The Effects Of Temperature On Methanogenic Metabolic Pathways And Microbial Community During The Processes Of Anaerobic Digestion

Since the Industrial Revolution,with the continuous enhancement of human activities,the concentration of greenhouse gases such as CO₂ and CH₄ in the atmosphere has increased,which has intensified the occurrence of global warming and extreme climate events.Among them,methane gas is the second most important greenhouse gas in global warming and has a profound impact on climate change.Methane gas emissions mainly come from agricultural production activities,organic wastes and energy utilization.Due to unreasonable stacking and landfill of organic wastes generated by human life,methane gas generated is directly discharged into the atmosphere,and the carbon emissions generated contribute to climate warming cannot be ignored.In addition,the organic pollution caused by organic waste in people's daily life is also one of the environmental problems need to be solved.Therefore,it is of great significance to study the conversion difference and generation mechanism of methane in different environments for solving the environmental pollution problem caused by organic wastes,quantitatively estimating methane emissions,controlling greenhouse gas emissions and slowing down the greenhouse effect.In this experiment,the stable carbon isotope ratio analysis method was used to reveal the evolution of methane generation and metabolism pathways,and the microbial molecular technology was used to monitor the changes of microbial community structure during methane generation and metabolism.Finally,the intrinsic relationship between the metabolic pathway of methane generation and the changes of microbial communities in the metabolic process is analyzed to clarify the metabolic mechanism of methane generation.As temperature is an important parameter that affects the degradation of organic matter,this study discussed the effect of temperature on methane production from organic wastes produced in human life by setting different temperature conditions.The main research conclusions are as follows:1.Under different environmental conditions,the methane conversion rate of organic wastes generated by human life is significantly different when methane is generated.The anaerobic digestion experiments of vegetable wastes and kitchen wastes are carried out by controlling the temperatures at 25?,35?and 40?,respectively.The gas production characteristics of organic matter degradation in anaerobic environment under the research temperature are as follows:vegetable wastes and kitchen wastes are two raw materials at 35?.Its total gas production,TS/VS gas production rate,average CH₄ content in stable period and total CH₄amount are all higher than those of the experimental group under the conditions of25?and 40?,which shows that when anaerobic digestion degrades organic matter,the gas production effect is better when the temperature is set to 35?.Under the same temperature condition,comparing the gas production potential of vegetable waste and kitchen waste,it is found that each index in the gas production process of kitchen waste fermentation is higher than that of vegetable waste,and the gas production effect is better.In addition,it was found that the volatile fatty acid?VFA?in the system was always at a high level and the daily gas production was low when the anaerobic digestion experiment was carried out with kitchen waste as raw material and the temperature was controlled at 40?.2.Through the study of carbon isotope in the gas when two types of organic wastes produce methane in different environments,it is found that the flow change of carbon isotope in the methane production process leads to the change of contribution of different methane metabolic pathways to the total methane production,and the contribution of methane metabolic pathways to the total methane production is also changing at different anaerobic digestion stages in the same system.The main manifestations are as follows:when vegetable wastes are used as raw materials,CO₂reduction pathway accounts for the largest proportion of methane production metabolic pathway in the early stage of fermentation,and its contribution to the total methane production is 50.82%,35.69%and 84.66%respectively at 25?,35?and40?.When kitchen waste is used as fermentation raw material,the contribution of CO₂ reduction pathway to the stable methane production phase of the system is greater.The contribution of CO₂ reduction pathway to the total methane production is46.12%,30.40%and 36.89%respectively under various temperature conditions,and the contribution rate is always less than 50%in the whole fermentation process.The contribution of CO₂ reduction pathway to the total amount of methane production in the anaerobic digestion process of each experimental group is 84.66%at the early stage of fermentation and less than 50%at the early stage of fermentation except when the vegetable waste is used as the raw material,which indicates that the acetic acid cracking pathway is the main pathway in the methane production process of each experimental group.3.In order to further study the methane conversion mechanism of organic wastes in the methane generation process,the diversity composition of microbial community structure in the system was studied.It was found that:?1?The composition of microbial community structure changed with the change of temperature,and the greater the temperature change,the greater the diversity difference of microbial community.?2?The dominant bacteria in each experimental group are mainly Bacteroidales,Methanosarcinales and Clostridiales,and the relative abundance is more than 10%in different fermentation stages,and the temperature change has little effect on the main dominant bacteria;The sub-dominant bacteria in each experimental group are mainly Synergistales,Pseudomonadales,Spirochaetales,Selenomonadales,Enterobacteriales,Burkholderiales and so on,and the change of temperature has a great influence on the sub-dominant bacteria.?3?Methanosarcinales and Methanomicrobiales were detected in each experimental group,of which Methanosarcinales was the main methanogenic archaea.Methanobacterium,Methanobrevibacter,Methanomassiliicoccus,Methanoculleus and Methanosarcina,which are all detected at the genus level,account for more than10%and are the main dominant bacteria.Methanobacteriales were detected at 25?in the experiment using kitchen waste as raw material,but not in other experimental groups.In the experiment using kitchen waste as raw material,the seventh methanogenous bacteria was detected at 40?,but not in other experimental groups.4.Changes in microbial community structure will affect daily gas production,methane content and carbon isotope fractionation in the system.The richer the diversity of microbial community structure,the greater the daily gas production and the lower the methane content.The change of temperature affects the gas production,methane production metabolic pathway and diversity of microbial community structure in the system.However,the gas production,methane production metabolic pathway and microbial community structure do not show significant regular changes with the change of temperature within the temperature range set by the research institute,because the temperature range set by the research institute is small and all are within the medium temperature range.In the same system,the relative abundance of microbial species changed with the extension of fermentation time,and the characteristics of gas production and methane metabolic pathway showed regular succession with the extension of fermentation time.