Environmental Significance Of Microbial Tetraether Lipids In Peatland Around China And Biogeochemical Processes In Deep Biosphere

One of the most important goals of paleoclimate reconstruction on land lies in the separation and extraction of paleo-temperature and paleo-hydrological signals from sediment records.Previous paleoclimate reconstruction studies focus mainly on the variation of summer monsoon intensity and the determination of its driving forces,with an ambiguous designation of the climate conditions driven by marine-continent interactions being either warm&humid or cool&dry without any specification of temperature and precipitation,which in turn comprise the most basic and independent components of climate.Thus,separation and extraction of temperature and precipitation signals from sediment records are vital in the understanding of paleoclimate change on land.Nowadays,deep biosphere has become a hot topic in the study of biogeochemistry.In extreme environment where there is no sunlight,microbial organisms constitute the vast majority of the ecosystem.These“dark ecosystems”in deep earth,therefore,offer great reference to the origin of life and to possible extraterrestrial forms of life.However,deep biosphere studies at present are mainly from marine drill core sediments.Peatland,as an extremely acidic and anoxic terrestrial sedimentary environment with enormous methane emission,represents similar deep biosphere on land as that in the marine environment and thus can contribute to our understanding of life forms in terrestrial deep biosphere and global carbon-nitrogen cycling.Due to its unique formation,peatland provides great study materials for terrestrial paleoclimate changes,deep biosphere on land and involved biogeochemical processes because of its strong acidic environment,abundant organic materials and extremely anoxic conditions.As the core composition of the membrane of organisms,tetraether lipids are the key elements connecting/separating the inner and outer worlds of organisms.Therefore,the study of tetraether lipids in peatland environment could provide insights into the following key scientific problems:?1?establishment of environmental proxies using tetraether lipids and paleoclimate reconstruction,?2?the understanding of deep biosphere on land and biogeochemical processes therein.As a contribution to resolving the above-mentioned scientific problems,this study aims to discover the relationship between the distribution of tetraether lipids and environmental factors,to have a discussion on the microbial community and on stable carbon isotopes of lipids by sampling surface sediments of several peatland and two peat cores within different climate zones around China.Main understandings and conclusions are as follows:1)In terms of environmental proxies,MBT'_5ME shows no relationship with pH but can be related to temperature after the separation of 5-methyl and 6-methyl bGDGTs whereas both cyclization indexes(CBT_5ME and CBT_6ME)exhibit significant negative correlation with pH,indicating a control effect of pH variation on the cyclization of6-methyl bGDGTs,which in turn validates the utility of CBT in pH reconstruction from peat samples.While both MBT'_5ME and MBT'_6ME can be related to temperature,the latter also shows discernable variation with pH changes,which makes MBT'_5ME a more reliable proxy for paleotemperature reconstruction.The fact that the relationship of5-methyl b GDGTs with temperature in peat is similar to that in soils,together with the similarity of the relationship of 6-methyl bGDGTs with temperature in peat to that in lacustrine systems,indicates that peat environment can be regarded as a transit environment between soil and lacustrine systems.2)Because of the scattered distribution of bGDGTs within each peatland studied,exclusive studies were conducted in the Dajiuhu peat core and the results show that MBT varies negatively with water content while CBT varies positively with oxygen availability,which has not been taken into consideration in previous peatland studies.3)Paleotemperature and pH values during the last 13 ka at the Dajiuhu peatland were reconstructed using the proxies of MBT'_5MEME and CBT_peat that have been established according to modern surface processes.Although the results lack sensitivity on a millennial scale,they are consistent with those reconstructions of paleotemperature and hydrology by other biomarkers in peat including Hopanoids.As to the low sensitivity of paleoclimate reconstruction using MBT'_5MEME and CBT_peat,it is considered to be compromised by water content,i.e.hydrological conditions:because of the opposite influence of water content on MBT and CBT,increase of temperature will