Study On Organic Matter Composition And Microbial Degradation In Response To Salinity In Northern Qinghai-tibetan Lakes

The Qinghai-Tibetan Plateau?QTP?harbors numerous and diverse types of lakes,whose average altitude,amount and total area are second to none all over the world.Due to its unique geographic location and climate characteristics,QTP has become a sensitive indicator and amplifier of global climate change.In the QTP lakes,microorganisms play an important role in maintaining the stability of the ecological network structure in lake habitats,and are important drivers of promoting the circulation of key biological elements?such as carbon,nitrogen,sulfur,phosphorus,etc.?and the energy flow of lake ecosystems.Owing to above reasons,large numbers of scholars have conducted researches on the microbial diversity and its response to environmental factors of QTP lakes.In recent years,in the context of global warming,the average annual temperature of QTP has accelerated,and the local climate has shown a shift from warm-dry to warm-humid,resulting in melting of glaciers and frozen soil,as well as the increased surface runoff.The above effect not only changes the physical and chemical properties of QTP lakes,but also introduces a large amount of terrestrial-origin organic carbon flowing into the lakes,which has a huge impact on the QTP lake ecosystem.However,studies focusing on the interaction between lake microorganisms and exogenous organic matters in this area are still scarce.Therefore,in the context of global climate change,it is of great ecological significance to study the interaction between microorganisms and organic matters and their ecological effects?such as CO₂ emission?responding to environmental factors in QTP lakes.In this study,lakes with different salinity in the northern part of QTP were selected as the main research objects,and some lakes in the eastern and northeastern regions of China were also selected as references.FT-ICR-MS,ToF-SIMS and other advanced geochemical analysis methods were used to analyze the composition of organic matters in QTP lakes with different salinity.The interaction between microorganisms related to carbon cycling and organic matters and their response to salinity was studied by using the combined methods of microcosmic and modern molecular biologic and bioinformatic techniques.Our results are as follows:1)In the QTP lakes with distinct salinity,the majority??70%?of the collected dissolved organic matters?DOM?samples may be of terrestrial origins and mainly consisted of assigned molecular formulae with high oxygen content,and rather low hydrogen to carbon?H/C?ratios,which is typical for polyphenols.A high percentage?>20%?of the assigned formulae belonged to aliphatic compounds,that showed higher saturation than that reported in most of estuarine,terrestrial freshwater,and glacial/marine habitats.The accumulation of highly saturated compounds that presumably indicates labile carbon could be caused by limited microbial metabolic activities under various high salinities.The chemical composition DOM showed statistically significant?P<0.01?correlation?R>0.7 or R<-0.7?with salinity,suggesting that microbes of different metabolic functions may play important roles in defining the DOM composition in saline/hypersaline lakes.Some key microbial groups?Proteobacteria,Bacteroidetes,Actinobacteria,Cyanobacteria,Firmicutes,Tenericutes?may play vital roles in degrading organic matters and maintaining the stability of lake ecological network.In addition,the total amount of DOC harbored in QTP saline/hypesaline lakes was very high and thus the DOC reservoir in global saline/hypesaline lakes may need to be adjusted for global carbon budget estimates.2)In response to terrestrial-origin DOM?t DOM?additions,total microbial cells increased while dissolved organic carbon?DOC?decreased during the microcosm incubation.Microbial community and DOM molecular compositions also exhibited slightly changes during the microcosm incubation.Abundant taxa?relative abundance>1%?and copiotrophs?that are often rare,relative abundance<0.1%in ambient environments but become abundant,relative abundance>1%,in response to nutrition increase?may play important roles in utilizing t DOMs in our studied lake microcosms.Correlation analysis indicated that different microbial taxa may have different correspondences to the transformation of different DOM compounds.Microbial communities in high salinity lake microcosms presented stronger abilities to utilize t DOMs of high carbon number and complex chemical structures than those in low salinity lake microcosms.3)Addition of exogenous organic matters resulted in increase of the CO₂ emission rate in different degrees,and the increment of CO₂ emission rate in freshwater lakes was much higher than that in saline/hypersaline lakes?one-way ANOVA:P<0.01?,indicating that salinity may be an important factor affecting the mineralization of organic matter mediated by microorganisms.The addition of organic matters did not result in significant changes in microbial community composition?ANOSIM:P>0.05?in a short period of time,but there were still some microbial groups that from different salinity responded to the addition of organic matters:Betaproteobacteria,Epsilonproteobacteria,Fusobacteria in freshwater lake sediments and Halobacteria in hypersaline lake sediments responded to organic matters addition sensitively,while Gammaproteobacteria,Deltaproteobacteria and Clostridia were sensitive to organic matters addition at all salinity levels.Furthermore,salinity was a key environmental factor that influenced carbon utilization ability of Gammaproteobacteria:strains belonging to each genus had stronger carbon utilization ability in low salinity environments than their counterpart in high salinity environments;strains belonging to one certain genus but isolated from lakes of different salinity shared similar carbon utilization preference,and thus occupy similar niche in distinct environments.4)Based on the microcosms cultivation experiment that constructed by using 58sediments collected along a salinity gradient?1-120 g/L?in hypersaline Xiaochaidan Lake and its sole feeding river,we found that organic carbon?OC?addition can increase microbial abundance,diversity and OC respiration rate?represented by CO₂ emission?of the studied lake sediments.In response to OC increase,microbial alpha diversity increased while the net CO₂ emission rate decreased with increasing salinity in the sediment microcosms.OC increase can also modify microbial community:copiotrophic predominated over oligotrophic species at low salinity,while the predominance trend was opposite at high salinity.