In the bottom section of the high-current PMFC, G. sulfurreducens or one of its close relatives (G. metalliredu- cens, G. grbiciae and G. hydrogenophilus) were observed frequently on the graphite granules. The roots in the bottom section of the high-current PMFC were also colonised, although less frequently than the graphite granules (Fig. 4c,f).Inthelow-currentPMFC, G. sulfurreducens or one of its close relatives (G. metallireducens, G. grbiciae and G. hydrogenophilus) were observed less frequently in the top section, and no signal at all was detected in the bottom section.