Identification of the soil microbial community structure of fuel cells amended with compost

Soil microbial communities have a key role in several ecosystem services, including the recovery of soil from contamination. In fact, microbial communities have a great metabolic versatility and adapt promptly to different environmental stress, including presence of contaminants, developing degrading capabilities (biodegradation). Microbial fuel cells (MFCs) are bioreactors that use the bacteria capabilities to oxidize organic and inorganic matter, generating electrical energy. Electrons produced by bacteria from substrates are transferred to an anode (negative terminal) and flow to a cathode (positive terminal) linked by a conductive material. Because MFCs can continuously generate power at normal temperature and atmospheric pressure without any additional maintenance, producing only CO2 and H2O, they are a promising alternative renewable energy sources. Terrestrial MFCs (TMFCs) are MFCs in which soil is used as electrolyte and, thanks to exo-electrogen microorganisms naturally occurring in soil, organic compounds are degraded, including organic contaminants. TMFC is a promising system for bioelectricity production as well as soil remediation. In TMFCs, the anode is buried in water-saturated soil connected via external circuit to cathode on top of soil. In the buried anode, a biofilm is generated and organic matter is degraded (oxidation) by exo-electrogen bacteria capable to donate electrons, which are then simultaneously transferred to the cathode compartment via an external circuit. Protons produced by exo-electrogen bacteria in the anode, diffuse through soil to the cathode where oxygen is primarily used as the oxidant, due to its abundance and high reduction potential. The flow of electrons generate electricity. Several studies reported that biofilms at anode are typically enriched by bacteria belonging to Geobacter, Skermanella, Desulfucapsa, Desulfobulbus and Desulfuromonas genera. However, few works have investigated the microbial community in MFCs. In this work, we have analysed the prokaryotic community structure of TMFCs amended with compost, at three different points: anode, cathode and bulk soil, at 2 months from the experimental set-up. Control TMFCs consisting of only soil were also performed. Electric measurements (power and voltage) were measured daily. Microbial abundance, viability, activity and structure (NGS analysis of the 16S rDNA) were evaluated at the start and end of the experiment. The results showed significant differences in the compost-TMFC microbial community at anode, cathode and bulk soil. At the anode, a higher cell abundance and activity than cathode and bulk soil were found. Moreover, an increase in the Bacillus, Clostridia, Bacteroidia and Deltaproteobacteria and a decrease in Alphaproteobacteria and Actinobacteria genera were observed. Gammaproteobacteria were also reduced in presence of compost at the cathode and bulk soil. Interestingly, microbial soil activity was positively correlated to voltage and power generated by the MFCs. The genus Geobacter was found at anode and bulk soil in presence of compost.