I. Hardewig

Metabolic Demand, Oxygen Supply, and Critical Temperatures in the Antarctic Bivalve Laternula elliptica

Oxygen consumption (Ṁo2), heartbeat rate and form, and circulating hemolymph oxygen content were measured in relation to temperature in the large Antarctic infaunal bivalve Laternula elliptica. After elevations in temperature from 0° to 3°, 6°, and then 9°C, Ṁo2 and heartbeat rate rose to new levels, whereas maximum circulating hemolymph oxygen content fell. At 0°C, Ṁo2 was 19.6 μmol O2 h−1 for a standard animal of 2‐g tissue ash‐free dry mass, which equates to a 8.95‐g tissue dry‐mass or 58.4‐g tissue wet‐mass animal. Elevation of metabolism following temperature change had acute Q10 values between 4.1 and 5, whereas acclimated figures declined from 3.4 (between 0° and 3°C) to 2.2 (3°–6°C) and 1.9 (6°–9°C). Heartbeat rate showed no acclimation following temperature elevations, with Q10 values of 3.9, 3.2, and 4.3, respectively. Circulating hemolymph oxygen content declined from 0° to 3° and 6°C but stayed at a constant Po2 (73–78 mmHg) and constant proportion (∼50%) of the oxygen content of the ambient water. At 9°C, Ṁo2 and heartbeat rate both peaked at values 3.3 times those measured at 0°C, which may indicate aerobic scope in this species. After these peaks, both measures declined rapidly over the ensuing 5 d to the lowest measured in the study, and the bivalves began to die. Hemolymph oxygen content fell dramatically at 9°C to values between 2% and 12% of ambient water O2 content and had a maximum Po2 of around 20 mmHg. These data indicate an experimental upper lethal temperature of 9°C and a critical temperature, where a long‐term switch to anaerobic metabolism probably occurs, of around 6°C for L. elliptica. Concurrent measures of mitochondrial function in the same species had indicated strong thermal sensitivity in proton leakage costs, and our data support the hypothesis that as temperature rises, mitochondrial maintenance costs rapidly outstrip oxygen supply mechanisms in cold stenothermal marine species.

Mitochondrial function and critical temperature in the Antarctic bivalve, Laternula elliptica

Thermal sensitivities of maximum respiration and proton leakage were compared in gill mitochondria of the Antarctic bivalve Laternula elliptica for an assessment of the contribution of mitochondrial mechanisms to limiting temperature tolerance. Proton leakage was measured as the oxygen consumption rate during blockage of oxidative phosphorylation (state IV respiration oligomycin). The maximum capacity of NADP dependent mitochondrial isocitrate dehydrogenase (IDH) was investigated as part of a proposed mitochondrial substrate cycle provoking proton leakage by the action of transhydrogenase. State III and IV respiration rose exponentially with temperature. Thermal sensitivities of proton leakage and IDH were unusually high, in accordance with the hypothesis that H leakage is an enzyme catalysed process with IDH being involved. In contrast to proton leakage, state III respiration exhibited an Arrhenius break temperature at 9°C, visible as a drop in thermal sensitivity close to, but still above the critical temperature of the species (3‐6°C). Progressive uncoupling of mitochondria led to a drop in RCR values and P:O ratios at high temperature. The same discontinuity as for state III respiration was found for the activity of mitochondrial IDH suggesting that this enzyme may influence the thermal control of mitochondrial respiration. In general, the high thermal sensitivity of proton leakage may cause an excessive rise in mitochondrial oxygen demand and a decreased efficiency of oxidative phosphorylation. This may exceed the whole animal capacity of oxygen uptake and distribution by ventilation and circulation and set a thermal limit, characterized by the transition to anaerobic metabolism.

Thermal sensitivity of mitochondrial function in the Antarctic Notothenioid Lepidonotothen nudifrons

Abstract The thermal sensitivity of mitochondrial function was investigated in the stenothermal Antarctic fish Lepidonotothen nudifrons. State 3 respiration increases with increasing temperature between 0 °C and 18 °C with a Q10 of 2.43–2.63. State 4 respiration in the presence of oligomycin, an inhibitor of mitochondrial ATP synthase, quantifies the leakage of protons through the inner mitochondrial membrane, which causes oxygen consumption without concomitant ATP production. This parameter shows an unusually high Q10 of 4.21 ± 0.42 (0–18 °C), which indicates that proton leakage does not depend merely on ion diffusion but is an enzyme-catalysed process. The differential thermal sensitivity of oxidative phosphorylation (=state 3) and proton leakage (=state 4 in the presence of oligomycin) leads to progressive uncoupling of the mitochondria and decreased efficiency of oxidative phosphorylation under in vivo conditions if the body temperature of L. nudifrons increases.

