Redox cycling of Fe(II) and Fe(III) in magnetite accelerates aceticlastic methanogenesis by Methanosarcina mazei

Summary It has been recently shown that magnetite nanoparticles (nanoFe 3 O 4 ) can facilitate methanogenic syntrophy but the effect of magnetite on methanogenesis alone remains elusive. Here we show that aceticlastic methanogenesis by Methanosarcina mazei is accelerated by magnetite and is correlated with the redox cycling of structural Fe(II) and Fe(III) in the mineral. An enrichment and its closest pure culture relative, Ms . mazei zm‐15, both obtained from a natural wetland of the Tibetan plateau were tested in this experiment. The Fe(II) to Fe(III) ratios in magnetite, as measured by multiple approaches, show an initial increase in both the methanogenic cultures and the blank preparations containing no microbes. The Fe(II)/Fe(III) ratio then displays a distinct decline followed by an increase towards the end of incubation only in the enrichment and pure culture cultivations. This redox cycling of magnetite is in accordance with the stimulation of aceticlastic methanogenesis. Microscopic observation reveals the precipitation of nanoFe 3 O 4 on methanogen cell surface. The genomic analysis predicts that in addition to electron transfer components essential for aceticlastic methanogenesis, Ms . mazei zm‐15 contains an outer‐surface multiheme c ‐type cytochrome (MHC) and a few function‐unknown surface proteins that harbour monoheme motif. We hypothesize that the redox cycling of nanoFe 3 O 4 delivers a positive influence via the MHC to the membrane electron transfer chain and hence promote the aceticlastic methanogenesis.