Short‐Term Effect of Ammonium Chloride on Nitrogen Fixation by Azotobacter vinelandii and by Bacteroids of Rhizobium leguminosarum

Abstract

Evidence is presented that the direct depressing effect of ammonium chloride on nitrogen fixation by Azotobacter vinelandii is due to inhibition of the electron transport system to nitrogenase. Furthermore, we were able to confirm the observation [Houwaard, F. (1979) Appl. Environ. Microbiol. in the press] that ammonium chloride has no short‐term effect on nitrogen fixation by isolated bacteroids of Rhizobium leguminosarum. By means of the flow dialysis technique it could be demonstrated that in A. vinelandii ammonium is taken up as a cation in response to the ΔΨ and that uptake of ammonium specifically inhibits the flow of reducing equivalents to nitrogenase by lowering the ΔΨ across the cytoplasmic membrane. In A. vinelandii, like in bacteroids, the generation of reducing equivalents at a potential low enough to reduce nitrogenase was found to be extremely sensitive towards changes in ΔΨ. At ΔΨ values less than 80 mV, interior negative, no such reducing equivalents are generated, while at a ΔΨ value of 110 mV nitrogenase is supplied optimally with reducing equivalents. The nature of the ammonium transport system in A. vinelandii and its significance as a regulator for the rapid ‘switch off/switch on’ of nitrogenase activity is discussed. Bacteroids of R. leguminosarum did not accumulate ammonium and no effect of ammonium on ΔΨ was observed. On the contrary, it could be demonstrated that bacteroids excrete ammonium in response to the ΔpH.