Steering the Chain-Elongating Microbiome to C8 or C6 Production with Ethanol and Lactate as Coelectron Donors

Microbial chain elongation is a sustainable process to convert organic residues into valuable biochemicals via anaerobic fermentation. The operating conditions for the bioreactor and the ecological interactions among functional populations are crucial in controlling this process, but they have not been completely ascertained. Here, we studied two operating conditions (i.e., environmental factors): (1) the substrate ratio of ethanol and lactate as coelectron donors, and (2) the temperature, which affected both the product specificity (i.e., function) and microbial dynamics of chain-elongating microbiomes in a continuously fed bioreactor with product extraction that is the third pertinent operating condition. Specifically, we found that the increase in the substrate ratio of ethanol to lactate shifted the microbiomes toward n-caprylate (C8) production, while the slightly higher operating temperatures of 37 or 42 °C were advantageous to n-caproate (C6) production. We detected a core microbiome that was similar for all operating conditions and the two bioreactors, consisting of populations from Sphaerochaeta spp., Caproiciproducens spp., and Oscillospiraceae. Besides the core microbiome, we observed positive correlations between Erysipelaclostridiaceae UCG-004, Bacteroides spp., Oscillospiraceae NK4A214, Rikenellaceae RC9, and Pseudoclavibacter spp. with n-caprylate production in a dormant microbiome. Similar populations compared to the core microbiome were positively correlated with n-caproate production for both bioreactors, because n-caproate must first be produced before conversion into n-caprylate. The results showed that we can steer microbiomes toward a high specificity of certain medium-chain carboxylates.