Community Structure And Function Of Anaerobic CO₂-fixing Microorganisms In Lake Sediments On Tibetan Plateau And Their Response To Environmental Factors

The aggravation of greenhouse effect has led to global warming,and biological carbon fixation is an important way to reduce greenhouse gas CO₂.Lake ecosystem play an important role in global carbon cycle,and microorganisms are the main drivers of carbon input and output in carbon cycle of lake.Qinghai-Tibet Plateau is the outpost of climate change,which lakes accounts for more than 50%of the total lake area in China.Qinghai-Tibet Plateau lakes show a trend of warming and salinization,which affects the absorption,transformation and release of carbon in the lake ecosystem.The temperature and salinity are key factors affecting the community structure and function of microbial community in lake ecosystem.In this study,the community structure and function of carbon-fixing microorganisms in lake sediments of Tibetan Plateau and their response to temperature and salinity was studied.The results are helpful to elucidate the role of CO₂-fixing microorganisms in carbon cycle of lake ecosystem on Tibetan Plateau,also study the response of ecosystem in Tibetan Plateau to climate change.The sediments of Dajiamangcuo Lake on the Qinghai-Tibetan Plateau were studied in this work,DNA-stable isotope probing（DNA-SIP）technology combined with metagenomics and highthrough sequencing were used to analyze community structure and function of CO₂-fixing microorganisms in the lake sedments using ¹³C labeled（H₂-）CO₂gas under two temperature（15°C and 20°C）and three salinity（0,40,and 80 g/L Na Cl）conditions.The autotrophic CO₂-fixing microorganisms in the sediments were enriched with H₂/CO₂ as the substrate and its community structure were also analyzed.The main results are as follows:（1）The temperature increasing from 15°C to 20°C significantly promoted the utilization of H₂ and CO₂ in the sediments,resulting in higher CO₂ fixation amount.However,temperature had no significant effect in the N₂/CO₂ incubations.The bacterial richness and diversity in the sediment increased significantly with the increase of temperature with the much higher alpha diversity index of 20°C than 15°C.Under autotrophic conditions,Clostridium sensu stricto 12domainated at 15°C,while Acetobacterium domainated at 20°C;under heterotrophic conditions,Prolixibacteraceae＿BSV13 and Cyanobium PCC-6307 were the main genus at 15°C,while Bacteroidetes vadin HA17＿norank,Pseudomona and Lentimicrobiaceae＿norank dominated at20°C.Elevated temperature increased the complexity of bacterial molecular ecological networks and microbial interactions,and the interactions between bacterial species were mainly positive correlations.The competition between bacterial species in the sediments was stronger at lower temperature.The bacterial ecological networks at 20°C were more structurally stable,better adapted to environmental change,and more resistant to disturbance than that at 15°C.Sequencing of heavy DNA from ¹³CO₂-labeled incubation samples shows that Clostridium sensu stricto 12 was the main functional CO₂-fixing bacteria under autotrophic conditions at15℃,while those under heterotrophic conditions were Sulfuricurvum and Desulfosporosinus.Metagenomic data show that carbon fixation in the lake sediment were mainly via Wood-Ljungdahl（WL）pathway,reductive tricarboxylic acid（r TCA）cycle and reductive glycine（r Gly）pathway.（2）Increased salinity significantly inhibited the activity of autotrophic CO₂-fixing microorganisms in the sediments and reduced the consumption rate and amount of H₂ and CO₂,but had less effect under heterotrophic conditions.The microbial abundance in the sediments increased with increasing salinity,and the microbial community structure was more similar.The microbial community structure was significantly different between the autotrophic conditions and heterotrophic conditions under low salinity（0 g/L Na Cl）,while the two microbial community structures were very similar the higher salinity（40 and 80 g/L Na Cl）.The complexity of bacterial molecular ecological network also enhanced,and the interactions between bacterial species were mainly positively correlated.Compared to low salinity,the bacterial ecological network under medium or high salinity was more stable,had a better adaptation to environmental change,and had stronger ability against disturbance.（3）During the enrichment of autotrophic CO₂-fixing microorganisms in the lake sediments,the consumption of H₂ and CO₂ was accompanied by the production of acetate,which accumulated gradually.With the enrichment time,the diversity and richness of bacterial community in the sediments significantly decreased,meanwhile,the microbial community structure tended to be stable,and the relative abundance of acetogen gradually increased.The dominant genus in the enrichment were Acetobacterium,Citrobacter,Desulfovibrio,Clostridium sensu stricto 12,Macellibacteroides,and Enterobacter.Metagenomic analysis shows that WL and r Gly pathway were the main carbon fixation pathways in the lake sediments,and Acetobacterium and Solidesulfovibrio were the main functional bacteria.In addition,CO₂-fixing microorganisms interacted with fermentative bacteria,sulfur oxidization/reduction bacteria,nitrate-reducing bacteria,and iron oxidization/reduction bacteria to commonly drive carbon,nitrogen,sulfur,and iron cycle in the sediments.