Temperature-dependent expression of cytochrome-c oxidase in Antarctic and temperate fish

Seasonal acclimation versus permanent adaptation to low temperatures leads to a differential response in the expression of cytochrome- c oxidase (CCO) in temperate and Antarctic eelpouts. Although eurythermal eelpout from the North Sea ( Zoarces viviparus) displayed a cold-induced rise of CCO activity in white muscle, enzyme activity in the cold stenothermal Antarctic eelpout Pachycara brachycephalum failed to reflect such a compensatory increase. In Antarctic eelpout, CCO activity correlates with transcript levels of mitochondrial encoded subunits of CCO (CCO I and CCO II), whereas cold-acclimated eelpout from the North Sea show lower enzyme activities than expected on the basis of mitochondrial mRNA levels. In these animals, CCO expression at low temperatures may be limited either by nuclear CCO transcripts or by posttranscriptional processes. These may comprise translation of the subunits or assembly of the CCO holoenzyme. mRNA levels of CCO IV, one of the nuclear encoded subunits, increased strongly during cold acclimation, indicating that the expression of CCO is likely not message limited in cold-acclimated Z. viviparus. Our data suggest that seasonal cold acclimation of Z. viviparus results in a modification of the relationship between transcription and translation or posttranslational processes. In permanently cold-adapted P. brachycephalum, on the other hand, CCO expression shows similar characteristics as in the warm-acclimated confamilial species, e.g., low levels of enzyme activity correlated with low levels of mitochondrial message.

High-energy turnover at low temperatures: recovery from exhaustive exercise in Antarctic and temperate eelpouts

Earlier work on Notothenioids led to the hypothesis that a reduced glycolytic capacity is a general adaptation to low temperatures in Antarctic fish. In our study this hypothesis was reinvestigated by comparing changes in the metabolic status of the white musculature in two related zoarcid species, the stenothermal Antarctic eelpout Pachycara brachycephalum and the eurythermal Zoarces viviparus during exercise and subsequent recovery at 0°C. In both species, strenuous exercise caused a similar increase in white muscle lactate, a drop in intracellular pH (pHi) by about 0.5 pH units, and a 90% depletion of phosphocreatine. This is the first study on Antarctic fish that shows an increase in white muscle lactate concentrations. Thus the hypothesis that a reduced importance of the glycolytic pathway is characteristic for cold-adapted polar fish cannot hold. The recovery process, especially the clearance of white muscle lactate, is significantly faster in the Antarctic than in temperate eelpout. Based on metabolite data, we calculated that during the first hour of recovery aerobic metabolism is increased 6.6-fold compared with resting rates in P. brachycephalum vs. an only 2.9-fold increase in Z. viviparus. This strong stimulation of aerobic metabolism despite low temperatures may be caused by a pronounced increase of free ADP levels, in the context of higher levels of pHi and ATP, which is observed in the Antarctic species. Although basal metabolic rates are identical in both species, the comparison of metabolic rates during situations of high-energy turnover reveals that the stenothermal P. brachycephalum shows a higher degree of metabolic cold compensation than the eurythermal Z. viviparus. Muscular fatigue after escape swimming may be caused by a drop of the free energy change of ATP hydrolysis, which is shown to fall below critical levels for cellular ATPases in exhausted animals of both species.

Exercise in the Cold: High Energy Turnover in Antarctic Fish

Antarctic fishes inhabit one of the world’s coldest marine habitats. At high latitudes the temperatures in the Southern Ocean are close to the freezing point of seawater at −1.86 °C and display little seasonal variation. The thermal conditions in the Antarctic have been relatively constant for several million years. The inhabiting fish fauna has therefore become highly specialized under the permanent cold conditions of this habitat which is reflected in a generally low upper lethal temperature of Antarctic fish at 5–6 °C [1]. In contrast to cold stenothermal polar fish, eurythermal boreal species must be able to survive a broader temperature range. While these species may be exposed to similarly cold water temperatures around 0 °C during winter, thermal conditions during summer require an elevated upper lethal temperature limit. On the other hand, eurythermal species are able to confine high cost metabolic activities like growth and reproduction to more favor able warmer seasons. Hence, acclimation to seasonal cold in eurythermal temperate species as opposed to adaptation to cold stenothermal conditions at high latitudes is associated with different requirements for the organism and may therefore result in distinct physiological features.

Interactions of Anaerobic Propionate Formation and Acid-Base Status in Arenicola marina: An Analysis of Propionyl-CoA Carboxylase

The contribution of propionyl-CoA carboxylase (PCC) to the control of anaerobic metabolism by acid-base parameters (pH, Pco₂, and [HCO₃⁻]) was investigated with a purified enzyme preparation and isolated mitochondria from the body wall musculature of Arenicola marina. The enzyme catalyzes the rate-limiting step in anaerobic propionate formation, namely, the carboxylation of methylmalonyl-CoA with concomitant formation of ATP and base equivalents (= HCO₃⁻). Propionyl-CoA carboxylase is likely not saturated with its substrates methylmalonyl-CoA, ADP, and Pi under in vivo conditions, and propionate formation is therefore activated by a decreasing energy charge of the cell (i. e., increasing ADP and Pi concentrations). The effects of the individual acid-base parameters pH, Pco₂, and [HCO₃⁻] on PCC activity have been determined. Stimulation of PCC by both high proton and low bicarbonate concentrations reflects an amplified control of propionate formation by the intracellular acid-base status. Nonrespiratory acidosis enhances the rate of decarboxylation of methylmalonyl-CoA, leading to a release of base equivalents. This mechanism has a strong stabilizing effect on the intracellular pH during long-term anaerobiosis. Without bicarbonate production by PCC, an additional pH drop of about 0. 03 pH units per hour of anaerobiosis would be observed in A. marina. Our data support the hypothesis that, besides ionic transport mechanisms, metabolism itself contributes to cellular pH regulation.