Extracellular Electron Uptake by TwoMethanosarcinaSpecies

Abstract

AbstractDirect electron uptake by prokaryotes is a recently described mechanism with a potential application for energy and CO2storage into value added chemicals. Members of Methanosarcinales, an environmentally and biotechnologically relevant group of methanogens, were previously shown to retrieve electrons from an extracellular electrogenic partner performing Direct Interspecies Electron Transfer (DIET) and were therefore proposed to be electroactive. However, their intrinsic electroactivity has never been examined. In this study, we tested two methanogens belonging to the genusMethanosarcina, M. barkeriandM. horonobensis,regarding their ability to accept electrons directly from insoluble electron donors like other cells, conductive particles and electrodes. Both methanogens were able to retrieve electrons fromGeobacter metallireducensvia DIET. Furthermore, DIET was also stimulated upon addition of electrically conductive granular activated carbon (GAC) when each was co-cultured withG. metallireducens. However, when provided with a cathode poised at −400 mV (vs. SHE), onlyM. barkericould perform electromethanogenesis. In contrast, the strict hydrogenotrophic methanogen,Methanobacterium formicicum, did not produce methane regardless of the type of insoluble electron donor provided (Geobactercells, GAC or electrodes). A comparison of functional gene categories between the twoMethanosarcinashowed differences regarding energy metabolism, which could explain dissimilarities concerning electromethanogenesis at fixed potentials. We suggest that these dissimilarities are minimized in the presence of an electrogenic DIET partner (e.g.Geobacter), which can modulate its surface redox potentials by adjusting the expression of electroactive surface proteins.