Microbial Response To Environmental Changes In Qinghai-Tibetan Lakes And Its Environmental Implications

Microorganism plays an important role in regulating geochemical element (carbon, nitrogen and sulfur etc.) cycles; and they sensitively respond to environmental change induced by regional and global climate change. Moreover, the understanding of relationship between microbial community and environmental factors avails to reconstruct paleo-environmental conditions of lakes based on microbial biomarkers. Qinghai-Tibetan lakes are unique aquatic environments in the world. These lakes contain multiple environmental gradients such as salinity (from 0.1 to 426.3 g/L) and pH (5.4-10.2), and their sediments record abundant historic climate events. Thus, Qinghai-Tibetan lakes are ideal for studying microbial response of current or past environmental changes. Microbial diversity has been thoroughly investigated in some lakes on the Qinghai-Tibetan Plateau during the past several years. However, it is poorly known about microbial difference between lake water and sediment and their relationships with environmental parameters, the characteristic of phylogenetic molecular ecological networks (pMENs), the relative influences of stochastic and deterministic processes, and microbial predicting models. In this study, The water and sediment samples of Qinghai-Tibetan lakes were used for investigating microbial diversity, followed by statistical assessment on the correlation between microbial community composition and environmental factors, community assembly process and microbial co-occurrence networks. Subsequently, we try to used ancient DNA method to reconstruct paleo-environmental changes in the Qinghai Lake and Kusai Lake according to microbial response of current environmental conditions. The results are as follows:1) Microbial communities in the sediment samples of nine lakes in Qinghai Province are dominated by Euryarchaeota, Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Cyanobacteria, Firmicutes, Gemmatimonadetes, Planctomycete, Proteobacteria, Thermi and Verrucomicrobia. Salinity plays a major role in shaping microbial community structure in these lake sediments. Relaive abundances of Gammaproteobacteria and Clostridia are significantly (p < 0.05) correlated with salinity. Significant correlations are observed between a and p diversity indices and salinity. In addition, Mantel test shows that microbial community compositions are significantly affected by salinity, pH and major ions (K+、Na+、Ca2-、Mg2+、Cl-and NO3-).2) Microbial diversity was further studied in the water and sediment samples collected from 21 lakes from three provinces (Qinghai, Tibet and Xinjiang). Microbial communities in the water samples are dominated by Bacteroidetes、Proteobacteria、Actinobacteria and Cyanobacteria; while Bacteroidetes、Proteobacteria and Cyanobacteria are dominant in the sediment communities. Statistical analyses showed that salinity and geographic distance are two major factors affecting microbial community composition in these lakes. Positive correlation (p<0.05) is observed between salinity and relative abundances of Halobacteria and Rhodothermi in water samples; whereas negative correlation (p< 0.05) is found between salinity and relative abundances of Actinobacteria、Betaproteobacteria、Saprospirae and Synechococcophycidea. Furthermore, significant correlations are also found between salinity and relative abundances of Clostridia, Gammaproteobacteria, Bacteroidia and Betaproteobacteria in sediment samples. Microbial α and β diversity indices among water and sediment samples are significantly correlated with salinity, geographic distance and elevation. Mantel test also shows significant correlations between microbial community composition and salinity, pH, geographic distance and elevation. Variation partitioning analyses indicate that 34.3% of the observed microbial variation is explained by major ions in water samples,9.0% for geographic distances and elevation,7.9% for pH, DOC and TN, and totally 41.8% of oberved variation cannot be explained; 27.5% variation is explained by major ions in sediment samples,6.3% for geographic distances and elevation,6.8% for pH and TOC, and total 56.2% variation cannot be explained.3) Some microbial functional communities were investigated in the lakes of Qinghai Province:[1] Co-occurrence of nitrite-dependent anaerobic methane oxidizing and anaerobic ammonia oxidizing bacteria was found in two Qinghai-Tibetan saline lakes (with salinity of 32 and 84 g/L, respectively). [2] Ammonia oxidizing archaea (AOA) community is primarily affiliated with Nitrososphaera、Nitrosopumilus and "low salinty" clusters in the water and sediment samples, and their community composition is affected by salinity. Cluster analysis shows that water and sediment samples are grouped into two separate clusters. [3] Phylogenetic analyses show that the carbon monoxide-oxidizing (COX) bacteria are dominated by Proteobacteria and Actinobacteria in the Qinghai-Tibet Plateau lakes, and their relative abundance varies with salinity:in the freshwater Erhai Lake, the COX bacteria in the water are dominated by the Betaproteobacteria, in contrast to the Actinobacteridae dominance in the sediment; in the saline and hypersaline lakes of Gahai Lakel, Gahai Lake2 and