THE OCCURRENCE, AND INFLUENCE ON PHOTOLITHOTROPHS, OF HIGH OXYGEN CONCENTRATIONS

Hyperoxia (O-2 in solution in excess of air-equilibrium values) occurs in certain photosynthesising cells which use inorganic C concentrating mechanisms, and as a result of abiological mechanisms. Geochemical evidence suggests that the atmosphere may have had significantly higher O-2 partial pressures in the past (e.g. the Upper Carboniferous) than occurs today. Biochemical effects of high O-2 concentrations in solution include inhibition of RUBISCO (competitive with CO2) and nitrogenase, as well as damage caused by higher levels of toxic O species (H2O2, O-2 and, especially, O-1(2) and OH.). The influence of high (twice the extant level) atmospheric O-2 on growth of non-N-2-fixers is as predicted from the properties of RUBISCO and the occurrence of inorganic C concentrating mechanisms. Acclimation of N-2-fixers to twice the extant O-2 level involves increased restriction on O-2 diffusion to nitrogenase so that growth is not inhibited (unless the high O-2 has access to the C-3 photosynthesis apparatus). Evidence as to the effect of hyperoxia on quenchers and scavengers of toxic O species is equivocal. Cells exposed to high O-2 probably have higher mutation rates as a result of higher levels of toxic O species, although the production and maintenance of 'stem' cells may occur in parts of the plants with relatively low O-2 levels.