Effect of oxygen and nitrate on nitrogen fixation and denitrification by Azospirillum brasilense grown in continuous culture

Abstract

A. brasilense was grown continuously at various levels of dissolved O2 in a N-free medium containing malate as the C source. Steady-state cultures were established only at O2 concentrations less than 0.0150 atm (1 atm = 101.325 Pa [Pascals]) and rates of acetylene reduction (N2 fixation) and efficiencies of N2 fixation were maximal between 0.0050-0.0075 atm dissolved O2. These cultures appeared to be O2- or N2-limited. There was no evidence of a respiratory protective mechanism in this organism. Anaerobic denitrifying steady-state cultures were established with nitrate (NO3-) as the sole N source with no detectable N2 fixation. N2 fixation, but no denitrification, was observed when NO3- was decreased to 10 .mu.g N/ml at 0.003 atm dissolved O2. In samples removed from the culture vessel, either activity could be induced with a lag of approximately 120 min by incubation under appropriate conditions.