Nitrous Oxide Reduction by an Obligate Aerobic Bacterium, Gemmatimonas aurantiaca Strain T-27.

Abstract

N2O-reducing organisms with nitrous oxide reductases (NosZ) are known as the only biological sink of N2O in the environment. Among the most abundant nosZ genes found in the environment are nosZ genes affiliated with the understudied Gemmatimonadetes phylum. In this study, a unique regulatory mechanism of N2O reduction in Gemmatimonas aurantiaca strain T-27, an isolate affiliated with the Gemmatimonadetes phylum, was examined. Strain T-27 was incubated with N2O and/or O2 as the electron acceptor. Significant N2O reduction was observed only when O2 was initially present. When batch cultures of strain T-27 were amended with O2 and N2O, N2O reduction commenced after O2 was depleted. In a long-term incubation with the addition of N2O upon depletion, the N2O reduction rate decreased over time and came to an eventual stop. Spiking of the culture with O2 resulted in the resuscitation of N2O reduction activity, supporting the hypothesis that N2O reduction by strain T-27 required the transient presence of O2 The highest level of nosZ transcription (8.97 nosZ transcripts/recA transcript) was observed immediately after O2 depletion, and transcription decreased ∼25-fold within 85 h, supporting the observed phenotype. The observed difference between responses of strain T-27 cultures amended with and without N2O to O2 starvation suggested that N2O helped sustain the viability of strain T-27 during temporary anoxia, although N2O reduction was not coupled to growth. The findings in this study suggest that obligate aerobic microorganisms with nosZ genes may utilize N2O as a temporary surrogate for O2 to survive periodic anoxia.IMPORTANCE Emission of N2O, a potent greenhouse gas and ozone depletion agent, from the soil environment is largely determined by microbial sources and sinks. N2O reduction by organisms with N2O reductases (NosZ) is the only known biological sink of N2O at environmentally relevant concentrations (up to ∼1,000 parts per million by volume [ppmv]). Although a large fraction of nosZ genes recovered from soil is affiliated with nosZ found in the genomes of the obligate aerobic phylum Gemmatimonadetes, N2O reduction has not yet been confirmed in any of these organisms. This study demonstrates that N2O is reduced by an obligate aerobic bacterium, Gemmatimonas aurantiaca strain T-27, and suggests a novel regulation mechanism for N2O reduction in this organism, which may also be applicable to other obligate aerobic organisms possessing nosZ genes. We expect that these findings will significantly advance the understanding of N2O dynamics in environments with frequent transitions between oxic and anoxic conditions.