ability of M. galloprovincialis to increase its thermal tolerance under the effect of elevated temperatures (24, 26 and 28 °C) through the "hardening" process. Our results demonstrate that this process can increase the heat tolerance and antioxidant defense of heat hardened mussels through more efficient ETS activity when exposed to temperatures beyond 24 °C, compared to non-hardened individuals. Enhanced cell protection is reflected in better adaptive strategies of heat hardened mussels, and thus decreased mortality. Although hardening seems a promising process for the maintenance of aquacultured populations under increased seasonal temperatures, further investigation of the molecular and cellular mechanisms regulating mussels' heat resistance is required.

Effects of warming rates on physiological and molecular components of response to CTMax heat stress in the Antarctic fish Harpagifer antarcticus

Maximum and minimum Critical thermal limits (CTMax and CTMin) have been studied extensively to assess thermal tolerance in ectotherms by means of ramping assays. Notothenioid fish have been proposed as particularly sensitive to temperature increases related to global climate change. However, there are large gaps in our understanding of the thermal responses of these extreme cold-adapted fish in assays with heating rates. We evaluated the effects of two commonly used heating rates (0.3 and 1 °C/min) on the cellular stress responses in the intertidal Antarctic fish Harpagifer antarcticus immediately after CTMax was reached, and at 2 and 4 h of recovery time in ambient water. We compared CTMax values, the relative transcript expression of genes relvant to heat shock response (Hsc70, Hsp70, Grp78), hypoxia (Hif1-α, LDHa, GR), ubiquitination (Ube2), and apoptosis (SMAC/DIABLO), and five plasma parameters - glucose, lactate, total protein, osmolality and cortisol. CTMax values between the two heating rates are not significantly different, and both rates elicited a similar stress response at molecular and physiological levels. We found a lack of up-regulated response of heat shock proteins, consistent with other Antarctic notothenioids. The general transcriptional pattern trended to downregulation, which was more evident in the slower 0.3 °C/min rate, and instances of upregulation were mainly related to ubiquitination. The faster 1 °C/min rate, rarely used for Antarctic fish, can be suitable for studying cold-adapted stenothermic fish without overestimating thermal tolerance or inducing damage from longer heat exposure.

Seasonal and environmental effects on upper thermal limits of eastern sand darter (Ammocrypta pellucida)

Anthropogenic stressors are predicted to increase water temperature, which can influence physiological, individual, and population processes in fishes. We assessed the critical thermal maximum (CT_max) of eastern sand darter (Ammocrypta pellucida), a small benthic fish listed as threatened under the Species at Risk Act in Canada. Field trials were conducted stream side June-November 2019 in the Grand River, Ontario, to encompass a range of ambient water temperatures (7-25°C) for which agitation temperature (T_ag) and CT_max were determined. Additional measures were taken in the comparatively more turbid Thames River to test the effect of turbidity on both measures. In the Grand, T_ag and CT_max ranged from 23°C to 33°C and 27°C to 37°C, respectively, and both significantly increased with ambient water temperature, with a high acclimation response ratio (0.49). The thermal safety margin (difference between ambient temperatures and CT_max) was smallest in July and August (~11°C) indicating that eastern sand darter lives closer to its physiological limit in summer. The between-river comparison indicated that turbidity had no significant influence on T_ag and CT_max. Comparison of CT_max with in-river temperatures suggested that mean stream temperature 24 hours before the trial was most important for determining CT_max. Fish mass, temperature variance and maximum temperature in the 24-hour period prior to the CT_max trial were also shown to have some effect on determining CT_max. Overall, study results better define the sensitivity of eastern sand darter to temperature changes across the growing season and provide information to assess the availability of suitable thermal habitat for conservation purposes.

