FIG 2 .

Hydrogen and formate formation by spent cell-free culture medium of M. maripaludis wild-type (MM901) and hydrogenase deletion mutant (MM1284) strains on graphite cathodes. (a) Formate formation on graphite cathodes; (b) hydrogen formation on graphite cathodes. Triangles, MM901; diamonds, MM1284; circles, medium control. Electrode potentials as indicated. (c) Surface attachment and long-term stability of the catalytic activity of MM901 SCM at a cathode potential of −600 mV. Above-background current consumption was observed for several weeks in a bioelectrochemical reactor amended with cell-free spent medium of strain MM901. Current consumption increased after the entire medium was exchanged in the cathode chamber (indicated by arrows). This current increase was likely due to replenished substrate supply (CO₂ and H⁺) and indicates surface attachment of the catalytic activity. Background current consumption without SCM is indicated by a gray line.