N-(3-oxohexanoyl)-homoserine lactone-assisted enrichment reshapes functional microbial consortia for chain elongation...

The functional microbial consortia supporting chain elongation determine medium-chain carboxylate recovery from organic wastes, but how signal-molecule-assisted enrichment shapes chain-elongating bacteria (CEB), electroactive bacteria (EAB), and competing guilds in electrofermentation remains unclear. Here, three N-acyl-homoserine lactones: N-butyryl-homoserine lactone (C4-HSL), N-octanoyl-homoserine lactone (C8-HSL), and N-(3-oxohexanoyl)-homoserine lactone (3OC6-HSL), were supplied during microbial enrichment, and the subsequent electrofermentation was conducted fed with sludge fermentation broth. Compared with the Control (without signaling molecules), 3OC6-HSL had the strongest response, increasing caproate production by 94.0%, compared with 16.9% and 27.3% for C4-HSL and C8-HSL, respectively. It also increased the apparent caproate electron transfer efficiency by 20.7 percentage points, increased the abundance of CEB (44.9% vs. 33.2%) and EAB (14.3% vs. 6.6%), and reduced the abundance of homoacetogens (12.1% vs. 33.7%). Co-occurrence network analysis revealed more modular and compact inferred associations, with 25.0% more modules and a 34.7-49.3% shorter average path length. Metagenomic analysis revealed enhanced reverse β-oxidation, QS, chemotaxis, and flagellar assembly potentials, and the expression levels of acetyl-CoA acyltransferase (ACAT/fadA) and acyl-CoA dehydrogenase (ACADS/ACADM) increased by 162.1% and 96.6%, respectively. Clostridium kluyveri dominated the ACAT contribution (85.9%). Overall, enrichment-phase 3OC6-HSL supplementation was associated with a caproate-oriented microbial consortium and improved caproate recovery without continuous signal dosing.