Thermal tolerance and routine oxygen consumption of convict cichlid, Archocentrus nigrofasciatus, acclimated to constant temperatures (20 °C and 30 °C) and a daily temperature cycle (20 °C → 30 °C)

Organismal temperature tolerance and metabolic responses are correlated to recent thermal history, but responses to thermal variability are less frequently assessed. There is great interest in whether organisms that experience greater thermal variability can gain metabolic or tolerance advantages through phenotypic plasticity. We compared thermal tolerance and routine aerobic metabolism of Convict cichlid acclimated for 2 weeks to constant 20 °C, constant 30 °C, or a daily cycle of 20 → 30 °C (1.7 °C/h). Acute routine mass-specific oxygen consumption ([Formula: see text]O₂) and critical thermal maxima/minima (CTMax/CTMin) were compared between groups, with cycle-acclimated fish sampled from the daily minimum (20 °C, 0900 h) and maximum (30 °C, 1600 h). Cycle-acclimated fish demonstrated statistically similar CTMax at the daily minimum and maximum (39.0 °C, 38.6 °C) but distinct CTMin values, with CTMin 2.4 °C higher for fish sampled from the daily 30 °C maximum (14.8 °C) compared to the daily 20 °C minimum (12.4 °C). Measured acutely at 30 °C, [Formula: see text]O₂ decreased with increasing acclimation temperature; 20 °C acclimated fish had an 85% higher average [Formula: see text]O₂ than 30 °C acclimated fish. Similarly, acute [Formula: see text]O₂ at 20 °C was 139% higher in 20 °C acclimated fish compared to 30 °C acclimated fish. Chronic [Formula: see text]O₂ was measured in separate fish continually across the 20 → 30 °C daily cycle for all 3 acclimation groups. Chronic [Formula: see text]O₂ responses were very similar between groups between average individual hourly values, as temperatures increased or decreased (1.7 °C/h). Acute [Formula: see text]O₂ and thermal tolerance responses highlight "classic" trends, but dynamic, chronic trials suggest acclimation history has little effect on the relative change in oxygen consumption during a thermal cycle. Our results strongly suggest that the minimum and maximum temperatures experienced more strongly influence fish physiology, rather than the thermal cycle itself. This research highlights the importance of collecting data in both cycling and static (constant) thermal conditions, and further research should seek to understand whether ectotherm metabolism does respond uniquely to fluctuating temperatures.

Heat hardening in a pair of Anolis lizards: constraints, dynamics and ecological consequences

Heat tolerance plasticity is predicted to be an important buffer against global warming. Nonetheless, basal heat tolerance often correlates negatively with tolerance plasticity ('trade-off hypothesis'), a constraint that could limit plasticity benefits. We tested the trade-off hypothesis at the individual level with respect to heat hardening in two lizard species, Anolis carolinensis and Anolis sagrei. Heat hardening is a rapid increase in heat tolerance after heat shock that is rarely measured in reptiles but is generally considered to be a first line of physiological defense against heat. We also employed a biophysical model of operative habitat temperatures to estimate the performance consequences of hardening under ecologically relevant conditions. Anolis carolinensis hardened by 2 h post-heat shock and maintained hardening for several hours. However, A. sagrei did not harden. Biophysical models showed that hardening in A. carolinensis reduces their overheating risk in the field. Therefore, while not all lizards heat harden, hardening has benefits for species that can. We initially found a negative relationship between basal tolerance and hardening within both species, consistent with the trade-off hypothesis. However, permutation analyses showed that the apparent trade-offs could not be differentiated from statistical artifact. We found the same result when we re-analyzed published data supporting the trade-off hypothesis in another lizard species. Our results show that false positives may be common when testing the trade-off hypothesis. Statistical approaches that account for this are critical to ensure that the hypothesis, which has broad implications for thermal adaptation and responses to warming, is assessed appropriately.

