Marine invertebrate mitochondria and oxidative stress

The marine environment confronts its inhabitants with a wide variety of O-2 concentrations, as well as with fluctuations of small scale local O-2 availability over time. This review analyzes the respiratory response of marine animal ectotherms to fluctuating environmental O-2 availability in their specific habitats and reactive O-2 species generation under environmental stress, with a special emphasis on temperature. Specifically we compare mitochondrial functioning and reactive O-2 species formation in these animals to what is known from endothermal, mammalian species. Among the strategies employed by marine invertebrates to maintain tissue PO2 low and within tolerable margins, the possible role of mitochondrial oxyconformity to control cellular PO2 in water breathers is discussed. Mitochondrial generation of reactive O-2 species in ectotherms has been shown to depend on the magnitude of the mitochondrial membrane potential and the mitochondrial H+ leak. Alternative mitochondrial oxidases are described in marine ectotherms and might add to ameliorate reactive O-2 species formation. The effect of nitric oxide, which in mammals controls the reduction state of the electron transport by lowering cytochrome oxidase O-2 affinity, remains to be investigated in marine invertebrates. A new concept is proposed, showing how the cross talk of reactive O-2 species in metabolically low marine invertebrates could support their outstanding hypoxia tolerance under non-stressed conditions.