Differential impacts of elevated CO2 and acidosis on the energy budget of gill and liver cells from Atlantic cod, Gadus morhua.

Ocean acidification impacts fish and other marine species through increased seawater PCO2 levels (hypercapnia). Knowledge of the physiological mechanisms mediating effects in various tissues of fish is incomplete. Here we tested the effects of extracellular hypercapnia and acidosis on energy metabolism of gill and liver cells of Atlantic cod. Exposure media mimicked blood conditions in vivo, either during normo- or hypercapnia and at control or acidic extracellular pH (pHe). We determined metabolic rate and energy expenditure for protein biosynthesis, Na(+)/K(+)-ATPase and H(+)-ATPase and considered nutrition status by measurements of metabolic rate and protein biosynthesis in media with and without free amino acids (FAA). Addition of FAA stimulated hepatic but not branchial oxygen consumption. Normo- and hypercapnic acidosis as well as hypercapnia at control pHe depressed metabolic stimulation of hepatocytes. In gill cells, acidosis depressed respiration independent of PCO2 and FAA levels. For both cell types, depressed respiration was not correlated with the same reduction in energy allocated to protein biosynthesis or Na(+)/K(+)-ATPase. Hepatic energy expenditure for protein synthesis and Na(+)/K(+)-ATPase was even elevated at acidic compared to control pHe suggesting increased costs for ion regulation and cellular reorganization. Hypercapnia at control pHe strongly reduced oxygen demand of branchial Na(+)/K(+)-ATPase with a similar trend for H(+)-ATPase. We conclude that extracellular acidosis triggers metabolic depression in gill and metabolically stimulated liver cells. Additionally, hypercapnia itself seems to limit capacities for metabolic usage of amino acids in liver cells while it decreases the use and costs of ion regulatory ATPases in gill cells.

Stress response or beneficial temperature acclimation: transcriptomic signatures in Antarctic fish (Pachycara brachycephalum).

Research on the thermal biology of Antarctic marine organisms has increased awareness of their vulnerability to climate change, as a flipside of their adaptation to life in the permanent cold and their limited capacity to acclimate to variable temperatures. Here, we employed a species-specific microarray of the Antarctic eelpout, Pachycara brachycephalum, to identify long-term shifts in gene expression after 2 months of acclimation to six temperatures between -1 and 9 °C. Changes in cellular processes comprised signalling, post-translational modification, cytoskeleton remodelling, metabolic shifts and alterations in the transcription as well as translation machinery. The magnitude of transcriptomic responses paralleled the change in whole animal performance. Optimal growth at 3 °C occurred at a minimum in gene expression changes indicative of a balanced steady state. The up-regulation of ribosomal transcripts at 5 °C and above was accompanied by the transcriptomic activation of differential protein degradation pathways, from proteasome-based degradation in the cold towards lysosomal protein degradation in the warmth. From 7 °C upwards, increasing transcript levels representing heat-shock proteins and an acute inflammatory response indicate cellular stress. Such patterns may contribute to a warm-induced energy deficit and a strong weight loss at temperatures above 6 °C. Together, cold or warm acclimation led to specific cellular rearrangements and the progressive development of functional imbalances beyond the optimum temperature. The observed temperature-specific expression profiles reveal the molecular basis of thermal plasticity and refine present understanding of the shape and positioning of the thermal performance curve of ectotherms on the temperature scale.

Overcoming the coupled climate and biodiversity crises and their societal impacts.

Earth's biodiversity and human societies face pollution, overconsumption of natural resources, urbanization, demographic shifts, social and economic inequalities, and habitat loss, many of which are exacerbated by climate change. Here, we review links among climate, biodiversity, and society and develop a roadmap toward sustainability. These include limiting warming to 1.5°C and effectively conserving and restoring functional ecosystems on 30 to 50% of land, freshwater, and ocean "scapes." We envision a mosaic of interconnected protected and shared spaces, including intensively used spaces, to strengthen self-sustaining biodiversity, the capacity of people and nature to adapt to and mitigate climate change, and nature's contributions to people. Fostering interlinked human, ecosystem, and planetary health for a livable future urgently requires bold implementation of transformative policy interventions through interconnected institutions, governance, and social systems from local to global levels.