Hypoxia inducible factor-1α knockout does not impair acute thermal tolerance or heat hardening in zebrafish

The rapid increase in critical thermal maximum (CTmax) in fish (or other animals) previously exposed to critically high temperature is termed 'heat hardening', which likely represents a key strategy to cope with increasingly extreme environments. The physiological mechanisms that determine acute thermal tolerance, and the underlying pathways facilitating heat hardening, remain debated. It has been posited, however, that exposure to high temperature is associated with tissue hypoxia and may be associated with the increased expression of hypoxia-inducible factor-1 (Hif-1). We studied acute thermal tolerance in zebrafish (Danio rerio) lacking functional Hif-1α paralogs (Hif-1aa and Hif-1ab double knockout; Hif-1α-/-), which are known to exhibit markedly reduced hypoxia tolerance. We hypothesized that Hif-1α-/- zebrafish would suffer reduced acute thermal tolerance relative to wild type and that the heat hardening ability would be lost. However, on the contrary, we observed that Hif-1α-/- and wild-type fish did not differ in CTmax, and both genotypes exhibited heat hardening of a similar degree when CTmax was re-tested 48 h later. Despite exhibiting impaired hypoxia tolerance, Hif-1α-/- zebrafish display unaltered thermal tolerance, suggesting that these traits are not necessarily functionally associated. Hif-1α is accordingly not required for short-term acclimation in the form of heat hardening.

A series of unfortunate events: characterizing the contingent nature of physiological extremes using long-term environmental records

Accelerating shifts in global climate have focused the attention of ecologists and physiologists on extreme environmental events. However, the dynamic process of physiological acclimatization complicates study of these events' consequences. Depending on the range of plasticity and the amplitude and speed of environmental variation, physiology can be either in tune with the surroundings or dangerously out of synch. We implement a modified quantitative approach to identifying extreme events in environmental records, proposing that organisms are stressed by deviations of the environment from the current level of acclimatization, rather than by the environment's absolute state. This approach facilitates an unambiguous null model for the consequences of environmental variation, identifying a unique subset of events as 'extremes'. Specifically, it allows one to examine how both the temporal extent (the acclimatization window) and type of an environmental signal affect the magnitude and timing of extreme environmental events. For example, if physiology responds to the moving average of past conditions, a longer acclimatization window generally results in greater imposed stress. If instead physiology responds to historical maxima, longer acclimatization windows reduce imposed stress, albeit perhaps at greater constitutive cost. This approach should be further informed and tested with empirical experiments addressing the history-dependent nature of acclimatization.

Modeling the influence of time and temperature on the levels of fatty acids in the liver of Antarctic fish Trematomus bernacchii

Antarctic fish (Trematomus bernacchii) are an ideal group for studying the effect of ocean warming on vital physiological and biochemical mechanisms of adaptation, including changes in the fatty acid composition to higher heat tolerance in the sub-zero waters of the Southern Ocean. Despite the awareness of the impact of ocean warming on marine life, bioclimatic models describing the effect of temperature and time on fatty acid levels in marine species have not been considered yet. The objective of the present study was to investigate changes in the concentrations of fatty acids in liver from T. bernacchii in response to an increase in temperature in the Antarctic region. Changes in the concentrations of fatty acids in liver from T. bernacchii were observed after varying simultaneously and systematically the temperature and time. The fatty acid profiles were determined by gas chromatography prior to acclimation (− 1.8 °C) and after acclimation (0.0, 1.0, and 2.0 °C) at different times (1, 5, and 10 days). The observed changes were graphically visualized by expressing the fatty acid concentration in absolute units (mg g−1) as a function of the temperature and time using polynomial models. Major changes in fatty acid composition were observed at day 1 of exposition at all temperatures. At day 5, the fish seem to tolerate the new temperature condition. The concentrations of saturated fatty acids were almost constant throughout the various conditions. The concentrations of monounsaturated fatty acids (in particular 18:1n − 9) decrease at day 1 for all temperatures. In contrast, there was an increase in the concentrations of polyunsaturated fatty acids (in particular 20:5n − 3 and 22:6n − 3) with increasing temperatures after 1, 5, and 10 days of exposure. The proposed models were in agreement with reported studies on polar and temperate fish, indicating possibly similar adaptation mechanisms for teleost to cope with global warming.

