AUTHOR=Gao Kailin , Lu Yahai 

TITLE=Putative Extracellular Electron Transfer in Methanogenic Archaea

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.611739

DOI=10.3389/fmicb.2021.611739

ISSN=1664-302X

ABSTRACT=<p>It has been suggested that a few methanogens are capable of extracellular electron transfers. For instance, <italic>Methanosarcina barkeri</italic> can directly capture electrons from the coexisting microbial cells of other species. <italic>Methanothrix harundinacea</italic> and <italic>Methanosarcina horonobensis</italic> retrieve electrons from <italic>Geobacter metallireducens via</italic> direct interspecies electron transfer (DIET). Recently, <italic>Methanobacterium</italic>, designated strain YSL, has been found to grow <italic>via</italic> DIET in the co-culture with <italic>Geobacter metallireducens</italic>. <italic>Methanosarcina acetivorans</italic> can perform anaerobic methane oxidation and respiratory growth relying on Fe(III) reduction through the extracellular electron transfer. <italic>Methanosarcina mazei</italic> is capable of electromethanogenesis under the conditions where electron-transfer mediators like H<sub>2</sub> 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 <italic>Geobacter</italic> and <italic>Shewanella</italic> 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.</p>