A biofilm anode acclimated with acetate, acetate+methane, and methane growth media for over three years produced a steady current density of 1.6-2.3 mA/m^2 in a microbial electrochemical cell (MxC) fed with methane as the sole electron donor. Geobacter was the dominant genus for the bacterial domain (93%) in the biofilm anode, while methanogens (Methanocorpusculum labreanum and Methanosaeta concilii) accounted for 82% of the total archaeal clones in the biofilm. A fluorescence in situ hybridization (FISH) image clearly showed a biofilm of bacteria and archaea, supporting a syntrophic interaction between them for performing anaerobic oxidation of methane (AOM) in the biofilm anode. Measured cumulative coulombs correlated linearly to the methane-gas concentration in the range of 10% to 99.97% (R^2 ≥ 0.99) when the measurement was sustained for at least 50 min. Thus, cumulative coulombs over 50 min. could be used to quantify the methane concentration in gas samples.