Evaluation of long-term performance of plant microbial fuel cells using agricultural plants under the controlled environment

Plant microbial fuel cell (PMFC) is a novel bioelectrochemical system that integrates the photosynthetic reaction from the living plants to generate electricity via microorganisms at the rhizosphere of the plant roots. To elucidate factors which are critical for PMFCs operation, this study investigated the effects of different plants and soil conditioners on PMFCs performance. The experiment was done in a controlled lighting incubator at 27 °C and 75% of humidity for 200 days. Two waterlogged agricultural plants, paddy (Oryza sativa) and water bamboo (Zizania latifolia), were applied in PMFC systems; besides, the compost made from food waste and biochar made from waste wood biomass were selected as soil conditioners. Results showed that varied electricity generation during the operation was observed for different PMFC systems, but the Paddy-PMFC with compost (PC-PMFC) demonstrated relatively more stable electricity generation for 200 days (15.57 ± 8.15 mW/m²) and significantly higher voltage production, reaching the highest output voltage of 894.39 ± 53.44 mV (34.78 mW/m²) among all PMFCs. It was observed that the output voltage of PMFCs was significantly higher than soil-MFC, and the output voltage of P-PMFC was significantly higher than water bamboo-PMFC, implying rhizodeposition of different plant roots could be important for the performance of electricity production in PMFCs. However, Paddy-PMFC with biochar (PB-PMFC) demonstrated significantly lower voltage production than those without biochar, likely due to the inhibitory effect of biochar made by waste wood biomass. The taxonomic identification of the microbial community at the anode showed that Proteobacteria was the most abundant phylum, and Gammaproteobacteria and Deltaproteobacteria were the most dominant classes of the microbial communities. Further analysis showed that the PB-PMFC had the most distinct anode microbial community structure, with the predominant family of Gallionellaceae, instead of Geobacteraceae as in other PMFCs. Geobacter was the major genus of the microbial population in all samples and showed the highest relative abundance in PC-PMFC, suggesting that it was the main exoelectrogen involved in electricity generation in our PMFC systems. This study has demonstrated that the power output of PMFC systems can be influenced by different agricultural plants and soil conditioners made from waste biomass, which warrants the need to better understand the underlying interaction among the anode microbial community, the rhizodeposition of different plant roots, and electrochemical mechanisms for the future scale-up application of PMFCs.