Biochar as a Terminal Electron Acceptor for Geobacter Metallireducens Respiration

• Ankit Singh, Chemistry, The University of North Carolina at Chapel Hill

• Pei Chiu, Civil and Environmental Engineering, University of Delaware

Wood biochars have been shown to support anaerobic microbial oxidation of organic substrates, resulting in the suppression of methanogenesis. This exemplifies how microbial utilization of black carbon can impact the climate. This study was conducted to understand how black carbon supports microbial metabolism. Geobacter metallireducens GS-15 was pre-grown on acetate as an electron donor and ferric citrate as an electron acceptor. When ferric citrate was replaced by 3 g of wood biochar having an electron accepting capacity of 4.62 mmol/g, the cell number increased from 2.8(±0.6)×10⁸ to 1.17(±0.14)×10¹⁰ in 8 days based on fluorescent cell counting. The result was confirmed by qPCR. The qPCR results also showed that most cells existed in solution, whereas cell attachment to biochar was minimal. Graphite, which conducts but does not store electrons, did not support growth. Using ¹³C-labeled acetate and through electron balance, we showed that (1) GS-15 utilized 0.86 mmol/g, or 19%, of the biochar’s electron accepting capacity for growth, (2) 84% and 16% of the acetate was used for energy and biosynthesis, respectively, when GS-15 grew on biochar, and (3) ~80 billion electrons were deposited into biochar for each GS-15 cell produced. These results show that biochar supported growth through electron storage rather than conduction.