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.

Characterization of thermally sensitive miRNAs reveals a central role of the FoxO signaling pathway in regulating the cellular stress response of an extreme stenotherm, Trematomus bernacchii

Despite the lack of an inducible heat shock response (HSR), the Antarctic notothenioid fish, Trematomus bernacchii, has retained a level of physiological plasticity that can at least partially compensate for the effects of acute heat stress. Over the last decade, both physiological and transcriptomic studies have signaled these fish can mitigate the effects of acute heat stress by employing other aspects of the cellular stress response (CSR) that help confer thermotolerance as well as drive homeostatic mechanisms during long-term thermal acclimations. However, the regulatory mechanisms that determine temperature-induced changes in gene expression remain largely unexplored in this species. Therefore, this study utilized next generation sequencing coupled with an in silico approach to explore the regulatory role of microRNAs in governing the transcriptomic level response observed in this Antarctic notothenioid with respect to the CSR. Using RNAseq, we characterized the expression of 125 distinct miRNA orthologues in T. bernacchii gill tissue. Additionally, we identified 12 miRNAs that appear to be thermally responsive based on differential expression (DE) analyses performed between fish acclimated to control (^-1.5 °C) and an acute heat stress (⁺4 °C). We further characterized the functional role of these DE miRNAs using bioinformatics pipelines to identify putative gene targets of the DE miRNAs and subsequent gene set enrichment analyses, which together suggest these miRNAs are involved in regulating diverse aspects of the CSR in T. bernacchii.

The effects of repeat acute thermal stress on the critical thermal maximum (CTmax) and physiology of juvenile shortnose sturgeon (Acipenser brevirostrum)

The shortnose sturgeon (Acipenser brevirostrum Lesueur, 1818) is a species of special concern in Canada, but little is known about their thermal biology. Information on the upper thermal tolerance of shortnose sturgeon becomes valuable for predicting future survival particularly with climate change and improving species management. Using a modified critical thermal maximum (CTmax) methodology, the objective is to determine whether previous thermal stress affects the thermal tolerance of juvenile shortnose sturgeon when exposed to a second thermal stress event. Prior exposure to thermal stress (CTmax1) did not affect the thermal tolerance (CTmax2) of juvenile shortnose sturgeon when a 24 h recovery period was allotted between tests. However, a significant increase in thermal tolerance occurred when the recovery time between the two thermal challenges was 1 h. Plasma glucose, lactate, and osmolality were all significantly affected by thermal stress, but values returned to control levels within 24 h. Hematocrit and plasma chloride concentrations were not significantly affected by thermal stress. All fish survived the CTmax testing. The data indicate that the thermal tolerance of juvenile shortnose sturgeon is modified when multiple thermal stresses occur closer together (1 h) but not if separated by a longer time period (24 h).

Waterborne copper is more toxic to the killifish Poecilia vivipara in elevated temperatures: Linking oxidative stress in the liver with reduced organismal thermal performance

