Is the role of aerobic methanotrophs underestimated in methane oxidation under hypoxic conditions?

Microbial methane oxidation is the major biological methane (CH ₄ ) sink in the carbon cycle. Methanotrophs can use various electron acceptors in addition to oxygen; understanding the role and contribution of methanotrophs is thus an important topic. However, anaerobic oxidation of methane (AOM) mediated by methanotrophs is poorly explored and understood. This article summarizes the role aerobic methanotrophic bacteria play in AOM. Though AOM was originally considered to be mediated by anaerobic methanotrophic archaea, intra-aerobic methane-oxidizing bacteria (Candidatus Methylomirabilis oxyfera) appear to be involved in nitrite-dependent AOM. In addition, aerobic methanotrophs of the Methylomonadaceae and Methylocystaceae, are more versatile than previously assumed and can also be involved in nitrate/nitrite- or mineral oxide-dependent AOM under oxygen-limitation. Furthermore, the simultaneous reduction of nitrous oxide and oxidation of CH ₄ may be another new metabolic trait of aerobic methanotrophs. We discuss the potential metabolic pathways of CH ₄ oxidation under hypoxic conditions. It is of great ecological importance not only for the quantification of CH ₄ oxidation and emissions, but also for the definition of a new function of aerobic methanotrophs in anaerobic/hypoxic environments.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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