Long-term Operation of Microbial Electrosynthesis Systems Improves Acetate Production by Autotrophic Microbiomes Christopher W. Marshall, † Daniel E. Ross, ‡ Erin B. Fichot, ‡ R. Sean Norman, ‡ and Harold D. May* ,† † Department of Microbiology & Immunology, Marine Biomedicine & Environmental Science Center, Medical University of South Carolina, Charleston, South Carolina 29425, United States ‡ Department of Environmental Health Sciences, University of South Carolina, Columbia, South Carolina 29208, United States * S Supporting Information ABSTRACT: Microbial electrosynthesis is the biocathode-driven pro- duction of chemicals from CO 2 and has the promise to be a sustainable, carbon-consuming technology. To date, microbial electrosynthesis of acetate, the first step in order to generate liquid fuels from CO 2 , has been characterized by low rates and yields. To improve performance, a previously established acetogenic biocathode was operated in semi-batch mode at a poised potential of -590 mV vs SHE for over 150 days beyond its initial development. Rates of acetate production reached a maximum of 17.25 mM day -1 (1.04 g L -1 d -1 ) with accumulation to 175 mM (10.5 g L -1 ) over 20 days. Hydrogen was also produced at high rates by the biocathode, reaching 100 mM d -1 (0.2 g L -1 d -1 ) and a total accumulation of 1164 mM (2.4 g L -1 ) over 20 days. Phylogenetic analysis of the active electrosynthetic microbiome revealed a similar community structure to what was observed during an earlier stage of development of the electroacetogenic microbiome. Acetobacterium spp. dominated the active microbial population on the cathodes. Also prevalent were Sulf urospirillum spp. and an unclassified Rhodobacteraceae. Taken together, these results demonstrate the stability, resilience, and improved performance of electrosynthetic biocathodes following long-term operation. Furthermore, sustained product formation at faster rates by a carbon-capturing microbiome is a key milestone addressed in this study that advances microbial electrosynthesis systems toward commercialization. ■ INTRODUCTION A recently described sustainable technology called microbial electrosynthesis is an enticing innovation to combat fossil fuel dependence and potential climate change because it combines carbon capture with the production of valuable chemicals and/ or fuels. Microbial electrosynthesis is an electrode-driven process that provides microorganisms with the reducing equivalents to fix CO 2 and generate a reduced end-product. A negatively poised cathode is the sole source of electrons that stimulates a microbial catalyst to consume CO 2 as the only carbon source. Several recent reviews have discussed the microbiological, 1 technological, 2 and economic 3 aspects of microbial electrosynthesis systems (MES), but only a handful of studies have demonstrated the technology in the laboratory. 4-7 The first example of biocathode-driven CO 2 fixation was a process called electromethanogenesis. As the nomenclature implies, current from a negatively poised cathode is converted to methane, the principal component of natural gas. Several studies utilized cathodes to stimulate methanogenesis, 8,9 but Cheng et al. first demonstrated methane production in the absence of exogenous electron-shuttling mediators by mixed biocathode communities and a pure culture methanogen, Methanobacterium palustre. 10 Subsequently, several studies by Villano et al. 11 and others 12 have shown electromethanogenesis through direct electron transfer and/or mediated electron transfer (via hydrogen) depending upon applied potentials and the capabilities of the microbial catalysts. Another important microbial electrosynthesis strategy is to store CO 2 in liquid fuels or chemicals, requiring the synthesis of carbon-carbon bonds. Nevin et al. described the first proof of principle of electroacetogenesis using pure cultures of acetogenic bacteria. 5 Subsequent studies by Nevin et al. 6 and Zhang et al. 7 demonstrated electroacetogenesis with a variety of pure culture acetogens and increases in rates with chemically modified cathodes, respectively. Further improvements in the rates of electroacetogenesis were detailed by Marshall et al. using mixed microbial communities (microbiomes). 4 In order to consider this technology for scaling, an essential question with MESs is if sustainably high rates and titers can be achieved. In the case of electrosynthetic microbiomes, it is feasible that long-term adaptation may result in improved performance, similar to what has been observed with MFCs. 13 Received: January 23, 2013 Revised: April 23, 2013 Accepted: April 26, 2013 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es400341b | Environ. Sci. Technol. XXXX, XXX, XXX-XXX