In this study, we measured the interactive effect of temperature (22 °C and 28 °C) and waterborne copper (Cu) contamination (9 μg/L and 20 μg/L) on the killifish Poecilia vivipara. Endpoints analyzed included parameters involved in Cu-accumulation, antioxidant capacity (antioxidant capacity against peroxyl radicals [ACAP] and total antioxidant capacity [TAC]), oxidative damage (lipid peroxidation [LPO]) and upper thermal tolerance (critical thermal maximum [CTMax]). Results show that Cu hepatic accumulation was elevated in 28 °C in comparison to 22 °C in both exposure groups. For gills, this was true only in 20 μg/L. Moreover, hepatic and brachial accumulation were concentration-dependent in both acclimation temperatures. Additionally, Hepatic ACAP and TAC were elevated in animals acclimated to 28 °C and only the animals kept at this temperature had reduced ACAP and TAC levels facing metal exposure (9 and 20 μg/L). Similarly, the combination of elevated temperature and Cu exposure raised hepatic LPO levels. Finally, animals acclimated to 28 °C had higher CTMax levels in comparison to fish acclimated to 22 °C both in control and exposed animals, however, CTMax of contaminated fish were only reduced in comparison to control in animals kept at 28 °C. Concluding, we show that the physiological mechanism besides the potentiating effect of elevated temperature in Cu toxicity is related to higher hepatic and branchial metal accumulation and elevated oxidative stress in the liver, outlined by reduced antioxidant capacity and elevated oxidative damage. We also show that these outcomes lead to compromised organismal performance, characterized by reduced CTMax. Finally, it is concluded that Cu exposure in warmer periods of the year or within global warming predictions may be more hazardous to fish populations.

Thermal windows and metabolic performance curves in a developing Antarctic fish

For ectotherms, temperature modifies the rate of physiological function across a temperature tolerance window depending on thermal history, ontogeny, and evolutionary history. Some adult Antarctic fishes, with comparatively narrow thermal windows, exhibit thermal plasticity in standard metabolic rate; however, little is known about the shape or breadth of thermal performance curves of earlier life stages of Antarctic fishes. We tested the effects of acute warming (- 1 to 8 °C) and temperature acclimation (2 weeks at - 1, 2, 4 °C) on survival and standard metabolic rate in early embryos of the dragonfish Gymnodraco acuticeps from McMurdo Sound, Ross Island, Antarctica. Contrary to predictions, embryos acclimated to warmer temperatures did not experience greater mortality and nearly all embryos survived acute warming to 8 °C. Metabolic performance curve height and shape were both significantly altered after 2 weeks of development at - 1 °C, with further increase in curve height, but not alteration of shape, with warm temperature acclimation. Overall metabolic rate temperature sensitivity (Q ₁₀) from - 1 to 8 °C varied from 2.6 to 3.6, with the greatest thermal sensitivity exhibited by embryos at earlier developmental stages. Interclutch variation in metabolic rates, mass, and development of simultaneously collected embryos was also documented. Taken together, metabolic performance curves provide insight into the costs of early development under warming temperatures, with the potential for thermal sensitivity to be modified by dragonfish phenology and magnitude of seasonal changes in temperature.

The role of stochastic thermal environments in modulating the thermal physiology of an intertidal limpet, Lottia digitalis

Much of our understanding of the thermal physiology of intertidal organisms comes from experiments with animals acclimated under constant conditions and exposed to a single heat stress. In nature, however, the thermal environment is more complex. Aerial exposure and the unpredictable nature of thermal stress during low tides may be critical factors in defining the thermal physiology of intertidal organisms. In the fingered limpet, Lottia digitalis, we investigated whether upper temperature tolerance and thermal sensitivity were influenced by the pattern of fluctuation with which thermal stress was applied. Specifically, we examined whether there was a differential response (measured as cardiac performance) to repeated heat stress of a constant and predictable magnitude compared with heat stress applied in a stochastic and unpredictable nature. We also investigated differences in cellular metabolism and damage following immersion for insights into biochemical mechanisms of tolerance. Upper temperature tolerance increased with aerial exposure, but no significant differences were found between predictable treatments of varying magnitudes (13°C versus 24°C versus 32°C). Significant differences in thermal tolerance were found between unpredictable trials with different heating patterns. There were no significant differences among treatments in basal citrate synthase activity, glycogen content, oxidative stress or antioxidants. Our results suggest that aerial exposure and recent thermal history, paired with relief from high low-tide temperatures, are important factors modulating the capacity of limpets to deal with thermal stress.

