2025 Volume 40 Issue 4 Article ID: ME25032
Since nitrogenase is intrinsically sensitive to oxygen (O2), diverse aerobic diazotrophs need strategies to cope with nitrogenase damage by O2. In the present study, we investigated the mechanisms by which aerobic methane-oxidizing bacteria (methanotrophs) enable the concurrent activities of methane monooxygenase, which uses O2, and nitrogenase in the cytoplasm of the same cell. By using 15N labeling, we confirmed the capacity of alphaproteobacterial methanotroph Methylosinus sp. 3S-1 for nitrogen fixation and diazotrophic growth across a wide range of O2 concentrations <20%. When the initial O2 concentration was increased from 2 to 20% in a diazotrophic culture, similar decreases were observed in fixed nitrogen and NifH protein levels. In contrast, the mRNA levels of nitrogen fixation genes (nif genes) markedly increased and remained elevated for the duration of slow growth at high O2 concentrations. This pattern of nif expression in response to O2 may be attributed to the properties of the nif-specific transcriptional regulator NifA. The present results suggest that the increase in nif transcription is one of the strategies by which this methanotroph maintains nitrogen fixation on the background of aerobic methane oxidation.
