Abstract
A unique microbiome that metabolizes lactate rather than ethanol for n-caproate production was obtained from a fermentation pit used for the production of Chinese strong-flavour liquor (CSFL). The microbiome was able to produce n-caproate at concentrations as high as 23.41 g/L at a maximum rate of 2.97 g/L/d in batch trials without in-line extraction. Compared with previous work using ethanol as the electron donor, the n-caproate concentration increased by 82.89%. High-throughput sequencing analysis showed that the microbiome was dominated by a Clostridium cluster IV, which accounted for 79.07% of total reads. A new process for n-caproate production was proposed, lactate oxidation coupled to chain elongation, which revealed new insight into the well-studied lactate conversion and carbon chain elongation. In addition, these findings indicated a new synthesis mechanism of n-caproate in CSFL. We believe that this efficient process will provide a promising opportunity for the innovation of waste recovery as well as for n-caproate biosynthesis.
Highlights
Have focused on the ethanol conversion pathway[2,3,4,5,6,7,8,9]
Phylogenetic analysis based on 16S rRNA gene showed that most dominant OTUs of the unique microbiome were more closely related to the species of the Clostridium cluster IV than other lactate-utilizing bacteria, e.g., the species of Clostridium cluster XIVa (Fig. S6)
Individual species classified in Clostridium cluster IV has been reported to be able to produce n-caproate from saccharides[17], but none of the species classified in Clostridium cluster IV or XIVa has been found to synthesize n-caproate from lactate
Summary
Have focused on the ethanol conversion pathway[2,3,4,5,6,7,8,9]. In a typical example of the new anaerobic digestion process for n-caproate recovery, municipal solid waste is converted to n-caproate through the addition of ethanol to the upflow anaerobic filter[3,7,8,9]. The fermentation pit used for Chinese strong-flavour liquor (CSFL) production is a unique artificial environment for n-caproate production that contains up to 20.0 g/kg (dry pit mud) n-caproate[20]. Because liquor fermentation is an ethanol-producing process, it is generally believed that the n-caproate in CSFL is synthesized from ethanol[21]. In our previous study, we observed that the significant increase in n-caproate production was accompanied by a continuous decline in lactate during the development of CSFL fermentation[20]. The present study found that lactate can serve as a sole carbon and energy source for n-caproate production by a unique microbiome obtained from the pits, whereas a small amount of n-caproate was produced when the microbiome was fed ethanol. Possible processes for the remarkably high n-caproate production by the unique microbiome from lactate were discussed
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