Suggested role of NosZ in preventing N ₂ O inhibition of dissimilatory nitrite reduction to ammonium

Abstract

Dissimilatory nitrate/nitrite reduction to ammonium (DNRA) is a microbial energy-conserving process that reduces NO₃ ^- and/or NO₂ ^- to NH₄ ⁺. Interestingly, DNRA-catalyzing microorganisms possessing <i>nrfA</i> genes are occasionally found harboring <i>nosZ</i> 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, <i>Bacillus</i> sp. DNRA2, we have discovered that N₂O 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 <i>nrfA</i>-possessing microorganisms have retained <i>nosZ</i> genes: to remove N₂O that may otherwise interfere with the transition from O₂ respiration to DNRA.