$N_2O$ -reducing organisms with Z-type nitrous oxide reductases are known as the only biological sink in the environment. Among the most abundance nosZ genes found in the environment are nosZ genes affiliated to the understudied Gemmatimonadetes phylum. We have examined the unique regulatory mechanism of $N_2O$ reduction in Gemmatiomonas aurantiaca strain T-27, an isolate affiliated to Gemmatimonadetes phylum. G. aurantiaca was initially incubated with air and/or $N_2O$ as the electron acceptor. $N_2O$ was repetitively added upon its depletion and air was added after $N_2O$ reduction halted. The change in headspace $O_2$ and $N_2O$ concentrations were monitored and culture samples were withdrawn at varying time points for quantification of nosZ genes and transcripts.
G. aurantiaca was unable to consume $N_2O$ in an anaerobic medium. When G. aurantiaca was initially grown with 0.051 mmoles $O_2$ (2.04 % in the headspace) and 0.121 mmoles $N_2O$ (3.3 % in the headspace), $N_2O$ reduction commenced after headspace $O_2$ concentration dropped below 0.02%. Initial batch of $N_2O$ was fully consumed within 5.3 days. Repeated $N_2O$ injections confirmed sustained but decelerating $N_2O$ reduction activity that lasted >900 hours. The maximum observed rate of $N_2O$ reduction was $3.19 \mu moles/hour$ and the rate eventually decreased to 0 after 954.5 hours. Spiking of the culture with $0.066 \mu moles$ $O_2$ resulted in resuscitation of $N_2O$ activity, suggesting that $N_2O$ reduction required transient presence of $O_2$ . qPCR confirmed that $N_2O$ reduction was initially coupled to cell growth with an yield of $8.9 \times 10^7cells/ \mu moles N_2O$ . The highest level (13.5 nosZ transcript / recA transcript) of nosZ transcription was observed immediately after $O_2$ depletion and decreased ~90-fold within 85 hours, indicating that elevated nosZ transcription was not sustained under anoxia. Temporary spikes in $N_2O$ emissions from soil are often observed upon transition to anoxia following precipitation events. The obligately aerobic Gemmatimonadetes may have gained the capability to temporarily exploit this $N_2O$ as its survival strategy through transient anoxia.
자연환경에서 $N_2O$ 환원효소 (NosZ)를 가지고 있는 미생물이 유일하게 $N2O$ 생제거 능력을 가지고 있다고 알려졌다. 환경 속에 존재하는 많은 nosZ 유전자가 Gemmatimonadetes문에 속한다. Gemmatimonadetes문에 속하는 Gemmatimonas aurantiaca strain T-27은 특이한 $N_2O$ 환원 메카니즘을 가지고 있다고 연구가 되어있다. G. aurantiaca는 완전호기성 미생물로써 nosZ 유전자가 있음에도 혐기성 배지조건에서 $N_2O$ 를 환원시키지 못하였다. 글루코스를 전자공여체로 사용하고, 2.1% $O_2$ 와 3 mL의 99.999% $N_2O$ 를 같이 넣어준 배지조건에서 $N_2O$ 를 환원시키는 것을 확인하였다. 계속적인 3mL $N_2O$ 를 넣어 주었을 때, $N_2O$ 환원속도는 줄어들다가 멈추었다. 이후, 추가적인 $O_2$ 를 넣어주었을 때, $N_2O$ 환원반응이 다시 재게되었다.
이러한 특성은 G. aurantiaca가 $N_2O$ 를 환원시킬 때, 순간적으로 $O_2$ 가 필요하다는 것을 암시한다. qPCR 결과에서 $N_2O$ 가 세포성장에 기여한다는 것을 확인하였고, nosZ 전사체의 개수는 $O_2$ 가 고갈되었을 당시 가장 큰 값을 갖다가 다시 줄어들었다. Gemmatimonadetes문은 일시적인 무산소 조건이 되었을 때, 생존전략으로써$N_2O$ 를 전자수용체로 사용할 수 있는 능력을 획득하게 되었을 것이다.
