Suggested role of NosZ in preventing N2O inhibition of dissimilatory nitrite reduction to ammonium

Climate change and nutrient pollution are among the most urgent environmental issues. Enhancing the abundance and/or the activity of beneficial organisms is an attractive strategy to counteract these problems. Dissimilatory nitrate reduction to ammonium (DNRA), which theoretically improves nitrogen retention in soils, has been suggested as a microbial process that may be harnessed, especially since many DNRA-catalyzing organisms have been found to possess clade II nosZ genes and the ability to respire N2O. However, the selective advantages that may favor these nosZ-harboring DNRA-catalyzing organisms is not well understood. Here, the effect of N2O on Nrf-mediated DNRA was examined in a recently isolated soil bacterium, Bacillus sp. DNRA2, possessing both nrfA and nosZ genes. The DNRA metabolism of this bacterium was observed in the presence of C2H2, a NosZ inhibitor, with or without N2O, and the results were compared with C2H2-free controls. Cultures were also exposed to repeated oxic-anoxic transitions in the sustained presence of N2O. The NO2--to-NH4+ reduction following oxic-to-anoxic transition was significantly delayed in NosZ-inhibited C2H2-amended cultures, and the inhibition was more pronounced with repeated oxic-anoxic transitions. The possible involvement of C2H2 was dismissed since the cultures continuously flushed with C2H2/N2 mixed gas after initial oxic incubation did not exhibit a similar delay in DNRA progression as that observed in the culture flushed with N2O-containing gas. The findings provide novel ecological and evolutionary insights into the oft-observed presence of nosZ genes in DNRA-catalyzing microorganisms. ImportanceDissimilatory nitrate/nitrite reduction to ammonium (DNRA) is a microbial energy-conserving process that reduces NO3- and/or NO2- to NH4+. Interestingly, many DNRA-catalyzing microorganisms possessing nrfA genes harbor nosZ genes encoding nitrous oxide reductases, i.e., the only group of enzymes capable of removing the potent greenhouse gas N2O. Here, through a series of physiological experiments examining DNRA metabolism in one of such microorganisms, Bacillus sp. DNRA2, we have discovered that N2O may delay transition to DNRA upon an oxic-to-anoxic transition, unless timely removed by the nitrous oxide reductases. These observations suggest a novel explanation as to why some nrfA-possessing microorganisms have retained nosZ genes that had probably been acquired via horizontal gene transfers: to remove N2O that may otherwise interfere with the transition from O2 respiration to DNRA.