Oxygen- and capacity-limitation of thermal tolerance: a matrix for integrating climate-related stressor effects in marine ecosystems.

The concept of oxygen- and capacity-dependent thermal tolerance in aquatic ectotherms has successfully explained climate-induced effects of rising temperatures on species abundance in the field. Oxygen supply to tissues and the resulting aerobic performance characters thus form a primary link between organismal fitness and its role and functioning at the ecosystem level. The thermal window of performance in water breathers matches their window of aerobic scope. Loss of performance reflects the earliest level of thermal stress, caused by hypoxaemia and the progressive mismatch of oxygen supply and demand at the borders of the thermal envelope. Oxygen deficiency elicits the transition to passive tolerance and associated systemic and cellular stress signals like hormonal responses or oxidative stress as well as the use of protection mechanisms like heat shock proteins at thermal extremes. Thermal acclimatization between seasons or adaptation to a climate regime involves shifting thermal windows and adjusting window widths. The need to specialize on a limited temperature range results from temperature-dependent trade-offs at several hierarchical levels, from molecular structure to whole-organism functioning, and may also support maximized energy efficiency. Various environmental factors like CO(2) (ocean acidification) and hypoxia interact with these principal relationships. Existing knowledge suggests that these factors elicit metabolic depression supporting passive tolerance to thermal extremes. However, they also exacerbate hypoxaemia, causing a narrowing of thermal performance windows and prematurely leading the organism to the limits of its thermal acclimation capacity. The conceptual analysis suggests that the relationships between energy turnover, the capacities of activity and other functions and the width of thermal windows may lead to an integrative understanding of specialization on climate and, as a thermal matrix, of sensitivity to climate change and the factors involved. Such functional relationships might also relate to climate-induced changes in species interactions and, thus, community responses at the ecosystem level.

Climate change effects on fishes and fisheries: towards a cause-and-effect understanding.

Ongoing climate change is predicted to affect individual organisms during all life stages, thereby affecting populations of a species, communities and the functioning of ecosystems. These effects of climate change can be direct, through changing water temperatures and associated phenologies, the lengths and frequency of hypoxia events, through ongoing ocean acidification trends or through shifts in hydrodynamics and in sea level. In some cases, climate interactions with a species will also, or mostly, be indirect and mediated through direct effects on key prey species which change the composition and dynamic coupling of food webs. Thus, the implications of climate change for marine fish populations can be seen to result from phenomena at four interlinked levels of biological organization: (1) organismal-level physiological changes will occur in response to changing environmental variables such as temperature, dissolved oxygen and ocean carbon dioxide levels. An integrated view of relevant effects, adaptation processes and tolerance limits is provided by the concept of oxygen and capacity-limited thermal tolerance (OCLT). (2) Individual-level behavioural changes may occur such as the avoidance of unfavourable conditions and, if possible, movement into suitable areas. (3) Population-level changes may be observed via changes in the balance between rates of mortality, growth and reproduction. This includes changes in the retention or dispersion of early life stages by ocean currents, which lead to the establishment of new populations in new areas or abandonment of traditional habitats. (4) Ecosystem-level changes in productivity and food web interactions will result from differing physiological responses by organisms at different levels of the food web. The shifts in biogeography and warming-induced biodiversity will affect species productivity and may, thus, explain changes in fisheries economies. This paper tries to establish links between various levels of biological organization by means of addressing the effective physiological principles at the cellular, tissue and whole organism levels.

The human imperative of stabilizing global climate change at 1.5°C.

Increased concentrations of atmospheric greenhouse gases have led to a global mean surface temperature 1.0°C higher than during the pre-industrial period. We expand on the recent IPCC Special Report on global warming of 1.5°C and review the additional risks associated with higher levels of warming, each having major implications for multiple geographies, climates, and ecosystems. Limiting warming to 1.5°C rather than 2.0°C would be required to maintain substantial proportions of ecosystems and would have clear benefits for human health and economies. These conclusions are relevant for people everywhere, particularly in low- and middle-income countries, where the escalation of climate-related risks may prevent the achievement of the United Nations Sustainable Development Goals.

