Putative Extracellular Electron Transfer in Methanogenic Archaea.

It has been suggested that a few methanogens are capable of extracellular electron transfers. For instance, can directly capture electrons from the coexisting microbial cells of other species. and retrieve electrons from direct interspecies electron transfer (DIET). Recently, , designated strain YSL, has been found to grow DIET in the co-culture with . can perform anaerobic methane oxidation and respiratory growth relying on Fe(III) reduction through the extracellular electron transfer. is capable of electromethanogenesis under the conditions where electron-transfer mediators like H or formate are limited. The membrane-bound multiheme c-type cytochromes (MHC) and electrically-conductive cellular appendages have been assumed to mediate the extracellular electron transfer in bacteria like and species. These molecules or structures are rare but have been recently identified in a few methanogens. Here, we review the current state of knowledge for the putative extracellular electron transfers in methanogens and highlight the opportunities and challenges for future research.