Low-temperature caproate production, microbial diversity, and metabolic pathway in xylose anaerobic fermentation

Mixed culture fermentation (MCF) is challenged by the unqualified activity of enriched bacteria and unwanted methane dissolution under low temperatures. In this work, caproate production from xylose was investigated by MCF at a low temperature (20 degrees C). The results showed that a 9 d long hydraulic retention time (HRT) in a continuously stirred tank reactor was necessary for caproate production (similar to 0.3 g/L, equal to 0.6 g COD/L) from xylose (10 g/L). The caproate concentration in the batch mode was further increased to 1.6 g/L. However, changing the substrate to ethanol did not promote caproate production, resulting in similar to 1.0 g/L after 45 d of operation. Four genera, Bifidobacterium, Caproiciproducens, Actinomyces, and Clostridium_sensu_stricto_12, were identified as the enriched caproate-producing bacteria. The enzymes in the fatty acid biosynthesis (FAB) pathway for caproate production were identified via metagenomic analysis. The enzymes for the conversion of (Cn+2)-2,3-Dehydroxyacyl-CoA to (Cn+2)-Acyl-CoA (i.e., EC 1.3.1.8 and EC 1.3.1.38) in the reverse beta-oxidation (RBO) pathway were not identified. These results could extend the understanding of low-temperature caproate production. (C) Higher Education Press 2023

Automated summary

This study investigated the production of caproate from xylose through mixed culture fermentation at a low temperature. Enriched bacteria and enzymes involved in caproate production were identified. These findings contribute to a better understanding of low-temperature caproate production.