Sample selection, preparation and DNA extraction
Samples for analysis were taken from PMFC generating the highest current (PMFC6) and the PMFC generating the lowest current (PMFC3). Both PMFCs were dismantled 225 days after the start of the experiment. The anode com- partments of both PMFCs were divided into bottom (0 to 0.15 m) and top (0.15 to 0.3) sections. Figure 1 shows a schematic representation of the PMFC and the sampling points. The root samples were taken from all of the sampling points in both PMFCs and stored at −80°C. DNA was extracted from a total of 500 mg of root material per sample following the protocol of Griffiths et al. (2000) and using a BIO101 lysing matrix (MP Biomedicals, Illkirch, France) and a final purification step with the AllPrep Mini Kit by Qiagen (Munich, Germany). The amount of DNA in solu- tion was quantified using a bioanalyser from Agilent (Böblingen, Germany).