Unexpected nondenitrifier nitrous oxide reductase gene diversity and abundance in soils

Agricultural and industrial practices more than doubled the intrinsic rate of terrestrial N fixation over the past century with drastic consequences, including increased atmospheric nitrous oxide (N₂O) concentrations. N₂O is a potent greenhouse gas and contributor to ozone layer destruction, and its release from fixed N is almost entirely controlled by microbial activities. Mitigation of N₂O emissions to the atmosphere has been attributed exclusively to denitrifiers possessing NosZ, the enzyme system catalyzing N₂O to N₂ reduction. We demonstrate that diverse microbial taxa possess divergent nos clusters with genes that are related yet evolutionarily distinct from the typical nos genes of denitirifers. nos clusters with atypical nosZ occur in Bacteria and Archaea that denitrify (44% of genomes), do not possess other denitrification genes (56%), or perform dissimilatory nitrate reduction to ammonium (DNRA; (31%). Experiments with the DNRA soil bacterium Anaeromyxobacter dehalogenans demonstrated that the atypical NosZ is an effective N₂O reductase, and PCR-based surveys suggested that atypical nosZ are abundant in terrestrial environments. Bioinformatic analyses revealed that atypical nos clusters possess distinctive regulatory and functional components (e.g., Sec vs. Tat secretion pathway in typical nos), and that previous nosZ-targeted PCR primers do not capture the atypical nosZ diversity. Collectively, our results suggest that nondenitrifying populations with a broad range of metabolisms and habitats are potentially significant contributors to N₂O consumption. Apparently, a large, previously unrecognized group of environmental nosZ has not been accounted for, and characterizing their contributions to N₂O consumption will advance understanding of the ecological controls on N₂O emissions and lead to refined greenhouse gas flux models.