Biocarrier-driven enhancement of caproate production via microbial chain elongation: Linking metabolic redirection and microbiome assembly.

This study investigated the effects of five representative biocarriers-biochar (BC), activated carbon (AC), nano-magnetite (NM), zero-valent iron (ZVI), and polyurethane sponge (PUS)-on chain elongation (CE) from ethanol/acetate in anaerobic systems. All carriers enhanced CE to varying extents. BC and NM significantly increased caproate yields (6032.6 and 5996.6 mg/L) and shortened lag phases (6.22 and 7.18 days) compared with Control (2226.6 mg/L, 11.61 days). In the second cycle, carbon selectivity rose to 58.9% and 58.0% with AC and BC, respectively, versus 18.9% in Control. Biocarrier addition enriched chain-elongating syntrophs, increased community diversity, and elevated predicted fatty acid biosynthesis gene abundance, shifting metabolism toward caproate over reduced byproducts. These findings demonstrate that carbonaceous biocarriers simultaneously alleviate product inhibition, restructure microbial networks, and reprogram metabolic pathways, providing a mechanistic basis for selective biocarrier use in microbiome engineering for medium-chain fatty acid production.