Electrophysiology of the Facultative Autotrophic Bacterium Desulfosporosinus orientis

Electroautotrophy is a novel and fascinating microbial metabolism, with tremendous potential for CO 2 storage and valorization into chemicals and materials made thereof. Research attention has been devoted toward the characterization of acetogenic and methanogenic electroautotrophs. In contrast, here we characterize the electrophysiology of a sulfate-reducing bacterium, Desulfosporosinus orientis , harboring the Wood-Ljungdahl pathway and, thus, capable of fixing CO 2 into acetyl-CoA. For most electroautotrophs the mode of electron uptake is still not fully clarified. Our electrochemical experiments at different polarization conditions and Fe 0 corrosion tests point to a H 2 - mediated electron uptake ability of this strain. This observation is in line with the lack of outer membrane and periplasmic multi-heme c -type cytochromes in this bacterium. Maximum planktonic biomass production and a maximum sulfate reduction rate of 2 ± 0.4 mM day –1 were obtained with an applied cathode potential of −900 mV vs. Ag/AgCl, resulting in an electron recovery in sulfate reduction of 37 ± 1.4%. Anaerobic sulfate respiration is more thermodynamically favorable than acetogenesis. Nevertheless, D. orientis strains adapted to sulfate-limiting conditions, could be tuned to electrosynthetic production of up to 8 mM of acetate, which compares well with other electroacetogens. The yield per biomass was very similar to H 2 /CO 2 based acetogenesis. Acetate bioelectrosynthesis was confirmed through stable isotope labeling experiments with Na-H 13 CO 3 . Our results highlight a great influence of the CO 2 feeding strategy and start-up H 2 level in the catholyte on planktonic biomass growth and acetate production. In serum bottles experiments, D. orientis also generated butyrate, which makes D. orientis even more attractive for bioelectrosynthesis application. A further optimization of these physiological pathways is needed to obtain electrosynthetic butyrate production in D. orientis biocathodes. This study expands the diversity of facultative autotrophs able to perform H 2 -mediated extracellular electron uptake in Bioelectrochemical Systems (BES). We characterized a sulfate-reducing and acetogenic bacterium, D. orientis , able to naturally produce acetate and butyrate from CO 2 and H 2 . For any future bioprocess, the exploitation of planktonic growing electroautotrophs with H 2 -mediated electron uptake would allow for a better use of the entire liquid volume of the cathodic re