| The Qinghai-Tibetan Plateau(QTP)is the geographical unit with the highest altitude on the earth.It has the largest number and the largest area of alpine lakes,and is the main body of the world’s third pole.In the context of global warming,the glacial melting on the QTP has accelerated,precipitation in the region has increased,and the area of lakes has increased rapidly.The lakes in the QTP region are less disturbed by human activities,and a variety of environmental parameters of the lakes are sensitive to regional climate and environmental changes,and have strong feedback effects on climate and environmental changes.Lake water volume and salinity changes are important indicators reflecting the degree of influence of climate and environmental changes,among which salinity changes are important factors affecting lake ecosystems.Microorganisms are the main drivers in lake ecosystems.They promoted the geochemical element(carbon,nitrogen,sulfur,phosphorus and iron,etc.)cycles and the decomposition and synthesis of organic matter through life metabolism activities in the lakes,which were sensitive indicators of salinity changes.At present,a large number of scholars have used various biomolecular technologies to study the microbial diversity,community composition and their response to salinity in lakes on the QTP.However,comprehensive investigation of the changes in ecological functions due to changes in microbial diversity,and the effects of salinity on the diversity and functional stability of microbial communities was still unclear.In addition,environmental changes and increased rainfall have led to increased nitrogen deposition in lakes,and the microbially driven nitrate reduction coupled with iron oxidation pathway in lakes can effectively remove nitrogen,which forms a cyclic process with iron reduction process and provides the possibility of continuous denitrification in lakes.However,studies on microbially-involved nitrate reducing iron oxidation processes under high salinity conditions were not fully understood.Therefore,in the context of increasing climate and environmental changes,it is important to study the influence of salinity on the structure and functional stability of lake microbial communities on the QTP to predict the response of lake ecosystems to climate and environmental changes.From the perspectives of the relationship between salinity changes and microbial functions,lakes with different salinities on the QTP were selected as the main research objects.Geochemical analysis,culture-dependent method,high-throughput sequencing technology,microcosms experiments,statistical analysis and other methods were applied to determine the onshore contributions on the stability of lake’s endogenous microbial community,the cultivable microbial community diversity and compositions in the QTP lakes,the salinity effect on the relationship between microbial diversity and ecosystem function.Finally,the key functional taxa in lakes-nitrate-reducing Fe(II)oxidizing and dissimilatory iron reducing microorganisms and their iron redox function in response to salinity was discussed.The main results are as follows:1)The effect of onshore microbial contribution on the stability of endogenous microbial communities in lakes was investigated.Results showed that after 50 days of in situ incubation,the salinity and total organic carbon(TOC)content increased significantly(P < 0.05)in the caged soils than the original soils in the studied lakes.The microbial community composition and predicted functions in the caged soils were significantly(P< 0.05)changed in comparison with their corresponding original soils,and such variation could be mainly explained by the succession of members of the Proteobacteria,Bacteroidetes and Actinomycetia from the original soils to their corresponding caged soils.It was expected that inoculation and salinity had a significant(P < 0.05)effect on microbial community composition in the caged soils.The onshore microbial contribution appeared to be limited(up to 11.2% for sediment and negligible for water,respectively)to nearby lake microbial communities,which also implied that the source of in situ lake community may be not from onshore contribution.Nevertheless,the survival of onshore soil microbial communities was mainly limited by the salinity of the receiving lakes.2)Research on the diversity,community composition and predicted ecological function of culturable microbial communities in the sediments of six saline lakes on the QTP(Erhai Lake,Qinghai Lake,Tuosu Lake,Gahai Lake,Xiaochaidan Lake and Chaka Lake)was explored.A total of 646 strains were isolated,belonging to 4 phyla(Firmicutes,Proteobacteria,Actinomycetes and Bacteroides),90 genera and 210 species.The compositions and prediction functions of microbial communities in lakes with different salinities were obviously different.Freshwater and low salinity lakes were dominated by freshwater and salinity-sensitive taxa(e.g.Paenibacillus,Pseudomonas),while high salinity lakes were dominated by salt-tolerant and halophilic taxa(such as Halomonas,Halobacillus).The osmotic pressure caused by salinity was the main factor influcing the composition of lake