Stroek, Gabriëlss and co-workers identify key microbial species driving CO₂ conversion in biofilm-based microbial electrosynthesis (MES) systems that produce acetate, butyrate, and caproate via the Wood–Ljungdahl and reverse β-oxidation pathways. Metagenomic and metaproteomic analyses revealed Eubacterium limosum, Sporomusa sphaeroides, and Clostridium aromativorans as main contributors, with C. aromativorans newly linked to enhanced butyrate and caproate production.