Temperature-dependent lipid levels and components in polar and temperate eelpout (Zoarcidae).

Total lipid content, lipid classes and fatty acid composition were analysed in tissues from two eelpout species fed on the same diet, the Antarctic Pachycara brachycephalum and the temperate Zoarces viviparus, with the aim of determining the role of lipids in fishes from different thermal habitats. The lipid content increased with decreasing temperature in the liver of both species, suggesting enhanced lipid storage under cold conditions. In P. brachycephalum, lipid composition in the liver and muscle was strongly dominated by triacylglycerols between 0 and 6 degrees C. In contrast, in the temperate species, lipid class composition changed with changes in the temperature. When acclimatized to 4 and 6 degrees C Z. viviparus not only displayed a shift to lipid anabolism and pronounced lipid storage, as indicated by high triacylglycerol levels, but also a shift to patterns of cold adaptation, as reflected by an increased content of polyunsaturated fatty acids in the lipid extract. Unsaturated fatty acids were also abundant in the Antarctic eelpout, but when compared to Z. viviparus at the same temperatures, the latter had significantly higher ratios of polyunsaturated to saturated fatty acid levels, whereas the Antarctic eelpout showed significantly higher ratios of monounsaturated to saturated fatty acid levels. High delta-15N values of the Antarctic eelpout reflect the high trophic level of this scavenger in the Weddell Sea food web. Stable carbon values suggest that lipid-enriched prey forms a major part of its diet. The strategy to accumulate storage lipids in the cold is interpreted to be adaptive behaviour at colder temperatures and during periods of irregular, pulsed food supply.

Intra-population variability of ocean acidification impacts on the physiology of Baltic blue mussels (Mytilus edulis): integrating tissue and organism response.

Increased maintenance costs at cellular, and consequently organism level, are thought to be involved in shaping the sensitivity of marine calcifiers to ocean acidification (OA). Yet, knowledge of the capacity of marine calcifiers to undergo metabolic adaptation is sparse. In Kiel Fjord, blue mussels thrive despite periodically high seawater PCO2, making this population interesting for studying metabolic adaptation under OA. Consequently, we conducted a multi-generation experiment and compared physiological responses of F1 mussels from 'tolerant' and 'sensitive' families exposed to OA for 1 year. Family classifications were based on larval survival; tolerant families settled at all PCO2 levels (700, 1120, 2400 µatm) while sensitive families did not settle at the highest PCO2 (≥99.8% mortality). We found similar filtration rates between family types at the control and intermediate PCO2 level. However, at 2400 µatm, filtration and metabolic scope of gill tissue decreased in tolerant families, indicating functional limitations at the tissue level. Routine metabolic rates (RMR) and summed tissue respiration (gill and outer mantle tissue) of tolerant families were increased at intermediate PCO2, indicating elevated cellular homeostatic costs in various tissues. By contrast, OA did not affect tissue and routine metabolism of sensitive families. However, tolerant mussels were characterised by lower RMR at control PCO2 than sensitive families, which had variable RMR. This might provide the energetic scope to cover increased energetic demands under OA, highlighting the importance of analysing intra-population variability. The mechanisms shaping such difference in RMR and scope, and thus species' adaptation potential, remain to be identified.

Exposure history determines pteropod vulnerability to ocean acidification along the US West Coast.

The pteropod Limacina helicina frequently experiences seasonal exposure to corrosive conditions (Ωar < 1) along the US West Coast and is recognized as one of the species most susceptible to ocean acidification (OA). Yet, little is known about their capacity to acclimatize to such conditions. We collected pteropods in the California Current Ecosystem (CCE) that differed in the severity of exposure to Ωar conditions in the natural environment. Combining field observations, high-CO2 perturbation experiment results, and retrospective ocean transport simulations, we investigated biological responses based on histories of magnitude and duration of exposure to Ωar < 1. Our results suggest that both exposure magnitude and duration affect pteropod responses in the natural environment. However, observed declines in calcification performance and survival probability under high CO2 experimental conditions do not show acclimatization capacity or physiological tolerance related to history of exposure to corrosive conditions. Pteropods from the coastal CCE appear to be at or near the limit of their physiological capacity, and consequently, are already at extinction risk under projected acceleration of OA over the next 30 years. Our results demonstrate that Ωar exposure history largely determines pteropod response to experimental conditions and is essential to the interpretation of biological observations and experimental results.

