The Response Of Microbial Community In Karst Cave System To Environmental Changes

Global environmental changes are proposed to have significant impact on terrestrial ecosystem in near future.Warming,enhanced frequency of floods and extreme drought events,increasing greenhourse?such as CO₂?concentration are included.Karst caves are important composition of terrestrial ecosystems.Due to geographical isolation and the lack of sunlight,caves generally considered as oligotrophic environments.Nevertheless,the caves contain a large number of microorganisms,making it as a natural laboratory for studying the underground biosphere.However,the research about the impact of global environmental change on the cave ecosystems is limited.Most of the studies focused on the reconstruction of the ancient environment by using the geochemical index and biomarker lipids in the stalagmite.At the same time,some factors limit the application of these indicators in a certain degree,including the effects of microbial community on these indicators,large samples required for lipid extraction and unclear microbial sources of biomarker lipids.Moreover,recent studies have demonstrated that autotrophic microorganisms also exist in the dark oligotrophic environment of the cave,and may be involved in the CO₂fixation process.However,studies on the response of cave microbial community to the increasing CO₂ concentration are very scarce.This would limite our understanding of the response of microbial community on cave environment under global changes.In order to fully explore the spatiotemporal changes of microbial community in karst cave ecosystem and its response to climate change,this paper selected Heshang cave in Hubei,central China as the study object.By using the 16S rRNA clone library method,biolog microbial metabiolic system,DNA stable isotope probing?DNA-SIP?about cbbL gene,and Phospholipid fatty acid-SIP?PLFA-SIP?technique,the microbial community in each habitat of the cave and their response to environmental factors were studied.The main results are listed below.At first,the results about the cave ecosystem revealed that bacterial community in each habitat was distinct and the distinctive community was significantly correlated with pH,total organic carbon,and potassium concentrations.Specifically,surface soils were dominated by Acidobacteria,Verrucomicrobia and Planctomycetes,and diversity significantly decreased with acidic pH values.Nitrospirae,Gemmatimonadetes,Firmicutes,and Chloroflexi were unique to cave sediments,while Actinobacteria and Proteobacteria dominated in weathered rocks and drip waters,respectively.The results demonstrated important implications of regarding the effects of acidification on bacterial communities throughout a karst system.Increased water flux into and through karst habitats due to monsoonal precipitation may result in deeper penetration of acidic solutions into karst and shift the bacterial communities inside the cave in the future.Secondly,the total number of cells and the number of living cells in the cave drip has a strong tendency to increase with the rainstorm and continuous rainfall.Proteobacteria was dominanted in both dark and twilight zone drip water.However,due to the different hydrodynamic of each drip water site,different hydrophilic and hydrophobic microorganisms were selected.Thiswould result in the difference of bacterial community composition below class level and their further response to environmental factors.The bacterial community in the dark zone drip water revealed seasonal changes and was significantly correlated with Yichang temperature.In particular,Gammaproteobacteria,the dominant group,was significantly negative correlation with temperature and showed seasonal dynamics with higher relative abundance in autumn and winter,and lower in spring and summer.Moreover,the relative abundance of Gammaproteobacteria was closely related to various metabolic functions,such as the significantly positive relationships with lipid metabiolism,metabolism of terpenoids and polyketid,and xenobiotics biodegradation and metabolism.Meanwhile,they had the significantly negtive relationships with enzyme family and carbonhydrate metabolism.Whereas,the bacterial communities in twilight zone drip water were dominanted by Gammaproteobacteria showing no significant seasonal difference.Instead,Acinetobacter and Pseudomonas showed a clear seasonal variation,and a close relationship with precipitation which explained about 27.6%of bacterial community variation.Besides,potential metabolic functions of bacterial communities in twilight zone drip water also show clear seasonal variation.Acinetobacter revealed a remarkable positive correlation with amino acid metabolism,lipid metabolism,metabolism of terpenoids and polyketid,xenobiotics biodegradation and metabolism,and carboxylic acids metabolism.Pseudomonas showed a significant negative correlation with lipid metabolism,metabolism of terpenoids and polyketid,xenobiotics biodegradation and metabolism,and amine metabiolism.Thirdly,Betaproteobacteria,Moracellaceae and Bacteroidetes are indicators in the wet speleothem surface samples,whereas Actinobacteria and Bacilli were distinctive in the dry speleothem surface samples.These indicator groups are further confirmed by contrasting microbial communities in the two Holocene stalagmite samples.Our results strongly suggest that microbial indicator groups can serve as new microbial proxies in paleohydrological reconstruction in cave systems,opening a new window to look into the speleothem paleoclimatology.Fourthly,bacteria were the main CO₂ fixer in cave.Among the all six known CO₂-fixation pathways,Calvin-Benson cycle was dominanted in weathered rocks,while HP/HB pathway in cave sediments.The groups involved in CO₂ fixation in the cave were mainly Proteobacteria,Actinobacteria and Planctomycetes.And within Proteobacteria,Gammaproteobacteria,Betaproteobacteria,Alphaproteobacteria,Burkholderiales and Peseudomonas were mainly contained.With the increase of CO₂concentration,??¹³C value of i15:0,16:0,i17:0?Bacillus?,10Me16:0?Actinomycetes?and cy19:0?Burkholderia?becomes higher.It suggested that CO₂ fixation ability of these bacterial groups highly improved as the CO₂ concentration increases.These bacterial groups might plays a huge role in carbon cycle of oligotrophic carbonate caves and help us to fundmental understand the response of bacterial community to greenhouse effect in karst area.Overall,microorganism indexes in response to temperature and precipitation in modern cave ecosystem are proposed to provide new ideas for paleoclimate index construction and future climate change prediction.Furthermore,the functional groups of autotrophic microorganisms involved in the process of CO₂ fixation,and their responses to CO₂ concentration in the oligotrophic alkaline cave ecosystem were explored.This would provid certain research basis for carbon cycle and greenhouse effect in karst area.