magnify the increase of MBT while dwarfing the changes in CBT values.All in all,hydrological changes in peatland may have played an important role in the large fluctuations of paleotemperature as well as in the flat pH profiles.4)Investigations in the peat surface processes show that concentrations of6-methyl bGDGTs varies with pH changes,e.g.both bGDGT-IIIa'and bGDGT-IIa'show positive correlation with pH values in peat surface samples.Comparison of the relationship between IR_6ME and pH in peatland around China with those in global peatland,global soil and some lacustrine systems in China demonstrate that the relationship between IR_6ME and pH in peatland both around China and in a global scale falls within the ranges of that in global soils but differs greatly from that in lacustrine systems in China,which could be the result of distinct biotic sources of bGDGTs in peat from those in lakes.Additionally,the presence of“shoulder”peaks before the diagnostic peaks for iGDGTs in most surface peat samples studied indicate either the presence of a double bond on the alkyl chain or the substitution of a cyclohexane for a cyclopentane.Interestingly,the concentration of compounds with“shoulder”peaks exhibits significant correlation with pH values,making it a promising pH proxy.5)Several Archaeal tetraether lipids?including iGDGTs,Archaeol,OH-Archaeol,BDGTs,Me-iGDGTs,Uns-iGDGTs,GMGTs and GDD?and their counterpart IPL with head groups?usually present as 1 or 2 glycose?were detected in the Hani peat core from northeast China.The concentration of all Archaeal tetraether lipids as well as that of specific compounds such as BDGTs,Uns-iGDGTs and Me-iGDGTs increases with the depth of the peat core.The ratio of IPL to CL also increases with depth,which together with the concentration of Archaeal tetraether lipids indicates abundant living Archaeal in the deep peat core.6)DNA sequencing of samples from different depth of the peat cores demonstrates the predominance of MCG which is seconded by methanogenic Achaea:the concentration of these two comprises 75%-90%of all microbial organisms detected but shows antipathetic relationship with depth,with MCG increasing towards depth and an indication of competitive interaction between MCG and Methanogenic Achaea.According to the distribution of tetraether lipids,iGDGT,especially iGDGT-0comprises the most in peat core,indicating a source from MCG and certain methanogenic Achaea,with a larger contribution from MCG.Carbon isotope analysis of iGDGT-0 shows similar values with that of TOC,supporting heterotrophic consumption of ambient organic materials for MCG and/or CH₄ genesis by Methanogenic Achaea utilizing acetate.Additionally,IPL/CL ratios for both Uns-iGDGTs and Me-iGDGTs show similar variation patterns with iGDGTs,indicating that Uns-iGDGTs,Me-iGDGTs and iGDGTs?mostly iGDGT-0?may have all sourced from MCG and/or certain Methanogenic Achaea.7)Previous studies have shown that BDGTs presents mainly in anoxic environments and has enormously large concentrations in the Order Methanomassiliicoccus luminyensisw within the Methanogenic Archaea family,which also shows great correlation with MCG.In the Hani peat core,IPL-BDGTs increase significantly with depth,indicating that relatively high pressure and temperature facilitates the thriving of Archaea that produces BDGTs.Correlation with the increasing concentration of NH₄⁺with depth further indicates that ammoniation in depth could have provided essential elements such as NH₄⁺for Archaea.Moreover,DNA data shows similar variation patterns of the relative concentration of Methanomassiliicoccaceae with that of the concentration of BDGTs within Archaea tetraether lipids in the peat core,which further validates Methanomassiliicoccaceae as the microbial source for BDGTs.8)Carbon isotope analysis of certain lipid in the Hani peat core indicates that Archaeol with different metabolic types exist in the deep peatland biosphere.Carbon isotope values of Archaeol are 10‰higher than those of TOC,which indicates that Methanogenic Archaea that produces Archaeol may have generated CH₄ through the H₂/CO₂ pathway.The crackates of iGDGTs?including 3 types of biphytanes?exhibit significantly different carbon isotope values,suggesting complex biotic sources for iGDGTs with distinct metabolism.The biphytane-0,crackate of iGDGT-0,has carbonate isotope values most close to those of TOC,indicating heterotrophic consumption of ambient organic matters by MCG or CH₄ genesis by Methanogenic Archaea using acetate.The carbonate isotopes pf biphytane-1?crackate of iGDGT-1?are more negatively deviated from carbon isotopes of TOC and may be sourced from Methanogenic Archaea using methylamine and methanol for CH₄ genesis.