Ocean warming and acidification modulate energy budget and gill ion regulatory mechanisms in Atlantic cod (Gadus morhua).

Ocean warming and acidification are threatening marine ecosystems. In marine animals, acidification is thought to enhance ion regulatory costs and thereby baseline energy demand, while elevated temperature also increases baseline metabolic rate. Here we investigated standard metabolic rates (SMR) and plasma parameters of Atlantic cod (Gadus morhua) after 3-4 weeks of exposure to ambient and future PCO2 levels (550, 1200 and 2200 µatm) and at two temperatures (10, 18 °C). In vivo branchial ion regulatory costs were studied in isolated, perfused gill preparations. Animals reared at 18 °C responded to increasing CO2 by elevating SMR, in contrast to specimens at 10 °C. Isolated gills at 10 °C and elevated PCO2 (≥1200 µatm) displayed increased soft tissue mass, in parallel to increased gill oxygen demand, indicating an increased fraction of gill in whole animal energy budget. Altered gill size was not found at 18 °C, where a shift in the use of ion regulation mechanisms occurred towards enhanced Na(+)/H(+)-exchange and HCO3 (-) transport at high PCO2 (2200 µatm), paralleled by higher Na(+)/K(+)-ATPase activities. This shift did not affect total gill energy consumption leaving whole animal energy budget unaffected. Higher Na(+)/K(+)-ATPase activities in the warmth might have compensated for enhanced branchial permeability and led to reduced plasma Na(+) and/or Cl(-) concentrations and slightly lowered osmolalities seen at 18 °C and 550 or 2200 µatm PCO2 in vivo. Overall, the gill as a key ion regulation organ seems to be highly effective in supporting the resilience of cod to effects of ocean warming and acidification.

OCEANOGRAPHY. Contrasting futures for ocean and society from different anthropogenic CO2 emissions scenarios.

The ocean moderates anthropogenic climate change at the cost of profound alterations of its physics, chemistry, ecology, and services. Here, we evaluate and compare the risks of impacts on marine and coastal ecosystems­and the goods and services they provide­for growing cumulative carbon emissions under two contrasting emissions scenarios. The current emissions trajectory would rapidly and significantly alter many ecosystems and the associated services on which humans heavily depend. A reduced emissions scenario­consistent with the Copenhagen Accord's goal of a global temperature increase of less than 2°C­is much more favorable to the ocean but still substantially alters important marine ecosystems and associated goods and services. The management options to address ocean impacts narrow as the ocean warms and acidifies. Consequently, any new climate regime that fails to minimize ocean impacts would be incomplete and inadequate.

A new function for lactate in the toad Bufo marinus.

In the amphibian Bufo marinus, progressive hypoxia below a critical PO2 elicits a transient 50% increase in O2 consumption that coincides with the onset of lactate formation. The present study was designed to test the hypothesis that lactate causes the observed rise in metabolic rate. Arterial bolus infusions of pH-neutral sodium lactate solutions (4 mmol/kg body wt) in toads maintained under hypoxia actually elicit a similar increase in metabolic rate. The application of adrenergic antagonists (bretylium tosylate, phentolamine, propranolol, and reserpine) inhibits this response, suggesting that catecholamines are involved. Moreover, animals injected with lactate move to a cooler environment (behavioral hypothermia), a behavioral response that is beneficial during hypoxia. We hypothesize that, in accordance with Cannon's concept of an emergency response, lactate may function as an alarm signal during hypoxia. However, the signal function of lactate is observed in animals both under hypoxia and under normoxia and should thus be considered in future studies whenever elevated lactate levels are present, e.g., during and after exercise.

Oxygen limited thermal tolerance in fish?--Answers obtained by nuclear magnetic resonance techniques.