The effects of elevated temperature and ocean acidification on the metabolic pathways of notothenioid fish

The adaptations used by notothenioid fish to combat extreme cold may have left these fish poorly poised to deal with a changing environment. As such, the expected environmental perturbations brought on by global climate change have the potential to significantly affect the energetic demands and subsequent cellular processes necessary for survival. Despite recent lines of evidence demonstrating that notothenioid fish retain the ability to acclimate to elevated temperatures, the underlying mechanisms responsible for temperature acclimation in these fish remain largely unknown. Furthermore, little information exists on the capacity of Antarctic fish to respond to changes in multiple environmental variables. We have examined the effects of increased temperature and pCO₂ on the rate of oxygen consumption in three notothenioid species, Trematomus bernacchii, Pagothenia borchgrevinki, and Trematomus newnesi. We combined these measurements with analysis of changes in aerobic and anaerobic capacity, lipid reserves, fish condition, and growth rates to gain insight into the metabolic cost associated with acclimation to this dual stress. Our findings indicated that temperature is the major driver of the metabolic responses observed in these fish and that increased pCO₂ plays a small, contributing role to the energetic costs of the acclimation response. All three species displayed varying levels of energetic compensation in response to the combination of elevated temperature and pCO₂. While P. borchgrevinki showed nearly complete compensation of whole animal oxygen consumption rates and aerobic capacity, T. newnesi and T. bernacchii displayed only partial compensation in these metrics, suggesting that at least some notothenioids may require physiological trade-offs to fully offset the energetic costs of long-term acclimation to climate change related stressors.

Antarctic notothenioid fish: what are the future consequences of 'losses' and 'gains' acquired during long-term evolution at cold and stable temperatures?

Antarctic notothenioids dominate the fish fauna of the Southern Ocean. Evolution for millions of years at cold and stable temperatures has led to the acquisition of numerous biochemical traits that allow these fishes to thrive in sub-zero waters. The gain of antifreeze glycoproteins has afforded notothenioids the ability to avert freezing and survive at temperatures often hovering near the freezing point of seawater. Additionally, possession of cold-adapted proteins and membranes permits them to sustain appropriate metabolic rates at exceptionally low body temperatures. The notothenioid genome is also distinguished by the disappearance of traits in some species, losses that might prove costly in a warmer environment. Perhaps the best-illustrated example is the lack of expression of hemoglobin in white-blooded icefishes from the family Channichthyidae. Loss of key elements of the cellular stress response, notably the heat shock response, has also been observed. Along with their attainment of cold tolerance, notothenioids have developed an extreme stenothermy and many species perish at temperatures only a few degrees above their habitat temperatures. Thus, in light of today's rapidly changing climate, it is critical to evaluate how these extreme stenotherms will respond to rising ocean temperatures. It is conceivable that the remarkable cold specialization of notothenioids may ultimately leave them vulnerable to future thermal increases and threaten their fitness and survival. Within this context, our review provides a current summary of the biochemical losses and gains that are known for notothenioids and examines these cold-adapted traits with a focus on processes underlying thermal tolerance and acclimation capacity.

RNA-seq reveals a diminished acclimation response to the combined effects of ocean acidification and elevated seawater temperature in Pagothenia borchgrevinki

PURPOSE

The IPCC has reasserted the strong influence of anthropogenic CO2 contributions on global climate change and highlighted the polar-regions as highly vulnerable. With these predictions the cold adapted fauna endemic to the Southern Ocean, which is dominated by fishes of the sub-order Notothenioidei, will face considerable challenges in the near future. Recent physiological studies have demonstrated that the synergistic stressors of elevated temperature and ocean acidification have a considerable, although variable, impact on notothenioid fishes. The present study explored the transcriptomic response of Pagothenia borchgrevinki to increased temperatures and pCO2 after 7, 28 and 56days of acclimation. We compared this response to short term studies assessing heat stress alone and foretell the potential impacts of these stressors on P. borchgrevinki's ability to survive a changing Southern Ocean.

