Methane (CH₄) is a potent greenhouse gas with a global warming potential 28-34 times that of carbon dioxide on a centennial time scale. Microbial CH₄ oxidation acts as a biofilter and prevents over 90% of CH₄ reaching the atmosphere. 

Traditionally, aerobic CH₄-oxidizing bacteria (MOB) is O₂-dependent to grow on CH₄ as their sole source of carbon and energy. Increasing studies have shown that MOB are present and even active in anoxic environments, without O₂ utilized as an electron acceptor by them, but their survival strategy and ecological contribution are still enigmatic. 

Recently, researchers led by Dr. LI Biao from Prof. WU Qinglong’s team at Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences (NIGLAS), along with their collaborators, have investigated the survival strategies of MOB under anoxic conditions. 

Their findings were published in Water Research on 3 March 2023. 

After two-year enrichment, the researchers obtained an enriched consortium dominated by γ-MOB, Methylomonas and several other several heterotrophic bacteria, but without anaerobic methanotrophs. 

They found the MOB consortium can couple CH₄ oxidation and Fe(III) reduction under anoxia with the help of electron shuttles such as riboflavin. Within the MOB consortium, MOB transformed CH₄ to low molecular weight organic matter such as acetate for consortium bacteria as a carbon source, while the latter secrete riboflavin to facilitate extracellular electron transfer. 

“A metabolic flexibility was observed in this conventional considered O2-depentend microbe, MOB. Given that iron is the fourth most abundant element on Earth and generally abundant in lacustrine sediment, using ferric oxides as electron acceptor maybe a critical lifestyle for MOB and an important CH₄ sink on early Earth, where anoxic conditions are ubiquitously present.” said Dr. LI. 

In the in situ anoxic sediments, multiple kinds of microbes associated to the consortium including Methylomonas were transcriptionally active. Moreover, Fe(III) reduction coupled to CH4 oxidation mediated by the MOB consortium reduced 40.3% of the CH₄ emission in the iron-rich sediments. 

“There are many iron-rich areas in Southern China, MOB in these iron-rich area may play critical roles in mitigating CH₄ emission even under anoxia there. Our study unveils how MOBs survive under anoxia and expands the knowledge of this previously overlooked CH4 sink in iron-rich sediments,” said Dr. LI. 

Schematic diagram of CH₄ metabolism mediated by MOB under oxic and anoxic conditions. (Image by LI Biao)

Contact 

TAN Lei 

Nanjing Institute of Geography and Limnology 

E-mail: ltan@niglas.ac.cn