Ottosen, L. D. M. | Search bioRxiv

bioRxiv2022

The Fate of DIET Consortium exposed to continuous and intermittent electrochemical stimulation

Electromethanogenesis is the electrochemical conversion of carbon dioxide (CO2) to methane (CH4) with microorganisms as catalysts, and it is a promising approach for biogas upgrading. Many studies have shown increased methane production by electrochemical stimulation, allegedly due to the enhancement of direct interspecies electron transfer (DIET) between electrogenic bacteria and methanogens. However, these studies were done with mixed cultures where the interspecies interactions were undefined, so the impact of an electrochemical stimulus on these interactions remains unresolved. Here we report that an electrochemical stimulus diminished the survival and activity of a defined DIET consortium. We investigated the impact of electrochemical stimulation on a co-culture of Geobacter metallireducens and Methanosarcina barkeri, where the two partners interact syntrophically via DIET. The electrochemical stimulus was a cathode poised at - 700 mV (versus the standard hydrogen electrode, SHE). Electrochemical stimulation was provided either continuously or intermittently, and so was the food substrate. Compared to the untreated DIET co-culture, all treatment strategies relying on cathodic current additions resulted in acetate accumulation, lower methane production and lower cell numbers for both partners, indicating a detrimental impact of electrochemical stimulation to a DIET consortium. These findings suggest that future bioelectrochemical technologies must guarantee the survival of the syntrophic partners during current additions.

microbiology↗

bioRxiv2018

Extracellular electron uptake by two Methanosarcina species

Direct electron uptake by prokaryotes is a recently described mechanism with a potential application for energy and CO2 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.

microbiology↗