Research On Carbon Metabolism Mechanism Of Ethanol Methanogenesis In Anaerobic Process

Anaerobic digestion is crucial for bio-energy recovery and organic waste management,which has already been widely applied to treat different types of wastes such as high strength wastewater,food waste,and manure,etc.In the anaerobic process,carbon metabolism in the ethanol methanogenesis process has always been the focus of research,and sludge retention times(SRT)is a key parameter affecting the performance of anaerobic digestion.In this study,acetate and ethanol were used as carbon sources,and SRT was set to 10 d and 25 d as the entry point.Three sets of anaerobic sequencing batch reactors(SBRs)were operated to explore the shifts in system performance,microbial population,metabolic pathway,and energy metabolism.The high-throughput 16s sequencing results show that when acetate(SBR_25-A)and ethanol(SBR_25-E)as carbon sources,and SRT of 25 days,methanogens Methanosarcina and Methanosaeta could be domesticated.When ethanol was used as a carbon source under two different SRTs,25 days(SBR_25-E)and 10 days(SBR_10-E),different ethanol-metabolizing bacteria and methanogens were observed.Ethanol was oxidized to acetate and propionate at SRT of 25 days due to the enrichment of ethanol oxidizers Geobacter and Pelobacter,propionate oxidizers Smithella,and methanogens Methanosaeta.However,ethanol was metabolized to only acetate by Desulfovibrio who outcompeted other ethanol oxidizers and established interspecies H₂transfer with H₂-utilizing methanogen Methanobacterium at SRT of 10 days.Acetoclastic methanogens were eliminated,resulting in the accumulation of acetate and low CH₄yield.Metagenomics results show that in SBR_25-A,Methanosarcina was the main bacterium involved in the hydrogenotrophic(CO₂reduction pathway)and aceticlastic pathways of methanogenesis.SBR_25-Eincluded conductive bacteria Geobacter and the obligate acetate-splitting methanogen Methanosaeta.The latter carried genes related to the hydrogen trophic methanogenesis pathway,suggesting ethanol methanogenesis partly performed by direct interspecies electron transfer(DIET)from Geobacter to Methanosaeta.Lastly,in SBR_10-E,the alcohol-degrading sulfate-reducing Desulfovibrio was detected,which may produce methane through interspecies hydrogen transmission(IHT)with the hydrogenotrophic methanogen Methanobacterium.Additionally,the major ATPase-gene owners were shifted from bacteria(F-type ATPase)to methanogens(V/A-type ATPase)with decreasing SRT and pH,suggesting the energy-metabolism turnover of anaerobic microbes when encountering the disadvantage conditions.