Methane production by Methanothrix thermoacetophila via direct interspecies electron transfer with Geobacter metallireducens

Methanothrix is widely distributed in natural and artificial anoxic environments and plays a major role in global methane emissions. It is one of only two genera that can form methane from acetate dismutation and through participation in direct interspecies electron transfer (DIET) with exoelectrogens. Although Methanothrix is a significant member of many methanogenic communities, little is known about its physiology. In this study, transcriptomics helped to identify potential routes of electron transfer during DIET between Geobacter metallireducens and Methanothrix thermoacetophila . Additions of magnetite to cultures significantly enhanced growth by acetoclastic methanogenesis and by DIET, while granular activated carbon (GAC) amendments impaired growth. Transcriptomics suggested that the OmaF-OmbF-OmcF porin complex and the octaheme outer membrane c -type cytochrome, Gmet_0930, were important for electron transport across the outer membrane of G. metallireducens during DIET with Mx. thermoacetophila . Clear differences in the metabolism of Mx. thermoacetophila when grown via DIET or acetate dismutation were not apparent. However, genes coding for proteins involved in carbon fixation and a surface associated quinoprotein, SqpA, were highly expressed in all conditions. Expression of gas vesicle genes was significantly lower in DIET- than acetate-grown cells, possibly to facilitate better contact between membrane associated redox proteins during DIET. These studies reveal potential electron transfer mechanisms utilized by both Geobacter and Methanothrix during DIET and provide important insights into the physiology of Methanothrix in anoxic environments.