Investigation On Structure And Function Of Carbon-fixing Microbial Communities In Sediments Of Northern Qinghai-Tibetan Plateau Lakes

As an important part of the inland waters,the lake environment has the characteristics of wide distribution,diverse physical and chemical parameters,and high carbon cycle rate,playing an important role in the carbon cycle on the regional and even global scale.Microbes are the main drivers of carbon cycle in lake ecosystems,one of which is microbial mediated carbon fixation.Therefore,exploring microbial mediated carbon fixation process is necessary for a comprehensive understanding of the lake carbon cycle.Previous studies have shown that in addition to cyanobacteria,eukaryotic algae and other photoautotrophic microbes,a variety of bacteria and archaea can fix carbon dioxide without light,that is,dark carbon fixation,and there is an essential contribution of dark carbon fixation in total carbon fixation among various environments.Therefore,it is of great ecological significance to have a thorough understanding of the lake's dark carbon fixation process and its contribution.There are lake groups of the highest altitude,the largest number,and the most extensive area in the world on the Qinghai-Tibet Plateau?Abbreviated as QTP?.Does the dark carbon fixation exist in these plateau lakes?What is its contribution to the total microbial mediated carbon fixation?How does the microbial mediated carbon fixation respond to changes of environmental factors?These issues are still inconclusive.In order to answer the above questions,six high-altitude lakes in the northern part of QTP with different physical and chemical parameters?e.g.Salinity?were selected as the research objects.High-throughput sequencing,cloning library and real-time quantitative PCR were used to profile the structure and abundance of functional gene community in nearshore surface sediments;the light and dark carbon fixation rates of nearshore surface sediments were determined by ¹⁴C isotope tracing technique;finally,the relationship among carbon-fixing microbial community structure and function and environmental factors was explored.Results show that:1)There are obvious differences in the algae community structure in nearshore surface sediments of six QTP lakes with different salilnity.On the whole,the Synechococcales of the cyanobacteria and the Ceramiales of the eukaryotic algae are dominant.The community structure of algae is significantly affected by various environmental factors?e.g.DOC,DIC,temperature,salinity and turbidity?.2)Functional genes of the Calvin-Benson-Bassham Cycle was the only one carbon fixation pathway detected in the non-algae?Prokaryotes other than Cyanobacteria?carbon-fixing microbial communities in the nearshore surface sediments of six QTP lakes,and it was detected in all sampling lakes.Therefore,the Calvin-Benson-Bassham Cycle may be the most important carbon fixation pathway for microbes in lake sediments located in the northern QTP,and this pathway may exist in the“full salinity environment?from freshwater to salinity saturation?”.3)In all lake samples,microbial communities detected by the Calvin-Benson-Bassham Cycle function genes cbbL and cbbM are mainly?-,?-,?-proteobacteria in Proteobacteria,including a large number Sulfur oxidizing bacteria?SOB?and a small amount of ammonia oxidizing bacteria?AOB?.Some functional gene communities are significantly affected by environmental factors such as salinity and pH.4)The microbial carbon fixation functional genes are extensive and abundant in QTP lake nearshore surface sediments.And the functional gene abundance distribution was cbbL?red?>cbbL?green?>cbbM on the whole.5)The dark carbon fixation is widespread in northern QTP lake sediments with the fixation rate of 0.50-6.02?g C g^-1 h^-1,and playing an important role in the total carbon fixation of lakes,especially high-salt lakes?The contribution rate is 0.7-41.1%?.6)pH,Salinity and other environmental factors have significant effects on light and dark carbon fixation rate and the ratio of dark carbon fixation in lake sediments.The above findings reveal the community structure and abundance characteristics of carbon-fixing microbes in the lake nearshore surface sediment environment of QTP,and prove that the dark carbon fixation is a considerable component in the carbon cycle of QTP lakes.In addition,the significant correlation between environmental factors and carbon-fixing microbial communities and functions suggests that the microbial mediated carbon fixation of lakes is significantly affected by the environment and climate,offering certain implications for the scientific and accurate assessment of the carbon fluxes of lake ecosystems.