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 x 10(8) cells/mL, slightly higher than those in the wastewater-fed MFC effluents (9.3 +/- 3.5 x 10(7) 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 x 10(4) cells/mL for acetate-fed; 1.4 +/- 0.3 x 10(5) 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.

Automated summary

Effluents from acetate- and wastewater-fed MFCs contain high numbers of Geobacter spp., but a high percentage of cells are inactive or dead, leading to lower counts of exoelectrogens using enumeration methods.