Metabolic adaptations of intertidal invertebrates to environmental hypoxia (a comparison of environmental anoxia to exercise anoxia).

By comparing environmental anaerobiosis with exercise anaerobiosis it appears that animals with high anoxia tolerance use (partly) different types of metabolic reactions to sustain energy metabolism, whereas low tolerance animals (Arthropoda, Echinodermata, Vertebrata) use the same pathway under both conditions. During exercise anaerobiosis the classical glycolysis (lactate pathway) is a main pathway among all multicellular organisms, although in marine invertebrates--except the Arthropoda and Echinodermata--it mostly does not terminate in lactate. During environmental anaerobiosis Cnidaria, Mollusca, Annelida and Sipunculida first couple additional pathways for energy extraction to the glycolytic pathway (the aspartate--succinate pathway) and later deviate the main carbon flow of glycogen at the level of phosphoenolpyruvate towards succinate, propionate and acetate production. Metabolic adaptations to anoxic cellular conditions in these groups are high fuel stores, increased ATP yield by anaerobic sources, formation of easily excretable (volatile) end products, an aspartate-dependent system for transport of hydrogen through the inner membrane of the mitochondrion and a rapid recovery from anaerobic metabolism. During anaerobic conditions three sources can contribute to the anaerobic power output, endogenous stores of both ATP and phosphagen and catabolism. Anaerobic power output rates have been calculated for a number of Mollusca, Annelida and Crustacea. Extreme anoxia resistance is coupled to a strongly reduced metabolic rate. In animals with high aspartate stores, the aspartate--succinate pathway and phosphagen hydrolysis can provide sufficient ATP during environmental anaerobiosis; however, with exercise anaerobiosis when ATP turnover rates may be increased by a factor of 20, pyruvate derivatives simultaneously accumulate in high amounts relative to succinate.