RESULTS

P. borchgrevinki did demonstrate a coordinated stress response to the dual-stressor condition, and even indicated that some level of inducible heat shock response may be conserved in this notothenioid species. However, the stress response of P. borchgrevinki was considerably less robust than that observed previously in the closely related notothenioid, Trematomus bernacchii, and varied considerably when compared across different acclimation time-points. Furthermore, the molecular response of these fish under multiple stressors displayed distinct differences compared to their response to short term heat stress alone.

CONCLUSIONS

When exposed to increased sea surface temperatures, combined with ocean acidification, P. borchgrevinki demonstrated a coordinated stress response that has already peaked by 7days of acclimation and quickly diminished over time. However, this response is less dramatic than other closely related notothenioids under identical conditions, supporting previous research suggesting that this notothenioid species is less sensitive to environmental variation.

Juvenile Antarctic rockcod (Trematomus bernacchii) are physiologically robust to CO2-acidified seawater

To date, numerous studies have shown negative impacts of CO2-acidified seawater (i.e. ocean acidification, OA) on marine organisms, including calcifying invertebrates and fishes; however, limited research has been conducted on the physiological effects of OA on polar fishes and even less on the impact of OA on early developmental stages of polar fishes. We evaluated aspects of aerobic metabolism and cardiorespiratory physiology of juvenile emerald rockcod, ITALIC! Trematomus bernacchii, an abundant fish in the Ross Sea, Antarctica, to elevated partial pressure of carbon dioxide ( ITALIC! PCO2 ) [420 (ambient), 650 (moderate) and 1050 (high) μatm ITALIC! PCO2 ] over a 1 month period. We examined cardiorespiratory physiology, including heart rate, stroke volume, cardiac output and ventilation rate, whole organism metabolism via oxygen consumption rate and sub-organismal aerobic capacity by citrate synthase enzyme activity. Juvenile fish showed an increase in ventilation rate under high ITALIC! PCO2 compared with ambient ITALIC! PCO2 , whereas cardiac performance, oxygen consumption and citrate synthase activity were not significantly affected by elevated ITALIC! PCO2 Acclimation time had a significant effect on ventilation rate, stroke volume, cardiac output and citrate synthase activity, such that all metrics increased over the 4 week exposure period. These results suggest that juvenile emerald rockcod are robust to near-future increases in OA and may have the capacity to adjust for future increases in ITALIC! PCO2  by increasing acid-base compensation through increased ventilation.

Transcriptome wide analyses reveal a sustained cellular stress response in the gill tissue of Trematomus bernacchii after acclimation to multiple stressors

BACKGROUND

As global climate change progresses, the Southern Ocean surrounding Antarctica is poised to undergo potentially rapid and substantial changes in temperature and pCO2. To survive in this challenging environment, the highly cold adapted endemic fauna of these waters must demonstrate sufficient plasticity to accommodate these changing conditions or face inexorable decline. Previous studies of notothenioids have focused upon the short-term response to heat stress; and more recently the longer-term physiological response to the combined stress of increasing temperatures and pCO2. This inquiry explores the transcriptomic response of Trematomus bernacchii to increased temperatures and pCO2 at 7, 28 and 56 days, in an attempt to discern the innate plasticity of T. bernacchii available to cope with a changing Southern Ocean.

RESULTS

Differential gene expression analysis supported previous research in that T. bernacchii exhibits no inducible heat shock response to stress conditions. However, T. bernacchii did demonstrate a strong stress response to the multi-stressor condition in the form of metabolic shifts, DNA damage repair, immune system processes, and activation of apoptotic pathways combined with negative regulation of cell proliferation. This response declined in magnitude over time, but aspects of this response remained detectable throughout the acclimation period.

CONCLUSIONS

When exposed to the multi-stressor condition, T. bernacchii demonstrates a cellular stress response that persists for a minimum of 7 days before returning to near basal levels of expression at longer acclimation times. However, subtle changes in expression persist in fish acclimated for 56 days that may significantly affect the fitness T. bernacchii over time.

