Significant enhancement by casamino acids of caproate production via chain elongation

In this study, the effects of casamino acids, as the main component in yeast extract, on the bioproduction of caproate were evaluated. A dosage of 5 g/L casamino acids addition significantly promoted caproate production and shortened the lag phase for caproate production by 7.6-folds compared with the control. Both the net carbon and electron conversion efficiencies were enhanced. The addition of casamino acids shaped a dissimilar microbial community from that of the control and that of the inoculum. The caproate-producing Clostridium and Caproiciproducens, the proteolytic Proteiniphilum and Brassicibacter, and the saccharide-utilizing Hydrogenoanaerobacteriu were enriched, and their abundance were distinguished between with and without the addition of casamino acids as fermentation progressed. Further metagenomics analysis recovered genome bins of Clostridium kluyveri, Clostridium cochlearium, Caprobacter fermentans and Proteiniphilum acetatigenes, representing the majority of the functional bacteria with casamino acids addition. The complete reverse β-oxidation and amino acids (serine/glycine) metabolic pathway for caproate production and butyrate formation, respectively, were fully recovered. Furthermore, all the genes involved were highly expressed and metabolically active in meta-transcriptomics analysis. The finding proved that casamino acids was able to enhance caproate formation by promoting ethanol utilization and butyrate formation. In general, yeast extract could not only support microbial growth but also contribute to the formation of medium chain fatty acids, therefore promoting the efficiency of chain elongation process.