In various phyla of marine invertebrates limited capacities of both ventilatory and circulatory performance were found to set the borders of the thermal tolerance window with limitations in aerobic scope and onset of hypoxia as a first line of sensitivity to both cold and warm temperature extremes. The hypothesis of oxygen limited thermal tolerance has recently been investigated in fish using a combination of non-invasive nuclear magnetic resonance (NMR) methodology with invasive techniques. In contrast to observations in marine invertebrates arterial oxygen tensions in fish were independent of temperature, while venous oxygen tensions displayed a thermal optimum. As the fish heart relies on venous oxygen supply, limited cardio-circulatory capacity is concluded to set the first level of thermal intolerance in fish. Nonetheless, maximized ventilatory capacity is seen to support circulation in maintaining the width of thermal tolerance windows. The interdependent setting of low and high tolerance limits is interpreted to result from trade-offs between optimized tissue functional capacity and baseline oxygen demand and energy turnover co-determined by the adjustment of mitochondrial densities and functional properties to a species-specific temperature range. At temperature extremes, systemic hypoxia will elicit metabolic depression, thereby widening the thermal window transiently sustained especially in those species preadapted to hypoxic environments.

Simultaneous observations of haemolymph flow and ventilation in marine spider crabs at different temperatures: a flow weighted MRI study.

In vivo magnetic resonance imaging (MRI) and angiography were applied to the marine spider crab Maja squinado for a study of temperature effects and thermal tolerance. Ventilation and haemolymph circulation were investigated during progressive cooling from 12 degrees C to 2 degrees C. The anatomical resolution of MR images from Maja squinado obtained with a standard spin echo sequence were suitable to resolve the structures of various internal organs. The heart of the animal could be depicted without movement artifacts. The use of a flow compensated gradient echo sequence allowed simultaneous observations of ventilation, reflected by water flow through the gill chambers as well as of haemolymph flow. Simultaneous investigation of various arteries was possible by use of flow weighted MRI. In addition to those accessible by standard invasive flow sensitive doppler sensors, flow changes in gill, leg arteries and the venous return could be observed. Both ventilation and haemolymph flow decreased during progressive cooling and changes in haemolymph flow varied between arteries. Haemolymph flow through the Arteria sternalis, some gill and leg arteries was maintained at low temperatures indicating a reduced thermal sensitivity of flow in selected vessels. In support of previous invasive studies of haemolymph flow as well as heart and ventilation rates, the results demonstrate that the operation of gills and the maintenance of locomotor activity are critical for cold tolerance. A shift in haemolymph flow between arteries likely occurs to ensure the functioning of locomotion and ventilation in the cold.

Oxidative stress and antioxidative defense in cephalopods: a function of metabolic rate or age?

Activities of the antioxidative enzymes superoxide dismutase (SOD), catalase, glutathione peroxidase (GPX) and glutathione reductase (GR) were measured in the cephalopods Sepia officinalis and Lolliguncula brevis. Maximal enzyme activities were higher in gill tissue than in the mantle musculature of both species. Activities were generally lower in tissues of L. brevis than in S. officinalis. Comparison with other ectothermic animals showed both cephalopod species to have a low enzymatic antioxidative status despite their high metabolic rate. Furthermore, changes in antioxidative enzyme activities were measured in the cuttlefish S. officinalis with increasing age. The concentrations of malondialdehyde (MDA) and lipofuscin were determined as indicators of lipid peroxidation. Investigated animals were between 1.5 months and over 12 months old. Changes of antioxidative enzyme activities with age were not uniform. SOD and GPX activities increased with age, while catalase activity declined. In contrast, GR activity remained almost unchanged in all age groups. The low level of antioxidative defense might allow for the significant age-induced rise in MDA levels in gills and mantle musculature and for the increase in lipofuscin levels in mantle and brain tissue. It might thereby contribute to increased oxidative damage and a short life span in these cephalopods.

Oxyconformity in the intertidal worm Sipunculus nudus: the mitochondrial background and energetic consequences.