communities and predicting functional differences.Culturedependent technology has limitations in understanding microbial diversity,but could provide germplasm resources for subsequent functional stability research and microbial resource development.3)Effect of salinity on microbial diversity and functional stability experiments showed that the 40 representative strains selected for this experiment were highly diverse and belonged to 4 phyla and 32 genera.The salinity tolerance traits of the 40 strains differed,and many taxa started to grow slowly or inactive after a certain salinity(e.g.salinity > 10%).Subsequently,mixed strain systems with different abundance(29 classes in total)were set up based on salt tolerance traits and incubated at five different salinities(0.9%,3.5%,7%,15% and 20% of culture salinity).The results showed that the mixed strain system exhibited strong enzyme activity at low culture salinities(0.9%,3.5% and7%)and decreased with increasing culture salinity(> 15%),but the increase in microbial species richness helped to slow down the loss of enzyme activity.Salinity significantly(P < 0.05)influenced the relationship between microbial species richness and enzyme activity expression.Microbial community analysis of the mixed strain system with the highest abundance(i.e.,containing 40 strains)at the end of the incubation revealed distinct clustering at different culture salinities,and the dominant taxa also differed with increasing culture salinity,evolving from salt intolerant to salt tolerant or halophilic taxa,possibly due to the selection effect of salinity based on the salt tolerance traits of microbial species.Displacement multivariate ANOVA verified the significant(P < 0.05)variability between microbial community composition at different culture salinities as described above.Thus,maintaining microbial diversity is very important.The higher the microbial diversity,the greater functional complementarity among microbial species,which helps microbial communities to increase salinity resistance and maintain function.4)Fe(II)and Fe(III)were present in the sediments of saline lakes on the QTP,and bioavailable iron,crystalline iron,nitrate and ammonium nitrogen could be detected.The presence of iron and nitrogen suggested that active iron and nitrogen cycling and iron mineral formation processes may exist in the sediments of these lakes.Most-probablenumber(MPN)results showed that the number of nitrate-reducing Fe(II)-oxidation(NRFe Ox)microorganisms decreased with the increasing lake salinity,suggesting that salinity may supress the number of NRFe Ox microorganisms in high salinity lakes.Highthroughput sequencing results revealed significant(P < 0.05)differences in the compositions of NRFe Ox communities enriched in MPN plates and clustered according to lake salinity.Linear regression model also indicated a significant(P < 0.05)relationship between salinity differences and Bray-Curtis difference in the NRFe Ox communities.In addition,two enrichments with nitrate reducing iron oxidation and stable transfers were enriched,named Fe N-EHL and Fe N-CKL,whose dominant taxa were Gallionella and Marinobacter,respectively.Kinetic experiments confirmed that they could undergo nitrate reduction coupled with the iron oxidation process,and the products after iron oxidation were all short-range order Fe(III)(hydroxy)oxides,which were more similar to the known ferrihydrite morphology.The majority of cells in the enrichments Fe N-EHL and Fe N-CKL were observed to be partially attached or completely encrusted by the formed minerals under the scanning electron microscope(SEM).This study described the community composition,kinetics,and mineral transformation of NRFe Ox microorganisms ranging from freshwater to salt-saturation.And a stable NRFe Ox enrichment Fe N-CKL was enriched in a salt-saturated lake,which provided evidence for the existence of nitrate reducing iron oxidation process at salt-saturated conditions.5)It was found that different iron fractions(bioavailable iron and crystalline iron)have been detected in the sediments of lakes on the QTP,and the concentration of crystalline iron was much higher than that of bioavailable iron.The key functional taxa in lakes-Dissimilatory iron reduction microorganisms(DIRM)in the QTP lakes were mainly dominated by Proteobacteria,Firmicutes,Bacteroidetes,Euryarchaeota and Actinomycetia.Salinity significantly(P < 0.05)affected the iron reduction abilities and community compositions of DIRM enrichments in lakes with different salinities.The DIRM enrichments formed amorphous or poorly crystalline Fe(II)minerals after performing iron reduction functions.The culturable strains isolated from the enrichment of low-salinity lakes were mainly distributed in the genera Clostridium,Ilyobacter and Shewanella.These groups were typical microorganisms with iron-reducing ability.Microbial dissimilatory iron reduction processes could be detected in six lakes with different salinities(from fresh water to salt-saturation),which expanded the natural salinity habitat boundary for the growth of this important functional mivroorganism. |
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