Selective Enrichment Establishes a Stable Performing Community for Microbial Electrosynthesis of Acetate from CO₂

Abstract

The advent of renewable energy conversion systems exacerbates the existing issue of intermittent excess power. Microbial electrosynthesis can use this power to capture CO 2 and produce multicarbon compounds as a form of energy storage. As catalysts, microbial populations can be used, provided side reactions such as methanogenesis are avoided. Here a simple but effective approach is presented based on enrichment of a robust microbial community via several culture transfers with H 2 :CO 2 conditions. This culture produced acetate at a concentration of 1.29 ± 0.15 g L −1 (maximum up to 1.5 g L −1 ; 25 mM) from CO 2 at a fixed current of −5 Am −2 in fed-batch bioelectrochemical reactors at high N 2 :CO 2 flow rates. Continuous supply of reducing equivalents enabled acetate production at a rate of 19 ± 2 gm −2 d −1 (projected cathode area) in several independent experiments. This is a considerably high rate compared with other unmodified carbon-based cathodes. 58 ± 5% of the electrons was recovered in acetate, whereas 30 ± 10% of the electrons was recovered in H 2 as a secondary product. The bioproduction was most likely H 2 based; however, electrochemical, confocal microscopy, and community analyses of the cathodes suggested the possible involvement of the cathodic biofilm. Together, the enrichment approach and galvanostatic operation enabled instant start-up of the electrosynthesis process and reproducible acetate production profiles.

Gas diffusion electrodes improve hydrogen gas mass transfer for a hydrogen oxidizing bioanode

“…Interestingly, after increasing bicarbonate levels, acetate was produced and detected in our bioanode even though the bioanode was inoculated with an enriched culture of electrogens and not with specific acetogenic microorganisms. It has been suggested, though, that a specific inoculum is required for acetate production in biocathodes 23. According to our results, under certain conditions, with sufficiently high H 2 pressure and a sufficiently high HCO 3 ‐ concentration, acetate formation will take place even if the reactors was not inoculated with specific acetogens.…”

Enhanced microbial electrosynthesis by using defined co-cultures

“…Often, sulfate reducers such as Desulfovibrio sp. are also present in these enrichments (Marshall et al, 2013;LaBelle et al, 2014;Patil et al, 2015), suggesting that hydrogen produced by the sulfate reducers might have a role in these biofilms. However, the role of individual strains in mixed communities and the molecular mechanism of microbial electron uptake from the cathode is unclear in most cases.…”

Biologically activated graphite fiber electrode for autotrophic acetate production from CO2in a bioelectrochemical system

“…Through this process, oxidized chemical molecules, such as CO 2 , can be converted to more reduced products, such as acetate and ethanol [4,5]. Since the first report of MESs with S. ovata [3,4], performance has been improved by efforts to optimize the reactor design, regulate the applied potential, and improve the bacterial enrichment method [6][7][8]. On the other hand, the interaction between microorganism and carbon materials is still unknown, which is the main factor limiting further improvement of the performance of the MES process.…”

“…These analyses provided information on how electro-active bacteria interact with the carbon electrode, and the appropriate range of applied potentials in BES [6,8]. For the first demonstration of electromethanogenesis, methane was produced from CO 2 at applied potentials below −700 mV vs Ag/AgCl.…”