The energetic consequences of strict oxyconformity in the intertidal worm S. nudus were studied by characterizing the Po2 dependence of respiration in mitochondria isolated from the body wall tissue. Mitochondrial respiration rose in a Po2 range between 2.8 and 31.3 kPa from a mean of 56.5 to 223.9 nmol O mg protein(-1) h(-1). Respiration was sensitive to both salicylhydroxamic acid (SHAM) and KCN. Po2 dependence remained unchanged with saturating and non-saturating substrate levels (malate, glutamate and ADP). A concomitant decrease of the ATP/O ratio revealed a lower ATP yield of aerobic metabolism at elevated Po2. Obviously, oxyconforming respiration implies progressive uncoupling of mitochondria. The decrease in ATP/O ratios at higher Po2 was completely reversible. Addition of 90.9 micromol H2O2 l(-1) did not inhibit ATP synthesis. Both observations suggest that oxidative injury did not contribute to oxyconformity. The contribution of the rates of mitochondrial ROS production and proton leakiness to mitochondrial oxygen consumption and uncoupling was investigated by using oligomycin as a specific inhibitor of the ATP synthase. The maximum contribution of oligomycin independent respiration to state 3 respiration remained below 6% and showed a minor, insignificant increase at elevated Po2, at a slope significantly lower than the increment of state 3 respiration. Therefore, Po2 dependent mitochondrial proton leakage or ROS production cannot explain oxyconformity. In conclusion Po2 dependent state 3 respiration likely relates to the progressive contribution of an alternative oxidase (cytochrome o), which is characterized by a low affinity to oxygen and an ATP/O ratio similar to the branched respiratory system of bacteria. The molecular nature of the alternative oxidase in lower invertebrates is still obscure.

High sensitivity to chronically elevated CO2 levels in a eurybathic marine sipunculid.

CO2 levels are expected to rise (a) in surface waters of the oceans as atmospheric accumulation continues or (b) in the deep sea, once industrial CO2 dumping is implemented. These scenarios suggest that CO2 will become a general stress factor in aquatic environments. The mechanisms of sensitivity to CO2 as well as adaptation capacity of marine animals are insufficiently understood. Here, we present data obtained in Sipunculus nudus, a sediment-dwelling marine worm that is able to undergo drastic metabolic depression to survive regular exposure to elevated CO2 levels within its natural habitat. We investigated animal survival and the proximate biochemical body composition during long-term CO2 exposure. Results indicate an unexpected and pronounced sensitivity characterized by the delayed onset of enhanced mortality at CO2 levels within the natural range of concentrations. Therefore, the present study contrasts the previously assumed high-CO2 tolerance of animals adapted to temporary hypercapnia. As a consequence, we expect future loss of species and, thereby, detrimental effects on marine benthic ecosystems with as yet poorly defined critical thresholds of long-term tolerance to CO2.

Temperature effects on hemocyanin oxygen binding in an antarctic cephalopod.

The functional relevance of oxygen transport by hemocyanin of the Antarctic octopod Megaleledone senoi and of the eurythermal cuttlefish Sepia officinalis was analyzed by continuous and simultaneous recordings of changes in pH and hemocyanin oxygen saturation in whole blood at various temperatures. These data were compared to literature data on other temperate and cold-water cephalopods (octopods and giant squid). In S. officinalis, the oxygen affinity of hemocyanin changed at deltaP50/degrees C = 0.12 kPa (pH 7.4) with increasing temperatures; this is similar to observations in temperate octopods. In M. senoi, thermal sensitivity was much smaller (<0.01 kPa, pH 7.2). Furthermore, M. senoi hemocyanin displayed one of the highest levels of oxygen affinity (P50 < 1 kPa, pH 7.6, 0 degrees C) found so far in cephalopods and a rather low cooperativity (n50 = 1.4 at 0 degrees C). The pH sensitivity of oxygen binding (delta log P50/delta pH) increased with increasing temperature in both the cuttlefish and the Antarctic octopod. At low PO2 (1.0 kPa) and pH (7.2), the presence of a large venous oxygen reserve (43% saturation) insensitive to pH reflects reduced pH sensitivity and high oxygen affinity in M. senoi hemocyanin at 0 degrees C. In S. officinalis, this reserve was 19% at pH 7.4, 20 degrees C, and 1.7 kPa O2, a level still higher than in squid. These findings suggest that the lower metabolic rate of octopods and cuttlefish compared to squid is reflected in less pH-dependent oxygen transport. Results of the hemocyanin analysis for the Antarctic octopod were similar to those reported for Vampyroteuthis--an extremely high oxygen affinity supporting a very low metabolic rate. In contrast to findings in cold-adapted giant squid, the minimized thermal sensitivity of oxygen transport in Antarctic octopods will reduce metabolic scope and thereby contribute to their stenothermality.

