Respiratory protection of nitrogenase in Azotobacter species: is a widely held hypothesis unequivocally supported by experimental evidence?

The hypothesis of respiratory protection, originally formulated on the basi s of results obtained with Azotobacter species, postulates that consumption of O-2 at the surface of diazotrophic prokaryotes protects nitrogenase fro m inactivation by O-2. Accordingly, it is assumed that, at increased ambien t O-2 concentrations, nitrogenase activity depends on increased activities of a largely uncoupled respiratory electron transport system. The present r eview compiles evidence indicating that cellular O-2 consumption as well as both the activity and the formation of the respiratory system of Azotobact er vinelandii are controlled by the C/N ratio, that is to say the ratio at which the organism consumes the substrate (i.e, the source of carbon, reduc ing equivalents and ATP) per sourer of compound nitrogen. The maximal respi ratory capacity which can br attained at increased C/N ratios, however, is controlled, within limits, by the ambient O-2 concentration. When growth be comes N-limited at increased CIN ratios, cells synthesize nitrogenase and f ix N-2. Under these diazotrophic conditions, cellular O-2 consumption remai ns constant at a level controlled by the O-2 concentration. Control by O-2 has been studied on the basis of both whole cell respiration and defined se gments of the respiratory electron transport chain. The results demonstrate that the effect of O-2 on the respiratory system is restricted to the lowe r range of O-2 concentrations up to about 70 mu M. Nevertheless, azotobacte rs are able to grow diazotrophically at dissolved O-2 concentrations of up to about 230 mu M indicating that respiratory protection is not warranted a t increased ambient O-2 concentrations. This conclusion is supported and ex tended by a number of results largely excluding an obvious relationship bet ween nitrogenase activity and the actual rate of cellular O-2 consumption. On the basis of theoretical calculations, it is assumed that the rate of O- 2 diffusion into the cells is not significantly affected by respiration. Al l of these results lead to the conclusion that, in the protection of nitrog enase fi om O-2 damage, O-2 consumption at the cell surface is less effecti ve than generally assumed. It is proposed that alternative factors like the supply of ATP and reducing equivalents are more important. (C) 2000 Federa tion of European Microbiological Societies. published by Elsevier Science B .V. All rights reserved.