Abstract
Author(s): Yee, Mon Oo; Snoeyenbos-West, Oona; Thamdrup, Bo; Ottosen, Lars DM; Rotaru, Amelia-Elena | Abstract: Direct electron uptake by prokaryotes is a recently described mechanism with a potential application for energy and CO 2 storage 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 genus Methanosarcina, M. barkeri and M. 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 from Geobacter metallireducens via DIET. Furthermore, DIET was also stimulated upon addition of electrically conductive granular activated carbon (GAC) when each was co-cultured with G. metallireducens . However, when provided with a cathode poised at −400 mV (vs. SHE), only M. barkeri could perform electromethanogenesis. In contrast, the strict hydrogenotrophic methanogen, Methanobacterium formicicum , did not produce methane regardless of the type of insoluble electron donor provided ( Geobacter cells, GAC or electrodes). A comparison of functional gene categories between the two Methanosarcina showed 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.
Highlights
Extracellular electron uptake by methanogens may impact carbon turnover in electron-acceptor limited environments (Morris et al, 2013)
We have shown that both Methanosarcina species grew by Direct Interspecies Electron Transfer (DIET), only M. barkeri performed methanogenesis on the cathode at −400 mV
When grown in pure cultures, Methanosarcina species were provided with 30 mM acetate and 20 mM methanol as methanogenic substrates, while M. formicicum was provided with 150 kPa of H2: CO2 (80:20) in the headspace
Summary
Extracellular electron uptake by methanogens may impact carbon turnover in electron-acceptor limited environments (Morris et al, 2013). In these environments, thermodynamically challenging processes become possible due to syntrophic interactions between bacteria and archaea. A syntrophic interaction requires a bacterium, which oxidizes organics to interspecies-transferrable molecules. Syntrophy requires a partner methanogen to scavenge the transferable molecules. We have assumed that interspecies-transferrable molecules were either H2 or formate (Stams and Plugge, 2009). We know that some species can transfer electrons
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