Mitochondrial function in seasonal acclimatization versus latitudinal adaptation to cold in the lugworm Arenicola marina (L.).

Previous studies in marine ectotherms from a latitudinal cline have led to the hypothesis that eurythermal adaptation to low mean annual temperatures is energetically costly. To obtain more information on the trade-offs and with that the constraints of thermal adaptation, mitochondrial functions were studied in subpolar lugworms (Arenicola marina L.) adapted to summer cold at the White Sea and were compared with those in boreal specimens from the North Sea, either acclimatized to summer temperatures or to winter cold. During summer, a comparison of mitochondria from subpolar and boreal worms revealed higher succinate oxidation rates and reduced Arrhenius activation energies (Ea) in state 3 respiration at low temperatures, as well as higher proton leakage rates in subpolar lugworms. These differences reflect a higher aerobic capacity in subpolar worms, which is required to maintain motor activity at low but variable environmental temperatures--however, at the expense of an elevated metabolic rate. The lower activity of citrate synthase (CS) found in subpolar worms may indicate a shift in metabolic control within mitochondria. In contrast, acclimatization of boreal lugworms to winter conditions elicited elevated mitochondrial CS activities in parallel with enhanced mitochondrial respiration rates. With falling acclimation temperatures, the significant Arrhenius break temperature in state 3 respiration (11 degrees C) became insignificant (5 degrees C) or even disappeared (0 degrees C) at lower levels of Arrhenius activation energies in the cold, similar to a phenomenon known from hibernating vertebrates. The efficiency of aerobic energy production in winter mitochondria rose as proton leakage in relation to state 3 decreased with cold acclimation, indicated by higher respiratory control ratio values and increased adenosine diphosphate/oxygen (ADP/O) ratios. These transitions indicate reduced metabolic flexibility, possibly paralleled by a loss in aerobic scope and metabolic depression during winter cold. Accordingly, these patterns contrast those found in summer-active, cold-adapted eurytherms at high latitudes.

Cold tolerance and the regulation of cardiac performance and hemolymph distribution in Maja squinado (Crustacea: decapoda).

Elevated Mg(2+) levels in the hemolymph ([Mg(2+)](HL)) of brachyuran crabs have recently been demonstrated to limit cold tolerance by reducing motor and circulatory activity. Therefore, the limiting function of elevated [Mg(2+)](HL) on circulatory performance and arterial hemolymph flow was investigated by the pulsed-Doppler technique in the spider crab Maja squinado during progressive cooling from 12 degrees to 0 degrees C. [Mg(2+)](HL) were reduced from control levels of 39.9 mmol L(-1) to levels of 6.1 mmol L(-1) by incubation in magnesium reduced seawater. At 12 degrees C cardiac output was 13.9+/-2.4 mL kg(-1) min(-1) and stroke volume 0.2+/-0.04 mL kg(-1) min(-1) in control animals. In [Mg(2+)](HL)-reduced animals cardiac output increased to 43.6+/-5.0 mL kg(-1) min(-1) and stroke volume rose to 0.6+/-0.1 mL kg(-1) min(-1). Temperature reduction in control animals revealed a break point at 8 degrees C linked to a major redirection of hemolymph flow from lateral to sternal and hepatic arteries. Cardiac output and heart rate dropped sharply during cooling until transiently constant values were reached. Further heart rate reduction occurred below 4.5 degrees C. Such a plateau was not detected in [Mg(2+)](HL)-reduced animals where the break point decreased to 6 degrees C, also indicated by a sharp drop in heart rate and cardiac output and the redirection of hemolymph flow. It is concluded that progressive cooling brings the animals from a temperature range of optimum cardiac performance into a deleterious range when aerobic scope for activity falls before critical temperatures are reached. Reduction of [Mg(2+)](HL) shifts this transition to lower temperatures. These findings support a limiting role for [Mg(2+)](HL) in thermal tolerance.