Methanothrix enhances biogas upgrading in microbial electrolysis cell via direct electron transfer.

•Bioelectrochemical methane upgrading was achieved via DET.•Biogas with >90% methane content was generated when −500 mV potential was applied.•8.2% of the CO2 was converted to methane under the applied potential condition.•Species from the genera Methanothrix and Azonexus dominated the cathode community.•Genes for DIET-based growth were highly transcribed by Methanothrix on cathode.