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.