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

Abstract

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.