Suggested role of NosZ in preventing N(2)O 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 nosZ genes and the ability to respire N(2)O. However, the selective advantage that may favor these nosZ-harboring DNRA-catalyzing organisms is not well understood. Here, the effect of N(2)O on Nrf-mediated DNRA was examined in a soil isolate, Bacillus sp. DNRA2, possessing both nrfA and nosZ genes. The DNRA metabolism of this bacterium was observed in the presence of C(2)H(2,) a NosZ inhibitor, with or without N(2)O, and the results were compared with C(2)H(2)-free controls. Cultures were also exposed to repeated oxic-anoxic transitions in the sustained presence of N(2)O. The NO(2) (−)-to-NH(4) (+) reduction following oxic-to-anoxic transition was significantly delayed in NosZ-inhibited C(2)H(2)-amended cultures, and the inhibition was more pronounced with repeated oxic-anoxic transitions. The possibility of C(2)H(2) involvement was dismissed since the cultures continuously flushed with C(2)H(2)/N(2) mixed gas after initial oxic incubation did not exhibit a similar delay in DNRA progression as that observed in the culture flushed with N(2)O-containing gas. The findings suggest a possibility that the oft-observed nosZ presence in DNRA-catalyzing microorganisms secures an early transcription of their DNRA genes by scavenging N(2)O, thus enhancing their capacity to compete with denitrifiers at oxic-anoxic interfaces. IMPORTANCE: Dissimilatory nitrate/nitrite reduction to ammonium (DNRA) is a microbial energy-conserving process that reduces NO(3) (−) and/or NO(2) (−) to NH(4) (+). Interestingly, DNRA-catalyzing microorganisms possessing nrfA genes are occasionally found harboring nosZ genes encoding nitrous oxide reductases, i.e., the only group of enzymes capable of removing the potent greenhouse gas N(2)O. Here, through a series of physiological experiments examining DNRA metabolism in one of such microorganisms, Bacillus sp. DNRA2, we have discovered that N(2)O may delay the 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: to remove N(2)O that may otherwise interfere with the transition from O(2) respiration to DNRA.