Current production in Methanoperedens enrichment culture

Anaerobic methanotrophic archaea (ANME) are important players in the global carbon cycle by mitigating emissions of the potent greenhouse gas methane. 'Candidatus (Ca.) Methanoperedens spp.' archaea are cytochrome-containing versatile members of the ANME-2d group, performing nitrate-dependent methane conversion. There is increasing evidence that 'Ca. Methanoperedens spp.' are able to perform extracellular electron transfer (EET), such as during methane-dependent reduction of Fe3+ and Mn4+. In this study, a culture highly enriched in 'Ca. Methanoperedens' was used to investigate its ability to perform EET. The enrichment culture oxidized 13CH4 with concomitant reduction of graphene oxide, supporting the metabolic possibility of EET by 'Ca. Methanoperedens'. Furthermore, a two-chamber electrochemical system with an applied potential of 400 mV vs standard hydrogen electrode was used to invoke current production. A biofilm developed on the working electrode and low-density electrical current was produced. The presence of redox centers at -140 to -160 mV and at -230 mV in the biofilm was confirmed by cyclic voltammetry scans. Metagenomic analysis and Fluorescence in situ hybridization of the biofilm showed that 'Ca. Methanoperedens sp.' was a significant member of the biofilm community. Other members of the community included Zoogloea sp. (Betaproteobacteria), Dechloromonas sp. (Betaproteobacteria), two members of the Bacteroidetes, and the Spirochete Leptonema illini. Current generation appeared to depend more on acetate than on methane. Genes encoding proteins potentially involved in EET were identified in Zoogloea sp., Dechloromonas sp. and one member of the Bacteroidetes. We therefore hypothesize that 'Ca. Methanoperedens sp.' produces acetate that is subsequently used as substrate for EET. Whether 'Ca. Methanoperedens spp.' can directly perform EET remains to be elucidated.