Production of butyrate and caproate from a coculture of Sporomusa ovata and Clostridium kluyveri during MES

Microbial electrosynthesis (MES) is the process in which microorganisms use electrons derivedfrom an electrode to reduce carbon dioxide via the Wood-Ljungdahl pathway to form organiccompounds. S. ovata, a Gram-negative acetogenic bacterium is the best electrotroph organismreported to date. The lack of a genetic system for S. ovata represents an obstacle that hampersengineering of this organism for the production of organic compounds other than acetate by MES.In this study, we attempted to produce longer and valuable compounds during MES by deploying acoculture of S. ovata and C. kluyveri to bypass the lack of genetic tools. S. ovata is able to produceethanol and acetate while C. kluyveri uses these two compounds as carbon source and producesbutyrate and caproate.The first step was to optimize the growth medium for S. ovata to increase ethanol production. Theeffect of trace metal ions such as SeO₄^- and WO₄^-, as well as Ni^₂+, Zn²⁺, Cu²⁺, and Fe²⁺ on growthand ethanol production was investigated. Growth and ethanol production under autotrophicconditions (H₂/CO₂) were improved by increasing tungstate and nickel (WO₄^- and Ni₂⁺)concentrations in the medium. The coculture S. ovata and C. kluyveri was thereafter tested underautotrophic condition (H₂/CO₂) using the optimised medium. A maximum of 6.0 mM and 2.0 mMof butyrate and caproate, respectively were produced after 18 days of incubation. In MES, onlytraces of butyrate were detected after 20 days. These results suggest that the amount of ethanolproduced in MES by S. ovata is not sufficient to support C. kluyveri growth; butyrate and caproateproduction. Further optimization of MES process is needed to increase ethanol production from S.ovata and subsequently increase butyrate and caproate production from C. kluyveri.