Enumeration of exoelectrogens in microbial fuel cell effluents fed acetate or wastewater substrates

Effluents from well-acclimated microbial fuel cells (MFCs) have been widely used as inocula to start up new MFC reactors. However, the actual cell concentrations and cell viability of exoelectrogens in these MFC effluents have not been well examined. In this study, concentrations of exoelectrogens in the effluent from acetate- or wastewater-fed MFCs were examined using a quantitative polymerase chain reaction (qPCR) method specific for Geobacter spp. that are usually the dominant genus in MFCs, and a non-specific WO₃ nanocluster/most probable number (WO₃/MPN) method for enumeration of viable exoelectrogens. Geobacter spp. concentrations in acetate-fed MFC effluents based on qPCR were 1.3 ± 0.2 × 10⁸ cells/mL, slightly higher than those in the wastewater-fed MFC effluents (9.3 ± 3.5 × 10⁷ cells/mL). However, exoelectrogen cell counts using the WO₃/MPN method were several orders of magnitude lower for both MFC effluents (1.1 ± 0.3 × 10⁴ cells/mL for acetate-fed; 1.4 ± 0.3 × 10⁵ cells/mL for wastewater-fed). Live/dead cell staining suggested that most cells (85 %) in the effluents were inactive or dead, which could partly explain the lower numbers using the WO₃/MPN method. These results suggest that both acetate- and wastewater-fed MFC effluents contain high numbers of Geobacter spp. although a high percentage of cells are not viable.