Deciphering mixotrophic microbial electrosynthesis with shifting product spectrum by genome-centric metagenomics

Microbial electrosynthesis (MES) is a promising platform for renewable energy storage, carbon fixing, and chemicals production. However, the generation of longer chain carboxylates of which their values are higher than acetate remains a grand challenge in traditional autotrophic MES feeding CO2 as the sole carbon source. The present study describes the employment of mixotrophic MES as an important strategy for the production of chemicals with longer carbon chain. With pre-enriched acetogens, the caproate (C6) specificity was increased from 0 % (autotrophic MES) to 12.8 +/- 1.5 % (mixotrophic MES). Without pre-enriched acetogens, no caproate was detected, while the propionate (C3) specificity was increased from 17.1 +/- 7.8 % (autotrophic MES) to 40.1 +/- 0.1 % (mixotrophic MES). Metagenomic sequencing indicated that this strategy increased the abundance of functional microbes capable of chain elongation via the fatty acid biosynthesis (FAB), although the mixotrophic condition slightly decreased the carbon metabolic pathways via Wood-Ljungdahl (WL) pathways and reverse beta-oxidation (RBO). Genome-centric analysis located 60 functional microbes who possessed WL and/or FAB pathways, and 27 of them were significantly enriched in mixotrophic conditions (p < 0.001). In addition, compared with the abundance of these functional microbes, it was proposed that the pre-enriched acetogens were more critical than that of mixotrophic conditions for a shift in product spectrum. These results demonstrate the great potential of mixotrophic MES, as a novel renewable-electricity-driven technology, to produce long -chain compounds from the simultaneous valorization of CO2 and organic waste.