Methane Cycle Process And Driven Mechanism In The Subtropical Subalpine Peat

Peatlands store a large amount of carbon and play an important role in the global carbon cycle,which just occupy about 3% of terrestrial aeras with a high carbon storage rate.Based on the “storaged carbon” and storaged water” ecosystem characteristics,the peat might contribute a potential threat to the global geochemical cycle,especially carbon cycle-methane cycle as the “huge carbon pool” with unbalance state.Owing to the presence of waterlogged and anaerobic conditions,peats are typically a source of methane,a very potent greenhouse gas.Considering the global serious greenhouse effect,the process of methane cycle in the peatlands should be considered as the spark issue in the peat study.Based on scientific target of methane cycle process,the subtropical subalpine peat comes to be an important setting.Different from high latitude polar peat and low latitude equatorial peat,middle latitude Dajiuhu peat in the middle reaches of Yangtze River with special geographical and climatic features might hide an unique and special process of carbon cycle and methane cycle.In the study,the author focused on microbial biomarkers combined with functional nucleic acid analysis as the technical methods,combined with modern process monitoring experiments and geological sedimentary drilling to reconstruct the paleoclimate and paleoenvironment process,in order to reveal the methane metabolism cycle process and its related driving mechanism from the modern process to geological history period.The specific progresses were as follows:Ecological function of peatlands in carbon stotage is highly connected with microbiomes,which are affected by various environmental variables.Despite of confirmed role of water table in the construction of microbial communities in peatlands,its impact on microbial functional groups involved in methane production and consumption still remained poorly dedined particularly at subtropical peatlands to date.To this end,prokaryotic communities and methane related functional groups from two peat profiles were investigated in the Dajiuhu Peatland,a subtropical peatland,Central China via high through put sequencing,quantitative PCR and clone library construction.Results showed that the hydrological condition(depth to water table,DWT)significantly shaped the vertical distribution of prokaryotic communities(dominated by Alphaproteobacteria,Acidobacteria and Bathyarchaeota)and methane functional groups.Specifically,the relative abundances of anaerobes/anaerobic functions significantly decreased with DWT,whereas those of aerobes/aerobic functions increased with DWT along the profiles.Moreover,DWT also significantly correlated with the absolute abundances of methanogens and methanotrophs negatively and positively as confirmed by quantification of functional genes.Structurally,methane functional groups showed a clear vertical distribution pattern.In upper samples along the profiles,methanogens were dominated by Methanoregulaceae,fen cluster and Methanocellales,and methanotrophs were mainly Methylocystis.On the contrast,methanogens in lower samples were dominated by fen cluster,rice cluster I and related rice cluster I and methanotrophs Methylispira.The co-occurrence network prokaryotic communities showed a well modularity,and interestingly each individual module was rather composed of aerobic OTUs or anaerobic OTUs,indicating a strong oxygendependent niche preference.RDA results confirmed that DWT significantly constructed total microbiomes and methanogenic communities,whereas methane concentration solely controlled methanotrophic communities.Collectively,these observations demonstrated important ecological regulation of DWT on total microbiomes and methane functional groups in the peatland,which expanded our understanding of peat methane cycle process in the context of environmental changes.Considering the correlationship between hydrological conditions and peat methane cycle in the subtropical Dajiuhu Peatland as above,how to reconstruct the hydrological condition during the geological periods should be an important issue for us to understand geological methane cycle in the past.Thus,the hopanoid producers might be a potential choice for hydrological index in the past.Hopanoids are bacteria membrane lipid biomarkers that are found ubiquitously across many diverse modern environments as well as in fossil records.To investigate the niche,composition and distribution of hopanoid producers,here we took the abundance of hopanoid producer members across different ecosystems and their correlation with environmental factors.Results indicated that acidic Dajiuhu Peatland harbored main Acidobacteria(59.16%)of hopanoid producers.However,main compositions of hopanoid producer groups were obviously different with that in the peatland,such as Alphaproteobacteria in soil(37.78%),cave(48.21%),hypersaline lagoon(34.04%)and marine(32.59%),and Beta-,Gamma-and Delta-Proteobacteria in reefs(100%),acid mine drainage(55.00%)and estuary(39.66%)respectively,and unknown groups in freshwater(29.43%)and hot spring(89.58%).Contrasting with other phyla or sub-phyla,Alpha-,Beta-and GammaProteobacteria were most widespread to 8 ecosystems.Statistical analysis indicated p H,DO,salinity and TOC had significant impacts on hopanoid producers across different ecosystems.Such results will be helpful to understand the structure and distribution of hopanoid producers and reponse to multiple environmental factors across different ecosystems.It might shed a new light on a stable and potential hydrological index based on hopanoids for reconstruction of paleohydrology in the Dajiuhu Peatland.According to the previous modern process,it has been proved that DWT value can significantly control the methane cycle in the Dajiuhu Peatland.Based on such proof,the auther attempted to explore how the methane metabolic cycle changed in the geological history period.Thus,ZK3 samples in the Dajiuhu Peatland across whole Holocene have been finished.The final results showed that: since the Holocene 13 kyr BP,the methane production process of Dajiuhu Peatland showed a two-stage model,which is basically consistent with the deposition trend of Dajiuhu Peatland.The Archaeol Flux,GDGT-0 Flux and mcr A quantificative abundance were significantly correlated with the Sedimentary Rate(p < 0.01),while the hydrological indexes and temperature curve have no stable and significantly correlation with methanogenesis indexes.Therefore,auther speculated that the Sedimentary Rate in the Dajiuhu Peatland had driven methanogenesis process rather than hydrology or temperature.Compared with methanogenesis,methanotrophy was obviously controlled by hydrological condition,and the pmo A quantificative abundance was significantly correlated with DWT(r = 0.81,p = 0.014);however,temperature and deposition rate had no significant correlation with methanotrophy.Thus,since the 13 kyr BP,the Sedimentary Rate and water table controlled microbial mediating methane cycle in the Dajiuhu Peatland.Based on the results of methane cycle during the Holocene,the methane metabolism indexes of Dajiuhu peat ZK7 since Late Pleistocene have been measured by biomarker analysis.The results showed that since Late Pleistocene,Archaeol Flux and GDGT-0 Flux were used as the indicators of methane metabolism in the Dajiuhu Peatland kept similar changes with hydrologic and redox conditions,suggesting that the fluctuation of hydrological conditions since the Late Pleistocene drived methane production process;especially,in the mid MIS3 period,about 41.10 kyr BP,the methane producing microbial index had the highest peak value,while GDGT-0/Cren and Hopanoid Flux were the most important indicators of methane production in the same period indicated that Dajiuhu Peatland was in the state of extreme anaerobic at high water level,which also confirmed that the hydrologic conditions had driven methane production and metabolism process of Dajiuhu Peatland since MIS5/4/3 of Late Pleistocene.In addition,temperature and TOC deposition had no significant effect on methane metabolism in Dajiuhu area since Late Pleistocene.Totally,such research based on subtropical Dajiuhu Peatland,combined molecular nucleric acid and biomarker,to understand microbial mediating methane cycle process in the mid-latitude subtropical peatland since the Holocene.Noticeably,the study analyzed microbial methane cycle by ancient DNA technology since 13 kyr BP,and different with traditional biomarker lipids.Besides,different with polar peat and equatorial peat,this study proposed a new mechanism,and contributed more data and evidence for a more comprehensive understanding of methane cycle across the global peatlands.