Fig. 5 Schematic presentation of the possible oxidation pathways of high molecular weight organic compounds (HMWO) and low molec- ular weight organic compounds (LMWO) in the anode of the plant microbial fuel cell (PMFC). The orange arrow represented the LMWO lost by the plant root; the yellow arrow represented the LMWO produced by cellulose-degrading bacteria (CDB). The green, light blue and brown arrows represented the possible oxidation pathways and products of the oxidation of LMWO compounds. The blue colour represents the volume of the PMFC anode where electrochemically active bacteria (EAB) are out-competed by acetate utilising denitrifiers (AUD) for LMWO. The green colour represents the volume of the PMFC anode where EAB are present and thus compete for LMWO with acetate-utilising methanogens (AUM)