Envirnmental Response Mechanism Of Abundant And Rare Bacterial Taxa To Thermal Stratification Evolution In Gangnan Reservoir

As an important source of drinking water in Shijiazhuang,the water quality of Gangnan Reservoir directly affects the safety of water supply in the city.Bacteria play an important role in water quality evolution.However,the existence of thermal stratification in Gangnan Reservoir results in the spatiotemporal heterogeneity of the interaction between water quality and microorganisms.Therefore,exploring the microbial driving mechanism of water quality evolution can provide a scientific basis for further understanding of water quality evolution and improvement of water quality.Therefore,taking the main reservoir area of Gangnan Reservoir as the research object,the spatio-temporal evolution characteristics of water quality and microorganisms were explored based on high-throughput absolute quantitative technology during the thermal stratification evolution.Moreover,the environmental responses mechanism of the abundant and rare bacteria to the thermal stratification were focused on in Gangnan Reservoir.The main conclusions are as follows:1)The response characteristics of environmental factors during the evolution of thermal stratification in Gangnan Reservoir were studied.There was thermal stratification in Gangnan Reservoir from April to October,and the total nitrogen（TN）,nitrate（NO₃^--N）,manganese（Mn）,protein-like components（C1）and humus-like components（C2+C3）exhibited significantly difference（P<0.05）in weak stratification period（WSP）,stable stratification period（SSP）and mixing period（MP）,respectively.Among them,WSP showed high concentration of dissolved organic matter（DOM）,SSP exhibited high Mn(0.24 mg·L^-1),and MP presented high TN(4.86 mg·L^-1)and NO₃^--N(4.42 mg·L^-1).Moreover,there were significant differences for environmental factors in the vertical stratification of SSP（P<0.05）.The correlation between DOM components and water quality factors is different in the evolution process of thermal stratification and in different vertical stratification.2)The spatial and temporal distribution characteristics of abundant and rare taxa during the evolution of thermal stratification were analyzed.The a-diversity of abundant and rare taxa both exhibited significantly difference during the thermal stratification periods（P<0.001）.In detail,a-diversity of abundant taxa was highest in MP,and that of rare taxa was highest in SSP.Moreover,the a-diversity of rare taxa was higher than that of abundant taxa（P<0.001）.Both abundant and rare taxa mainly belonged to Proteobacteria.Moreover,the highest abundance of Proteobacteria for abundant taxa and rare taxa presented MP（38.95±6.46%）and SSP（4.13±1.52%）,respectively.For abundant taxa,the dominant genus was Candidatus＿Pelagibacter,and its absolute abundance was the highest in MP.For rare taxa,Flavobacterium,Parcubacteria and Fluviicola are the dominant genera in WSP,SSP and MP,respectively.The community distance of abundant and rare taxa both exhibited significantly difference in different water periods（P<0.001）,and the rare taxa was significantly higher than the abundant taxa（P<0.001）.In SSP,the community distance of abundant and rare taxa in the epilimnion was significantly higher than that in the isothermal layer（P<0.05）.The correlation between community distance and environmental factors differences showed obvious temporal and spatial differences and taxa differences.In detail,the abundant taxa had the highest correlation in WSP,while the rare taxa had the highest correlation in MP.Moreover,for the vertical perspective,the correlation of thermal stratification period was lower than that of mixing period,the correlation of bottom was higher than that of the surface,and the correlation of rare taxa was higher than that of the abundant taxa.3)The environmental response mechanism of abundant and rare taxa to the evolution of thermal stratification was explored.The correlation between a-diversity of abundant and rare taxa and environmental factors was significantly different in spatiotemporal and taxa perspective.Meanwhile,the environmental driving factors of microbial community evolution also showed significantly spatio-temporal differences.In detail,C2 and T were the most critical factors to drive the thermal stratification evolution of different subcommunities,and NO₃^--N and T were the most critical factors to drive the vertical distribution of different subcommunities in water stratification and mixing period,respectively.Network analysis showed that water stratification increased the association between rare taxa and environmental factors,but decreased the association between abundant taxa and environmental factors.The correlation between key environmental factors and species showed obvious thermal stratification and vertical evolution characteristics in the following perspectives,including associated environmental factors,abundant/rare taxa,and species.Structural equation model showed that NO₃^--N obtained the highest total effect onβ-diversity of abundant and rare taxa in water stratification period,while the T presented the highest total effect in water mixing period.Meanwhile,there were also vertical and taxa differences in the quantitative effects of environmental factors for driving the evolution of abundant and rare taxa in different thermal stratification periods.4)The community construction process and driving mechanism of abundant and rare taxa during the evolution of thermal stratification were investigated.The communities of abundant and rare taxa were mainly controlled by dispersal limitation.The effect of dispersal limitation on abundant taxa was reduced by water stratification,while that on rare taxa was enhanced by water stratification.Moreover,the effect of dispersal limitation on abundant and rare taxa was different in different vertical stratification.The phylogenetic distance of abundant and rare taxa both exhibited significantly difference in different thermal stratification periods（P<0.001）,and the rare taxa was significantly hjgher than the abundant taxa（P<0.001）.Moreover,the stratification led to the increase of phylogenetic distance between abundant and rare taxa.The correlation between phylogenetic distance and environmental factors and the driving factors of community construction process showed obvious spatio-temporal and taxa differences.Among them,C1,TP and NH₄⁺-N were the most critical environmental factors for driving the assembly of abundant taxa in WSP,SSP and MP,respectively.C2,NO₂^--N and T are the most important environmental factors for driving the assembly of rare taxa in the WSP,SSP and MP,respectively.