A Membrane-Bound Cytochrome EnablesMethanosarcina acetivoransto Conserve Energy to Support Growth from Extracellular Electron Transfer
AbstractConservation of energy to support growth solely from extracellular electron transfer was demonstrated for the first time in a methanogen.Methanosarcina acetivoransgrew with methanol as the sole electron donor and the extracellular electron acceptor anthraquione-2,6-disulfonate (AQDS) as the sole electron acceptor when methane production was inhibited with bromoethanesulfonate. Transcriptomics revealed that transcripts for the gene for the transmembrane, multi-heme,c-type cytochrome MmcA were 4-fold higher in AQDS-respiring cells versus methanogenic cells. A strain in which the gene for MmcA was deleted failed to grow via AQDS reduction whereas strains in which other cytochrome genes were deleted grew as well as the wild-type strain. The MmcA-deficient strain grew with the conversion of methanol or acetate to methane, suggesting that MmcA has a specialized role as a conduit for extracellular electron transfer. Enhanced expression of genes for methanol conversion to methyl-coenzyme M and components of the Rnf complex suggested that methanol is oxidized to carbon dioxide in AQDS-respiring cells through a pathway that is similar to methyl-coenezyme M oxidation in methanogenic cells. However, during AQDS respiration the Rnf complex and reduced methanophenazine probably transfer electrons to MmcA, which functions as the terminal reductase for AQDS reduction. Extracellular electron transfer may enable survival of methanogens in dynamic environments in which oxidized humic substances and Fe(III) oxides are intermittently available. The availability of tools for genetic manipulation ofM. acetivoransmakes it an excellent model microbe for evaluatingc-type cytochrome-dependent extracellular electron transfer in Archaea.ImportanceExtracellular electron exchange inMethanosarcinaspecies and closely related Archaea plays an important role in the global carbon cycle and can enhance the speed and stability of anaerobic digestion, an important bioenergy strategy. The potential importance ofc-type cytochromes for extracellular electron transfer to syntrophic bacterial partners and/or Fe(III) minerals in some Archaea has been suspected for some time, but the studies withMethanosarcina acetivoransreported here provide the first genetic evidence supporting this hypothesis. The results suggest parallels with Gram-negative bacteria, such asShewanellaandGeobacterspecies, in which outer-surfacec-type cytochromes are an essential component for electrical communication with the extracellular environment.M. acetivoransoffers an unprecedented opportunity to study mechanisms for energy conservation from the anaerobic oxidation of one-carbon organic compounds coupled to extracellular electron transfer in Archaea with implications not only for methanogens, but possibly also for anaerobic methane oxidation.