Anaerobic Methane Oxidation And Associated Influencing Mechanisms In The Sediments Of Yangtze Estuary

Estuarine and coastal ecosystems are the critical transitional zone between land and sea,playing an important role in the biogeochemical cycles and ecological service function,and is thus considered as one of critical zones.Methane（CH₄）is an important greenhouse gas with a global-warming potential being 28-34 times of carbon dioxide（CO₂）on a 100-year time scale.Estuarine and coastal ecosystems are also the importance sites for CH₄ production that contributes mostly to atmospheric CH₄ concentration.Therefore,increasing concerns regarding the biogeochemical cycling of methane in estuarine and coastal ecosystems have attracted many attentions for decades.Recently,large amount of nutrients have transported into estuarine and coastal ecosystems,which has leaded to the occurrence of hypoxia as a significant environmental issue.In addition,physicochemical properties and biological variables in these ecosystems generally experience dramatically under the interaction of riverine runoff and tidal current,the changes in environmental properties could potentially affect CH₄ biogeochemical cycling in estuarine and coastal environments.Overall,carrying out CH₄ biogeochemical cycling process of estuarine coastal environment is an international hot topic and frontier science issue.Of the CH₄ removal processes,aerobic oxidation of CH₄,has traditionally been considered the only important mechanism of CH₄ removal.However,the anaerobic oxidation of CH₄ and associated environmental influences have been reported recently,and thus attracting considerable concerns,mainly due to the developments of research techniques and means.Anaerobic oxidation of methane（AOM）via sulfate,iron/manganese and nitrite/nitrate reductions has been identified and considered as a novel microbial process regulating CH₄ emission in these aquatic environments.Therefore,this process has influence on ecosystem environment through alleviating the greenhouse effect by converting CH₄ to CO₂.To date,AOM has been discovered in various freshwater wetland and lake environments.However,the occurrence of AOM,environmental variables and environmental implication remain unclear in estuarine and coastal ecosystems.Thus,the Yangtze estuary was selected as a typical study area to study the spatiotemporal differentiation of AOM processes,environment variables impacts control and microbial influence mechanism.In addition,the status and ecological environment effect of AOM in the estuarine environment were identified.The present work can provide insights into biogeochemical cycling of CH₄ in estuarine and coastal environments,and it can also help to propose theoretical basis for evaluating CH₄ emission accurately.The main findings are as following.（1）The salinity was the key variable regulating the microbial biomass carbon and dissolved organic carbon in the Yangtze Estuary intertidal sediment.The significantly spatial and temporal variations in methane production were observed.Specifically,methane production rates were significantly higher in high salinity sites than in low salinity sites,and higher in summer than in winter.The sulfate reduction coupled to methane oxidation（termed as SAMO）rates were significantly higher in nitrate reduction coupled to methane oxidation（termed as NAMO）.The NAMO rates were significantly higher in summer than in winter,and significantly higher in high salinity sites than in low salinity sites.The SAMO rates were significantly higher in summer than in winter,and significantly higher in low salinity sites than in high salinity sites.The NAMO and SAMO contributed 1.01-11.09%and 28.7-58.9%of total methane remival amount,respectively.MBC,DOC and S2-were the main factors affecting methane production,NO3-,S2-and SO42-were the key variables regulating NAMO,and MBC,DOC,S2’,and SO42-had great influence on SAMO.The methan production was affected by Gram-positive bacteria,total bacteria,arbuscular mycorrhizal fungi and total biomass.The NAMO was influenced by ratio of bacteria/fungi,and arbuscular mycorrhizal fungi and total biomass had significant influences on SAMO.（2）Tidal cycling process had great influence on sediment physicochemical properties.The redox potential decreased with the tidal inundation,which regulates the reductions of iron and sulfate.The concentrations of ferrous iron and sulphide were higher due to the high redox potential in low intertial flat.The production and oxidation of methane were affected obviously by tidal processes.The concentrations in methane of sediment and porewater changed with tidal inundation,and flood tide accelerate the production and oxidation of methane.Overall,the tidal cycling was the key variable regulating methane emission.The abundances of methanogens and methane-oxidizing bacteria were higher in deep sediments,while these of sulfate and iron reducing bacteria were higher in superficial sediment.（3）The NAMO process was detected in the Yangtze estuary sediments,with potential rates of 0.06-0.39 nmol CO₂ g-1 d-1 dry sediment based on isotope tracer technique.The measured NAMO rates decreased from inshore sites to offshore sites.Clone library analyses of M.oxyfera-like bacterial 16S rRNA and pmoA genes indicated the occurrence of the NAMO process in the estuarine and coastal sediments.Most 16S rRNA gene sequences of M.oxyfera-like bacteria were affiliated with group B,and a few detected sequences were affiliated with group A.Saility was the key property regulating changes in community composition from inshore sites to offshore sites.The group B of M.oxyfera-like bacteria may have been responsible for the NAMO process in the study area The estimated NAMO rates and M.oxyfera-like bacterial abundance were higher at the inshore sites than at the offshore sites.Sediment geochemical variables affected M.oxyfera-like bacteria,and thus mediated the activity of NAMO and associated CH₄ removal.It is estimated that a loss of approximately 0.12 Gg CH₄ yr-1 was linked to the NAMO process,accounting for 1.2-6.9%of total annual CH₄ emission from the sediments in the study area.On the basis of theoretical ratio of 5 or 3 moles of CH₄ oxidized by 8 mole of NO3-/NO2-reduction,respectively,approximately 0.17-0.28 Gg yr-1 of nitrogen loss could be attributed to NAMO,accounting for 0.11-0.19%of total nitrogen removal（sum of denitrification and anammox）from the study area.Overall,these results indicate that the NAMO process is of significance in regulating CH₄ sink,and highlight the.importance of the geochemical gradients in shaping the NAMO process in estuarine and coastal ecosystems.（4）The anaerobic methane oxidation（AMO）rates decreased with the decreasing in substrate methane in the study area.The AMO rates in anaerobic and un-anaerobic sediments ranged from 7.38 to 10.29 nmol g-1 d-1 and 2.84 from to 2.93 nmol g-1 d-1.The anaerobic process can promote AMO of sediments.The community composition of methanogens and methane-oxidizing bacteria differed between anaerobic and un-anaerobic sediments.Methanogens in anaerobic sediments were dominated by Methanosarcinales,Methanococcales and Methanohalophius,while Methanobacteriales,Methanococcoides and Methanococcales were the main communities in un-anaerobic sediments.The methane-oxidizing bacteria in anaerobic sediments were Methylomicrobium,Methylosphaera and Methylocaldum,while Methylocella and Methylocapsa in un-anaerobic sediments.The DO was the key variable regulating the AMO process.The abundances of mcrA、pmoA、dsrB、Geo.and She.were affected significantly by sediment TOC,sediment sulphide had great influence on dsrB.In addition,abundances of mcrA、pmoA、dsrB、Geo.and She.were related significantly with AMO rates.