Intracellular pH and energy metabolism in the highly stenothermal Antarctic bivalve Limopsis marionensis as a function of ambient temperature

Changes in oxygen consumption, ammonia excretion and in the acid-base and e nergy status of various tissues were investigated in the cold stenothermal Antarctic bivalve, Limopsis marionensis, and compared to similar data in th e limpet, Nacella concinna, for an assessment of thermal sensitivity. Oxyge n consumption of L. marionensis varied between -1.5 and 2 degrees C with a Q(10) of 2.2. Ammonia excretion could only be detected in animals exposed t o elevated temperature for periods in excess of 45 days and close to death and it is interpreted as the onset of protein and amino acid catabolism wit h starvation under temperature stress. In L. marionensis any change in temp erature as well as starvation stress at constant temperature induced a decr ease in phospho-L-arginine and ATP levels. However, only temperature stress resulted in a drop in the Gibb's free energy change of ATP hydrolysis. Int racellular pH rose in all tissues during upward or downward temperature cha nges of only 1.5 or 2 degrees C for 24 h with a concomitant trend to accumu late succinate and acetate in the: tissues. These changes are seen to refle ct disturbances of the tissue acid-base and energy status with any under- o r overshoot in aerobic metabolic rate during a temperature decrease or incr ease. Elevated temperature: at 2 degrees C during 2 weeks of incubation res ulted in continued net ATP depletion, at low levels of ATP free energy. Thi s indicates long-term stress, which was also mirrored in the inability to e stablish a new steady-state mean rate of oxygen consumption. Incubation at even higher temperatures of 4 and 7 degrees C led to an aggravation of ener getic stress and transition to an intracellular acidosis, as well as a fall in oxygen consumption. In N. concinna a drop in energy levels was also vis ible at 2 degrees C but was compensated for during long-term incubation. In conclusion, L. marionensis will be able to compensate for a temperature ch ange only in a very narrow range whereas the thermal tolerance window is mu ch wider in N. concinna. The inability of the metabolic rate to rise contin ually and the concomitant transition to anaerobic metabolism and long-term energetic stress characterize the upper critical temperature. Stenothermali ty is discussed, not only as reflecting the permanent and very stable low t emperature in the natural environment, but also regarding differences in th e level of activity and aerobic scope.