Hyperoxia Does Not Extend Critical Thermal Maxima (CTmax) in White- or Red-Blooded Antarctic Notothenioid Fishes

Understanding what limits the capacity of organisms to tolerate increasing temperatures is a critical objective in comparative biology. Using an experimental system of Antarctic notothenioid fishes, we sought to determine whether a mismatch between oxygen demand and oxygen supply was responsible for setting thermal tolerance limits. Previous studies have shown that Antarctic icefishes (family Channichthyidae), which lack hemoglobin, have lower critical thermal maxima (CTmax) than red-blooded notothenioids collected from the same region of the Antarctic (Western Antarctic Peninsula). In addition, within the notothenioid fishes there exists a positive correlation between CTmax and hematocrit. We tested the hypothesis that the lower CTmax of icefishes is associated with reduced oxygen supply. We employed an experimental heat ramp (4°C h(-1)) to determine CTmax under both normoxic and hyperoxic conditions and quantified correlates of oxygen limitation (lactate levels and expression of hypoxia-inducible factor-1α) in white-blooded Chaenocephalus aceratus and red-blooded Notothenia coriiceps. Hyperoxia, corresponding to a three- to fourfold increase in seawater Po2, did not extend CTmax in either species despite an overall mitigation in the rise of plasma and muscle lactate compared with the normoxic treatment. Our results also indicate that cardiac HIF-1α mRNA levels were insensitive to changes in both temperature and oxygen treatments. The absence of a change in CTmax with hyperoxia is likely to represent the contribution of factors beyond oxygen supply to physiological failure at elevated temperatures.

Is warmer better? Decreased oxidative damage in notothenioid fish after long-term acclimation to multiple stressors

Antarctic fish of the suborder Notothenioidei have evolved several unique adaptations to deal with subzero temperatures. However, these adaptations may come with physiological trade-offs, such as an increased susceptibility to oxidative damage. As such, the expected environmental perturbations brought on by global climate change have the potential to significantly increase the level of oxidative stress and cellular damage in these endemic fish. Previous single stressor studies of the notothenioids have shown they possess the capacity to acclimate to increased temperatures, but the cellular-level effects remain largely unknown. Additionally, there is little information on the ability of Antarctic fish to respond to ecologically relevant environmental changes where multiple variables change concomitantly. We have examined the potential synergistic effects that increased temperature and Ṗ(CO2) have on the level of protein damage in Trematomus bernacchii, Pagothenia borchgrevinki and Trematomus newnesi, and combined these measurements with changes in total enzymatic activity of catalase (CAT) and superoxide dismutase (SOD) in order to gauge tissue-specific changes in antioxidant capacity. Our findings indicate that total SOD and CAT activity levels displayed only small changes across treatments and tissues. Short-term acclimation to decreased seawater pH and increased temperature resulted in significant increases in oxidative damage. Surprisingly, despite no significant change in antioxidant capacity, cellular damage returned to near-basal levels, and significantly decreased in T. bernacchii, after long-term acclimation. Overall, these data suggest that notothenioid fish currently maintain the antioxidant capacity necessary to offset predicted future ocean conditions, but it remains unclear whether this capacity comes with physiological trade-offs.

Arctic marine fishes and their fisheries in light of global change

In light of ocean warming and loss of Arctic sea ice, harvested marine fishes of boreal origin (and their fisheries) move poleward into yet unexploited parts of the Arctic seas. Industrial fisheries, already in place on many Arctic shelves, will radically affect the local fish species as they turn up as unprecedented bycatch. Arctic marine fishes are indispensable to ecosystem structuring and functioning, but they are still beyond credible assessment due to lack of basic biological data. The time for conservation actions is now, and precautionary management practices by the Arctic coastal states are needed to mitigate the impact of industrial fisheries in Arctic waters. We outline four possible conservation actions: scientific credibility, 'green technology', legitimate management and overarching coordination.