Genomics and Ecology of Novel N₂O-Reducing Microorganisms

Microorganisms with the capacity to reduce the greenhouse gas nitrous oxide (N₂O) to harmless dinitrogen gas are receiving increased attention due to increasing N₂O emissions (and our need to mitigate climate change) and to recent discoveries of novel N₂O-reducing bacteria and archaea. The diversity of denitrifying and nondenitrifying microorganisms with capacity for N₂O reduction was recently shown to be greater than previously expected. A formerly overlooked group (clade II) in the environment include a large fraction of nondenitrifying N₂O reducers, which could be N₂O sinks without major contribution to N₂O formation. We review the recent advances about fundamental understanding of the genomics, physiology, and ecology of N₂O reducers and the importance of these findings for curbing N₂O emissions. N₂O is produced in several microbial processes, but N₂O reduction is limited to a single process catalyzed by the N₂O reductase present among taxonomically diverse bacteria and archaea. This enzyme is used for energy conservation through anaerobic respiration, detoxification, or removal of excess electrons. The N₂O reductase has evolved into two lineages, clade I and the recently described clade II. Surveys indicate that clade II N₂O reductase (NosZ) is abundant in many biomes. Genome analyses and culture-based studies suggest that the physiology of N₂O reduction differs between the clades. Evidence from community ecology supports niche differentiation of organisms with either type of N₂O reductase, and within the two clades. Identifying factors that drive physiological and ecological responses of N₂O-reducing communities could lead to effective and innovative strategies to curb N₂O emissions.