How Bacteria Find the Optimal Oxygen Concentration for Nitrogen Fixation

Abstract

Biological nitrogen fixation occurs under low oxygen concentrations since high oxygen concentrations inactivate the nitrogenase. Nitrogen-fixing bacteria employ different strategies in order to maintain low intracellular oxygen concentrations. Rhizobia take advantage of the low oxygen concentration inside the plant roots where they fix nitrogen. Azotobacter vinelandii cells have a high concentration of respiratory complexes and intensive respiration that significantly lowers the intracellular oxygen concentration. Bacteria of the genus Azospirillum quickly move away from high oxygen concentrations by means of the aerotactic response. Aerotaxis is a migratory response that guides bacteria to niches where the oxygen concentration is optimal for their metabolic life style (Taylor 1983a). Azospirillum brasilense is a microaerophilic bacterium, whose growth is enhanced at low oxygen concentrations under nitrogen-fixing conditions as well as in the presence of combined nitrogen. We have recently demonstrated that A. brasilense cells are attracted to a specific oxygen concentration (4 µM dissolved oxygen; extracellular concentration) (Zhulin et al., 1996). Cells placed in a capillary tube, where a spatial oxygen gradient was created, accumulated at the optimal oxygen concentration within 30–60 seconds. Direct measurements of the proton motive force revealed that the bacteria generated a maximum energy level at the same oxygen concentration that causes positive aerotaxis (Zhulin et al., 1996). Both attraction to a specific oxygen concentration and generation of the maximal energy level occured under nitrogen-fixing conditions as well as in the presence of combined nitrogen.