Microbial Conversion Mechanism Of Thermophilic Syngas Fermentation By Mixed Culture

Syngas?mainly CO,H₂ and CO₂?can be gasificated from a variety of sources.Syngas fermentation is a promising biotechnology to produce valuable biofuels and biochemicals.Syngas fermentation by mixed culture has attracted more and more attention due to its unique advantages.As the microbial catalyst for the bioconvension of syngas,knowing the microbial conversion mechanism is crucial to enhance the efficiency of syngas fermentation.In this study,CO-,H₂/CO₂-,formate-uitilizing mixed cultures were enriched from cow manure for thermophilic syngas fermentation,and the effect of syngas components and its intermediate on the bioconversion process and associated microbial community were studied;The CO-,H₂/CO₂-,formate-uitilizing enrichments were mixed for thermophilic syngas fermentation,and their bioconversion efficiency and microbial succession process were studied;furthermore,amixedcultureconsistedofMoorellathermoautotrophica,Thermoanaerobacterium thermosaccharolyticum and Moorella perchloratireducens was screened from the above enrichments,the bioconversion of syngas and the metabolic activity of this defined mixed culture also were investigated.The main results and conclusions are as follows:?1?2-bromoethanesulfonate?BES?was added to inhibit methanogenesis during the enrichment process.Using cow manure as an inoculum,CO-,H₂/CO₂-,formate-and syngas-converting enrichments were obtained after 4-5 transfers with CO-,H₂/CO₂-,formate-and syngas as substrate,respectively.In CO-converting enrichments,CO was mainly converted to ethanol and acetate via biological water-gas reaction process and acetogenesis;In H₂/CO₂,formate and syngas-converting enrichments,acetate was the main product via acetogenesis.The number of homoacetogen increased by 1-2 orders of magnitude after all the enrichments,Thermoanaerobacterium,Ruminococcus,Moorella and Clostridium were significantly enriched under CO,H₂/CO₂ and syngas incubation,while Clostridium and Ruminococcus were dominant in the formate-converting enrichment.?2?The results of thermophilic syngas fermentation by mixed CO-,H₂/CO₂-,formate-uitilizing enrichments show that,compared with the acetate concentration in CO-,H₂/CO₂-and formate-enrichments of 9.77,7.48 and 9.24 mmol/L,respectively,the acetate accumulation in mixed CO-formate enrichments increased slightly or keep silimar?9.10 mmol/L?,and that of mixed CO-H₂/CO₂ enrichments and syngas enrichment was higher with the value of 10.36 and10.06 mmol/L,respectively.Biological water-gas reaction process and acetogenesis occurred in mixed CO-formate enrichments with acetate as main products,and acetogenesis dominated gradually in the metabolic pathways with the incubation time.Acetate was mainly generated from syngas via acetogenesis in mixed CO-H₂/CO₂ and syngas enrichment,and the H₂-dependent acetogenesis enhanced gradually.The copy number of fhs gene in mixed CO-formate enrichments maintained at about 10^7.33-10^7.90 copies/mL,but slightly decreased in the mixed CO-H₂/CO₂ enrichments.The succession of microbial community structure in the mixed CO-H₂/CO₂,CO-formate enrichments and syngas enrichment occurred and resulted in a similar microbial composition with Thermoanaerobacterium,Eubacterium and Clostridium as the dominant microorganisms.The microbial composition of CO,H₂/CO₂,and formate enrichment was similar,and the dominant microorganisms were Thermoanaerobacterium,Moorella and Acetonema.?3?A defined mixed culture mainly consisted of Moorella thermoautotrophica,Thermoanaerobacterium thermosaccharolyticum and Moorella perchloratireducens was screened from the H₂/CO₂ enrichments.The results show this mixed culture converted efficiently H₂/CO₂ to acetate with a H₂ and CO₂ conversion rate of 3.88 and 1.75 mmol/L·d,respectively.This mixed culture could not grow on CO,and the presence of CO inhibited its utilization of H₂/CO₂.