AUGMENTED RATES OF RESPIRATION AND EFFICIENT NITROGEN-FIXATION AT NANOMOLAR CONCENTRATIONS OF DISSOLVED O-2 IN HYPERINDUCED AZOARCUS SP STRAIN BH72

Azoarcus sp. strain BH72 is an aerobic diazotrophic bacterium that was originally found as an endophyte in Kallar grass. Anticipating that t hese bacteria are exposed to dissolved O-2 concentrations (DOCs) in th e nanomolar range during their life cycle, we studied the impact of in creasing O-2 deprivation on N-2 fixation and respiration. Bacteria wer e grown in batch cultures, where they shifted into conditions of low p O(2) upon depletion of O-2 by respiration. During incubation, specific rates of respiration (qO(2)) and efficiencies of carbon source utiliz ation for N-2 reduction increased greatly, while the growth rate did n ot change significantly, a phenomenon that we called ''hyperinduction. '' To evaluate this transition from high- to low-test N-2 fixation in terms of respiratory kinetics and nitrogenase activities at nanomolar DOC, bacteria which had shifted to different gas-phase pO(2)s in batch cultures were subjected to assays using leghemoglobin as the O-2 carr ier. As O-2 deprivation in batch cultures proceeded, respiratory K-m ( O-2) decreased and V-max increased. Nitrogenase activity at nanomolar DOC increased to a specific rate of 180 nmol of C2H4 min(-1) mg of pro tein(-1) at 32 nM O-2. Nitrogenase activity was proportional to respir ation but not to DOC in the range of 12 to 86 nM O-2. Respiration supp orted N-2 fixation more efficiently at high than at low respiratory ra tes, the respiratory efficiency increasing from 0.14 to 0.47 mol of C2 H4 mol of O-2 consumed(-1). We conclude that (i) during hyperinduction , strain BH72 used an increasing amount of energy generated by respira tion for N-2 fixation, and (ii) these bacteria have a high respiratory capacity, enabling them to develop ecological niches at very low pO(2 ), in which they may respire actively and fix nitrogen efficiently at comparatively high rates.