Shaping microbial consortia in coupling glycerol fermentation and carboxylate chain elongation for Co-production of 1,3-propanediol and caproate: Pathways and mechanisms

Glycerol is presently being generated in surplus with the rapid growth of the biodiesel industry and seeks ways to be upcycled, rather than to be treated with costs. Glycerol for the co-production of 1,3-propanediol (1,3-PDO) and caproate has a great prospect. Yet, its technical difficulty lies in the enhancement of caproate productivity, which requires the presence of ethanol as a co-substrate and necessitates the co-existence of functional microbes for glycerol fermentation and chain elongation. This study successfully achieved 6.38 mM C 1,3-PDO d^-1 and 2.95 mM C caproate d^-1 in a 2-L mixed-cultured semi-continuous fermenter with a glycerol-ethanol-acetate stoichiometric ratio of 4:3:1. Such conversions were mainly facilitated by a microbial community of Eubacterium limosum, Clostridium kluyveri and Massilibacterium senegalense. With such a synergistic microbiome, the co-production of 1,3-PDO and caproate was achieved from glycerol without ethanol addition. Based on metagenomics, E. limosum is capable of converting glycerol to 1,3-PDO, ethanol and H₂, and also redirecting the electron potential of H₂ into acetate via the Wood-Ljungdahl pathway, which is then used for chain elongation. C. kluyveri worked synergistically with E. limosum by consuming ethanol and acetate for caproate production. M. senegalense encodes for ethanol oxidation to acetate and butyrate, facilitating the generation of these intermediates for C. kluyveri elongation to caproate. During the transition between fermentation and elongation, an unexpected observation of poly-β-hydroxybutyrate (PHB) formation and reutilization by M. senegalense may be associated with butyrate formation for further caproate generation. The knowledge gleaned from the substrate constitute, microbial consortium and their synergetic metabolism demonstrates a resource upgrade potential for crude glycerol or glycerol-containing wastewater generated from the biodiesel industry.