Linking structure to function in high performing electrosynthetic biofilm communities

Biofilm-based microbial electrosynthesis (MES) is a promising technology that converts CO2 into industrially relevant organic compounds using renewable energy sources. The highest performing MES systems reported to date consist of biofilm-driven microbial communities. However, for a successful deployment of this technology, we still need to overcome key challenges, including the long colonization time of the biocathode and the difficulty of controlling the product profile. To address these challenges, it is crucial to better understand the key microbial components responsible for the desired metabolic products, and how they assemble and function as a community. In this study, we conducted an in-depth characterisation of three high-performing mixed MES communities using metagenomics, metaproteomics and advanced analysis of metagenome-derived metabolic networks. Our findings identified Eubacterium limosum, Sporomusa sphaeroides and Clostridium aromativorans as key contributors to the production of acetate, butyrate and caproate via the Wood-Ljungdahl and the reverse {beta}-oxidation pathways. A higher production of butyrate and caproate was observed in reactors with higher abundance of C. aromativorans, an organism only recently discovered and never reported in gas-fermenting systems before. The recovered genome of C. aromativorans, reconstructed in a single circular fragment, provides a more comprehensive genomic representation than the current reference. In addition, we found genes related to lactate and ethanol production from acetyl-CoA in the metagenomes, including proteomic evidence for lactate production. This study provides key insights into the microbial players, metabolic processes and microbial community features driving product formation in biofilm-based MES, bringing this technology closer to industrial application.