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 basis of results obtained with Azotobacter species, postulates that consumption of O(2) at the surface of diazotrophic prokaryotes protects nitrogenase from inactivation by O(2). Accordingly, it is assumed that, at increased ambient O(2) concentrations, nitrogenase activity depends on increased activities of a largely uncoupled respiratory electron transport system. The present review compiles evidence indicating that cellular O(2) consumption as well as both the activity and the formation of the respiratory system of Azotobacter vinelandii are controlled by the C/N ratio, that is to say the ratio at which the organism consumes the substrate (i.

Nitrogenase activity and regeneration of the cellular ATP pool in Azotobacter vinelandii adapted to different oxygen concentrations.

The in vivo activity of nitrogenase under aerobiosis was studied with diazotrophic chemostat cultures of Azotobacter vinelandii grown under glucose- or phosphate-limited conditions at different dilution rates (Ds, representing the growth rate mu) and different dissolved oxygen concentrations. Under steady-state conditions, the concentration as well as the cellular level of ATP increased in glucose-limited cultures when D was increased. Irrespective of the type of growth limitation or the dissolved oxygen concentration, the steady-state concentrations of ATP and of dinitrogen fixed by nitrogenase increased in direct proportion to each other.

Control of nitrogen fixation by oxygen in purple nonsulfur bacteria.

Some members of the facultatively phototrophic bacteria are able to grow diazotrophically in the presence of oxygen. As in other diazotrophs, the nitrogenase of the phototrophic bacteria is highly sensitive to oxygen; therefore, both the function and the expression of nitrogenase are strictly controlled by oxygen. This review focuses on the different levels of oxygen control in the two most extensively studied facultatively phototrophic bacteria, Rhodospirillum rubrum and Rhodobacter capsulatus.

Hydrogenase does not confer significant benefits to Azotobacter vinelandii growing diazotrophically under conditions of glucose limitation.

The presumed beneficial effect of hydrogenase on growth of diazotrophic bacteria was reinvestigated with carbon-limited chemostat cultures of the hydrogenase-deficient mutant hoxKG of Azotobacter vinelandii and its parent. The results revealed that hydrogen recycling was too low to benefit the cellular energy metabolism or activities of nitrogenase and respiration.

Cellular ATP levels and nitrogenase switchoff upon oxygen stress in chemostat cultures of Azotobacter vinelandii.

When Azotobacter vinelandii, growing diazotrophically in chemostat culture, was subjected to sudden increases in the ambient oxygen concentration (oxygen stress), nitrogenase activity was switched off and cellular ATP pools decreased at rates depending on the stress level. Following a fast decrease, the ATP pool approached a lower level. When the stress was released, these effects were reversed.