Changes Of Microbial Community Structure During Anaerobic Fermentation Of Food Waste

Efficient disposal of food waste is an important challenge for the research and application of organic solid waste recycling.Anaerobic digestion can not only make harmless treatment of food waste,but also generate methane.However,there are many problems in the anaerobic digestion process,such as poor operation and low methane production efficiency due to the unclear microbial mechanism.Characteristics on the changes of microbial community structure and functional genes and the relation between microbial community and environmental factors during the anaerobic digestion of food waste is critical for the regulation of anaerobic digestion of food waste and efficient methane production.We collected food waste as the substrate and activated sludge from biogas digester as the inoculum to simulate the anaerobic fermentation process in sequencing batch reactors.Through the monitoring of physical and chemical indicators and the observation by scanning electron microscope,the reaction and the morphology of substrates before and after anaerobic fermentation were characterized.The samples from different stages were analyzed by molecular biology technologies（qPCR,RT-PCR and metagenomics）.In addition,the effect of acetic acid on the lower gas production stage（IIB）and higher gas production stage（IIC）of food waste were explored,with emphasis on the effect of acetic acid on methanogens.The main conclusions are as follows:（1）Although the reaction of the food waste fermentation is not stable,the system still has a certain self-recovery ability.The specific surface area of food waste increased after anaerobic fermentation so that the microorganisms attached to the surface.The results of quantitative analysis showed that the expression of mcrA gene suited for methane production indication.（2）The results of metagenomics technology showed that the concentration of acetic acid had no affect the variety of microorganism,but had a great impact on the abundance distribution in the fermentation process.Proteobacteria is the most abundant phyla in stage IA and IC,accounting for 26.2%and 39.1%respectively,and Firmicutes is the most abundant phyla in stage IB,accounting for 49.2%;The dominant populations of archaea at different stages of fermentation are Methanosaeta（55.9%,19.3%,and 21.5%）.（3）The abundance of the key enzyme genes of glycolytic pathway is IB（12.7）>IC（11.4）>IA（10.3）,which indicates that the higher the gas production,the higher the ATP demand of anaerobic microorganisms.The abundance of related enzyme genes in the the acetic methanogenic pathway is much greater than that of H₂/CO₂ methanogenic pathway in the methane metabolism pathway,suggesting that acetoclastic methanogens are the main contributors.The lipid metabolism pathway lacks EC:1.3.99.3,a key enzyme gene,suggesting that the incomplete lipid metabolism pathway lead to destabilize in the reaction of late stage.In sulfur metabolism,the biggest abundance of the key enzyme genes is IA（15.8）,indicating that the stronger the sulfur metabolism,the inhibition of the expression of methanogens more obvious.（4）The results of quantitative analysis showed that the activity of methanogens increased gradually with the reaction,indicating that the fermentation process was in a relatively stable inhibition state,and the concentration of acetic acid had a certain promotion effect on the activity of methanogens.（5）The results of high-throughput sequencing showed that the addition of acetic acid had a great impact on the number of microbial communities in different fermentation stages.Among them,the dominant populations of archaea at stage IIB and IIC are Methanosaeta（74.0%,3.3%）and Methanosarcina（0.03%,48.0%）.It can be concluded that the addition of acetic acid had an significant effect on the abundance of acetoclastic methanogens,which may be the reason for the low gas production efficiency in the initial stage of the reaction.