The effect of temperature on metabolic characteristics and biochemical structure: Indices of thermal sensitivity in mesopelagic fishes

Mesopelagic fishes spend daytime hours in deep cold water. Many species vertically migrate into shallow water at night. The thermal gradients experienced during the course of migration depend on geographic location. Migrating species in the Southern Ocean are stenothermal in contrast to those in tropical-subtropical regions that may be widely eurythermal. Three biochemical indices were used to assess the thermal sensitivity of fishes from different geographic regions.; Kinetic and thermal stability measurements of the muscle-type intermediary metabolic enzyme lactate dehydrogenase (A₄-LDH, EC 1.1.1.27, NAD +: lactate oxidoreductase) were positively correlated with the upper environmental temperature that mesopelagic fishes experience. Metabolic performance was negatively correlated with species' depth of occurrence. Reduced A ₄-LDH activity and turnover rates (k_cat) were found in deep-living species, a pattern consistent with the greater thermal stability exhibited by their enzymes. All mesopelagic species studied had lower A₄-LDH activity and turnover rates than those of shallow-living species living at similar habitat temperature. The inefficient enzymes of mesopelagic fishes may be one factor of importance in establishing the upper limits of their distributions.; Because temperature has a profound effect on the marginal stability of structure that is characteristic of enzymatic and structural proteins, endogenous and stress-induced levels of molecular chaperones were also used as indices of thermal sensitivity. Endogenous levels of the 70 kDa hsps were highest in the tissues of species that are strong vertical migrators. Levels of hsp70 were correlated with species' minimum depth of occurrence (MDO). Muscle tissue showed a strong negative correlation with MDO, and hepatic tissue showed a positive correlation with MDO.; Upon exposure of gill to temperatures between 8 and 40°C, proteins with apparent molecular masses of 27 and 70 kDa were ubiquitous among species. Hsp104 was also prevalent. The magnitude of the stress response was lowest in species with diurnal migration habits, suggesting they are more thermotolerant than asynchronously migrating congeners and confamilials.; Together, the proxies of metabolic performance and biochemical structure indicate that the physiological systems of vertically migrating mesopelagic fishes are adapted to upper habitat temperature. Furthermore, strong vertical migrators are poised to function at greater maximal temperatures and over a wider range of habitat temperatures than weak migrators. Indices of binding affinity between enzyme and ligand, rates of enzyme performance and thermal stability, levels of endogenous hsp70, and induction profiles of heat-shock proteins all suggest that strong vertical migrators are more thermotolerant than asynchronous and non-migrating species.