Absence of ammonia excretion during free-living, nitrogen-fixing growth of Azorhizobium caulinodans

Abstract

Azorhizobium caulinodans ORS571 was grown under nitrogen-fixing conditions in the chemostat at low pH, low specific growth rate (μ) and in the presence of excess energy. None of these conditions resulted in excretion of ammonia. In order to prevent negative feedback of fixed N2 on nitrogenase, mixed cultures with non-nitrogen-fixing organisms (NNFOs) were carried out. Increase of NNFOs wasfound in recycling cultures, but that was caused by distribution of nitrogen over the daughter cells. No indication has been found for differentiation into specialized non-growing, N2-fixing cells. The nitrogenase-capacity was found to be proportionally related to μ. We propose that during ex-planta, N2-fixing growth of A. caulinodans, no diffusion of the permeable NH3-molecule takes place as a result of rapid and efficient ammonia metabolism. © 1991.