Less Biomass and Intracellular Glutamate in Anodic Biofilms Lead to Efficient Electricity Generation by Microbial Fuel Cells

Background: Using a microbial fuel cell (MFC), we observed that a complex microbial community decomposed starch and transferred electrons to a graphite felt anode to generate current. In spite of the same reactor configuration, inoculum, substrate, temperature, and pH, MFCs produced different current and power density. To understand which factor(s) affected electricity generation, here, we analyzed a complex microbial community in an anodic biofilm and fermentation broth using Illumina MiSeq sequencing and metabolomics.

Results: Microbial biomass on the anode was lower in MFCs generating more electricity (0.09-0.16 mg cm-anode) than in those generating less electricity (0.60-2.80 mg cm-anode), while being equal (3890-4196 mg L-broth) in the fermentation broth over the same operational period. Chemical oxygen demand removal and acetate concentration were also similar in fermentation broths. MFCs generating more electricity had relatively more exoelectrogenic bacteria, such as sp., but fewer acetate-utilizing sp. and/or sp. in anodic biofilms. Accordingly, anodic biofilms generating more electricity presented higher levels of most intracellular metabolites related to the tricarboxylic acid cycle and a higher intracellular ATP/ADP ratio, but a lower intracellular NADH/NAD ratio. Moreover, the level of intracellular glutamate, an essential metabolite for microbial anabolic reactions, correlated negatively with current density.

Conclusion: Microbial growth on the anode and intracellular glutamate levels negatively affect electricity generation by MFCs. Reduced formation of anodic biofilm, in which intracellular glutamate concentration is 33.9 μmol g-cell or less, favors the growth of acetate-utilizing sp. on the anode and improves current generation.