Xiaochaidan Lake, alphaproteobacterial COX bacteria are dominant in the water, whereas Actinobacteridae and alphaproteobacterial COX bacteria are dominant in the sediment, In the hypersaline Lake Chaka, an unknown COX bacterial clade and alphaproteobacterial COX bacteria are dominant in the water and sediment, respectively. Statistical analyses show that salinity, pH, and major ions (e.g. K+, Na+, Ca2+, Mg2+, SO42-, and Cl-) are important factors affecting the COX bacterial community compositions in the investigated lakes. [4] Phylogenetic analysis show that Alpha-and Betaproteobacteria were dominant sulfur-oxidizing bacteria (SOB) in the investigated lakes, and the composition of proteobacterial subgroups varied with salinity:in freshwater Erhai Lake and low-salinity Gahai Lake 1, the SOB populations were dominated by the Betaproteobacteria, whereas in hypersaline Lake Gahai 2 and Xiaochaidan Lake, the SOB populations were dominated by Alphaproteobacteria. Overall, salinity plays a key role in controlling the diversity and distribution of SOB populations in the investigated Qinghai-Tibetan lakes. [5] The diversity of dinoflagellate and haptophyte is low in these lake water samples, and their community structures are affected by salinity.4) Microbial community assembly processes were quantified in lake water and sediment samples. The results are as follows:in the freshwater lake water,11.0% and 89.0% of turnover in community composition are primarily due to Selection and Dispersal Limitation acting in concert with Drift, respectively; in the saline lake water,10.0%,86.3%,2.1%, and 1.6%of micirobial composition turnover are primarily due to Selection, Dispersal Limitation acting in concert with Drift, Homogenizing Dispersal, and Drift, respectivley; in the hypersaline lake water,22.2%, 15.6%,6.7% and 55.6% of community composition turnover are primarily due to Selection, Dispersal Limitation acting in concert with Drift, Homogenizing Dispersal and Drift, respectively, In the freshwater lake sediment,30.8%,67.9%, and 1.3% of community composition turnover are primarily due to Selection, Dispersal Limitation acting in concert with Drift and Drift, respectively; in the saline lake sediment,29.4%,56.2%,8.5% and 5.9% of community composition turnover is primarily due to Selection, Dispersal Limitation acting in concert with Drift, Homogenizing Dispersal and Drift, respectively; in the hypersaline lake sediment,30.3%, 13.6%,13.6% and 42.6% of community composition turnover are primarily due to Selection, Dispersal Limitation acting in concert with Drift, Homogenizing Dispersal and Drift, respectively.5) Salinity affects microbial ecological network structure:network nodes and connectivity decrease in response to salinity increase. High relative abundance of certain microbial groups corresponds to high network connectivity. The composition of node connectivity varies significantly(Bray-Curtis dissimilarity more than 0.65) in the lakes of different salinity.6) Microbial community assemblages were predicted based on MAP (microbial assemblage prediction) model. The model was used to interpolate and extrapolate community structure among water and sediment samples with an average Bray-Curtis similarity of 0.84 and 0.52, respectively.7) Field microcosm experiments were successfully performed to mimic salinity increase and decrease scenarios in different evolutionary phases of a lake. Microbial communities in these samples are dominated by Crenarchaeota, Euryarchaeota, Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Cyanobacteria, Firmicutes, Planctomycetes, Proteobacteria and Verrucomicrobia. Salinity shapes microbial community composition along lake salinization and desalinitization. Many classes of microbial community are significantly correlated with salinity, but few studies observed significant correlations between salinity and the relative abundances of Bacilli, Actinobacteria, Deltaproteobacteria, Flavobacteria, and Halobacteria. Additionally, Bray-Curtis dissimilarity among experimentl and 2 samples are significantly correlated (salinization:r= 0.618, desalinization:r=0.923) with salinity differences.8) Diversity and abundances of haptophyte, dinoflagellate, AOA and total eukaryotes of the Kusai Lake and Qinghai Lake show systematic temporal variations along the depth of the sediment cores, which corresponds to some global climate events (such as Younger Dryas, Heinrich event, Bond events).In summary, this study systematically revealed microbial diversity in the Qinghai-Tibetan lakes and their response to environmental factors, the characteristics of microbial co-occurrence networks in Tibetan lakes of different salinity levels, the relative influences of Selection, Dispersal Limitation acting in concert with Drift, Homogenizing Dispersal, and Drift, and successfully constructs microbial predicting models. Furthermore, we successfully reconstructed historic lake environmental conditions on the basis of the abundances of haptophyte, dinoflagellate, AOA and total eukaryotes in the sediment cores. The results presented in this thesis will favor to fully understand the ecological and biological evolution in response to the global climate change, and provide theoretical foundation for assessing the impact of global environmental change on the biosphere.