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 these bacteria are exposed to dissolved O-2 concentrations (DOCs) in the nanomolar range during their life cycle, we studied the impact of increasing O-2 deprivation on N-2 fixation and respiration. Bacteria were grown in batch cultures, where they shifted into conditions of low pO(2) upon depletion of O-2 by respiration. During incubation, specific rates of respiration (qO(2)) and efficiencies of carbon source utilization for N-2 reduction increased greatly, while the growth rate did not 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 carrier. 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 protein(-1) at 32 nM O-2. Nitrogenase activity was proportional to respiration but not to DOC in the range of 12 to 86 nM O-2. Respiration supported N-2 fixation more efficiently at high than at low respiratory rates, the respiratory efficiency increasing from 0.14 to 0.47 mol of C2H4 mol of O-2 consumed(-1). We conclude that (i) during hyperinduction, strain BH72 used an increasing amount of energy generated by respiration 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.