Hydrogen-based syntrophy in an electrically conductive biofilm anode

•The electrical conductivity of the butyrate-fed biofilm was as high as 0.67 mS/cm.•H2-based syntrophy played an important role in the electrically conductive biofilm.•Aqueous H2 concentration decreased as current increased.•Acetogens (Sphaerochaeta and Treponema) were primary H2 consumers in the biofilm.•Intracellular electron transport mainly limited current in the biofilm.