Mechanisms of adaptation to seasonal temperature variation in marine fish (Gadus morhua and Zoarces viviparus): Physiological, biochemical and theoretical aspects

Oxygen availability to mitochondria and their functional capacity are the key-factors in setting the thermal tolerance limits. Onset of the limitations in the aerobic scope at pejus temperatures (Tp) limits the geographical distribution. Further changes in temperature towards its extremes results in loss of aerobic scope at critical temperature.; Measurements of the temperature dependence of the whole body rate of oxygen consumption (Mo₂) showed that with acclimation to cold the Mot level was compensated by 50–60%, but at the same time the temperature dependence decreases 2 folds. These changes are probably related to the changes in concentration and the functional capacities of mitochondria in cold acclimation. The studied populations showed differences in temperature dependence of Mo ₂ only at temperatures above 13–15°C.; Critical oxygen concentration ([O₂]_c) shows an exponential temperature dependence and reaches ambient oxygen concentration at high temperature, implying the complete loss of aerobic scope for the whole animal, which is 22.5°C for eelpout. A theoretical evaluation of total oxygen conductance showed that it has a hyperbolic dependence on Mo₂ and at 15°C it is used at 50% of its capacity. Consequently, the capacity to lower the [O₂]_c is limited above this temperature. Most likely, 15°C is the pejus temperature for eelpout.; Analysis of the functional capacity of lactate dehydrogenase (LDH) revealed that this enzyme has no temperature induced functional limitation at physiological conditions in vivo below 16°C. We did not find an effect of LDH-B allozymes on the kinetic and thermodynamic parameters in crude homogenates. We also did not observe expression of new LDH isozymes during acclimation to different temperatures. The constant of substrate inhibition as well as the Arrhenius activation energy were significantly changed with acclimation to cold, which implies changes in enzyme functioning. Further thermodynamic analysis suggested that this could be explained by an introduction of additional hydrogen bonds in the protein structure (non-genetic post translational protein modification). This mechanism may significantly shift pyruvate distribution between aerobic and anaerobic metabolism in cold acclimation in favour of the aerobic one. This suggestion was also supported by Mo₂ measurements and may be one of the mechanisms contributing to the acclimation to the seasonal temperature variation.