The Effect Of Precipitation Events On Carbon Fixed Microbial Community Structure In Karst Groundwater

Karst groundwater microbes,as one of the key driving forces of the carbon cycle in karst ecosystem,play a role in driving the transformation and recycling of bioactive elements such as carbon and nitrogen.Therefore,understanding the community change of carbon fixed microorganisms in karst groundwater during rainfall and uncovering the link between carbonate ions and the quantity of carbon fixed microorganisms is beneficial to develop the process of carbon cycling in karst groundwater and its microbial mechanism.At present,progress has been made on the carbon sink effect of rivers,plants,and soils in karst area,but little is known about the carbon sink in groundwater by microorganisms.For this purpose,three karst springs located in Guilin,Guangxi,were selected for our study,and a representative rainy day in summer was selected for groundwater sampling,resulting in a total of 114groundwater samples.Used field measurements combined with laboratory measurements to derive the dynamics of water chemistry during rainfall,and clarified the structural differences in the microbial community of karst groundwater under different rainfall conditions based on bioinformatics technology methods to determine the changes and amounts of the carbon fixed gene cbb L gene during rainfall,and characterized the quantitative changes in solid carbon microorganisms.Multiple regression models were used to model the quantitative relationship of cbb L gene with Ca²⁺and HCO₃^-.The findings suggest that:（1）All of the groundwater samples composed of Ca²⁺and HCO₃^-,and the hydrological characteristics,hydrogeological conditions,and hydrochemical characteristics of CF1,S31,and CF4 are different.In CF1 and S31,Ca²⁺,conductivity and calcite saturation index all decrease with rainfall,while p H,HCO₃^-and CO₂partial pressure increase with rainfall.The conductivity change of CF4 is opposite.Karst springs with similar aquifer conditions have similar chemical characteristics,and the proportion of ions formed tends to be the same.The hydrochemical dynamics are not only influenced by rainfall,but also by the combined action of water-rock-gas.（2）During rainfall,cbb L bacterial community of groundwater in different karst springα、βThe diversity is significantly affected by environmental factors,and the specific factors in different karst spring have different impacts.Overall,Ca²⁺,conductivity,and rainfall promoted diversity,while oxygen content and p H inhibited it.The dominant bacterial phyla of cbb L bacterial community in groundwater of different karst spring are also different.Chloroflexi,Proteobacteria and Bacteroidetes are the dominant bacteria with the highest abundance in the total groundwater samples during rainfall.Although their relative abundance exhibits certain regularity during the rainfall process,due to the influence of accumulated rainfall and the time of microbial community succession,the relative abundance of dominant bacteria corresponding to the same rainfall is generally different.（3）The fluorescence quantitative PCR results showed consistency between the number of cbb L genes and the cumulative rainfall peak,and the number of carbon fixing microorganisms also increased with the progress of rainfall.The multiple regression model based on the number of Ca²⁺-HCO₃^-cbb L genes during the rainfall process shows that CF1 and S31 are significantly affected by Ca²⁺,CF4 is significantly affected by HCO₃^-,and the changes in the cbb L bacterial community in surface karst fissure groundwater are dominated by Ca²⁺,while the changes in the cbb L community in karst pipeline groundwater are dominated by HCO₃^-.The multiple regression prediction model based on CF1 data shows a good fit compared with the actual situation,indicating that although there are differences in specific karst spring,the number of carbon fixing bacteria in karst groundwater can be predicted and evaluated by mathematical models in general.