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<p>The electrosynthesis of valuable compounds by biofilms on electrodes is intensively studied since few years. However, the actual biofilms growing so far on cathode produce mainly small inexpensive compounds such as acetate or ethanol. A novel Knallgas bacteria, <em>Kyrpidia spormannii</em> have been recently described to grow on cathode in thermophilic and microaerophilic conditions, producing significant amount of PolyHydroxyAlkanoates (PHAs) (Reiner et al., 2018). These PHA are promising sustainable bioplastic polymers with the potential to replace petroleum-derived plastics in a variety of applications. However, the effect of culture conditions and electrode properties on the growth of <em>K. spormannii</em> biofilm and PHA production is still unclear.</p> <p>We present in this study the successful development and operation of autotrophic biocathode whereby the electroactive biofilm was able to grow by utilizing CO<sub>2</sub> and a cathode as the sole carbon and electron source, respectively. We report for the first time, the effect of operating conditions of the Bioelectrochemical system (BES), cathode materials and cathode surface modification on current consumption, biofilm formation, PHA productivity and overall coulombic efficiency of a <em>K. spormannii</em> culture growing on electrodes. In particular, the focus of this study lies on optimization of three main operating conditions, which are the applied cathode potential, pH buffer and the oxygen concentration in the feed gas. Increased biofilm formation and PHA production was observed at an applied potential of -844mV vs. SCE, pH 6.5, O<sub>2</sub> saturation of 2.5%, and for a graphite cathode modified by CO<sub>2</sub> activation. The PHA concentration in the biofilm reached a maximum of ≈40 μg·cm<sup>-2</sup> after optimization. The resultant PHA yield reported after optimization is increased by 12.2 times in comparison to previous results. In conclusion, these findings take microbial electrosynthesis of PHA a step forward towards practical implementation.</p>

The electrosynthesis of valuable compounds by biofilms on electrodes is being intensively studied since few years. However, so far, the actual biofilms growing on cathodes produce mainly small and relatively inexpensive compounds such as acetate or ethanol. Recently, a novel Knallgas bacterium, Kyrpidia spormannii EA-1 has been described to grow on cathodes under thermophilic and microaerophilic conditions, producing significant amounts of PolyHydroxyAlkanoates (PHAs). These PHA are promising sustainable bioplastic polymers with the potential to replace petroleum-derived plastics in a variety of applications. However, the effect of culture conditions and electrode properties on the growth of K. spormannii EA-1 biofilms and PHA production is still unclear. In this study, we report on the optimization of growth and PHA production in liquid culture and on the cathode of a Microbial Electrosynthesis System. Optimization of the preculture allows to obtain high cell density of up to 8.5 Log10 cells·ml-1 in 48h, decreasing the time necessary by a factor of 2.5. With respect to cathodic biofilm formation, this study was focused on the optimization of three main operating parameters, which are the applied cathode potential, buffer pH, and the oxygen concentration in the feed gas. Maximum biofilm formation and PHA production was observed at an applied potential of -844mV vs. SCE, pH 6.5, O2 saturation of 2.5%. The PHA concentration in the biofilm reached a maximum of ≈26.8 μg·cm-2 after optimization, but at 2.9% the coulombic efficiency remains relatively low. We expect that further nutrient limitation will allow the accumulation of more PHA, based on a dense biofilm growth. In conclusion, these findings take microbial electrosynthesis of PHA a step forward towards practical implementation.