| In recent decades,with the change of global climate conditions and the disturbance of human activities,the input of allochthonous dissolved organic carbon(DOC)in freshwater lakes has been increasing year by year,and the concentration of DOC in lake water keeps rising,which affects the carbon balance of lake water,and then affects regional or global climate change.Aquatic microorganisms mediate the DOC transformation process and contribute greatly to the carbon dynamics and energy flow of aquatic ecosystems.However,there is a lack of research on how the continuous input of allochthonous DOC affects the microbial transformation process of DOC in lake water and whether there is a close relationship between the composition of DOC in water and the community structure of bacterioplankton have seriously hindered our understanding of the mechanism of allochthonous DOC transformation driven by microorganisms.Based on the above scientific problems,in this study,the microcosmic and mesocosmic experimental systems were used to simulate the response of bacterioplankton community composition to DOC input from different sources and different concentrations,as well as the relationship between the concentration of DOC components in water and the bacterial species.Two aspects were mainly studied: 1)the response of the bacterioplankton community to the input of DOC from different sources(sucrose,plants and soil extracts);2)the response of bacterioplankton community to DOC input of different concentrations.In this study,microcosmic and mesocosmic experiment,combined with three-dimensional fluorescence spectroscopy and highthroughput sequencing technology,were used to explore the response rules of the bacterioplankton community to the input of exogenic DOC.The main conclusions were as follows:1.Response of the bacterioplankton community to the input of DOC from different sources(1)The input of DOC from different sources significantly changed the composition of DOC,p H,DO and the concentration of nitrogen and phosphorus nutrients in the water within a short time(24 h),and then the differences between the treatments and control gradually narrowed with the extension of time.In addition,with the input of soil source DOC,the content of humus-like components in water was higher,and the aromaticity,hydrophobicity and molecular weight of DOC in water increased.The content of protein-like components was higher under the input of sucrose source DOC,and the DOC component in water had lower molecular weight and hydrophilicity,and its bioavailability is enhanced.The spectral characteristics of water DOC under the input of plant source DOC were in between sucrose and soil sources(sucrose < plant < soil).(2)The input of DOC from different sources promoted the increase of the number,richness and diversity of bacterioplankton,among which the plant and soil sources had the greater promoting effect.The input of sucrose source DOC significantly increased the relative abundance of Actinobacteria,the input of plant source DOC promoted the relative abundance of Bacteroidetes,and the input of soil source DOC significantly increased the relative abundance of Patescibacteria.In addition,the input of DOC from different sources enhanced the functions related to bacterial carbon metabolism(sucrose > plant > soil).Co-occurrence network analysis of bacteria-DOC components showed that the main bacteria involved in DOC transformation were Proteobacteria and Actinobacteria.Among them,The proportion of “generalist” species using different carbon components was higher than that of "specialist" species using specific carbon components,and the proportion of “generalist” species decreased with the increase of humus-like components in the DOC component(sucrose < plant < soil).2.Response of bacterioplankton community to DOC input of different concentrations.(1)After the input of different concentrations of DOC,the total DOC concentration of the water remained stable,but the internal DOC component changed.The content of humus-like components remained stable,but the protein-like components changed significantly.When the concentration of DOC in water was low(below 15 mg/L),the protein-like components increased with the increase of DOC concentration,and its molecular weight decreased and the degree of humification decreased.However,when the concentration of DOC was above 15 mg/L,the opposite trend appeared.(2)The number and diversity of bacteria increased at first and then decreased with the input of DOC at different concentrations,and reached the highest at 20 mg/L.The relative abundance of Cyanobacteria decreased with the increase of DOC concentration,while the relative abundance of Actinobacteria and Proteobacteria showed an opposite pattern.In addition,the carbon metabolism of the bacterioplankton community increased first and then decreased with the increase of DOC concentration,and reached the peak at about 15 mg/L.(3)Random forest regression analysis showed that the OTU of the bacteria associated with DOC concentration belonged to rich species(Proteobacteria,Actinobacteria and Bacteroidetes),and the inflection point of their response to DOC concentration appeared in the range of 15-20 mg/L.With the increase of DOC concentration,the number of bacterial species involved in the metabolism of DOC components showed a trend of first decrease and then increase(" U-shaped " curve distribution),and reached the lowest point at 15-20 mg/L concentration.Moreover,the number of "specialist" species showed a consistent pattern with the change of DOC concentration.These results indicate that there is a nonlinear correlation between DOC concentration and bacterioplankton community,and there is a threshold effect(DOC concentration in the range of 15-20 mg/L).In conclusion,our results showed that allochthonous DOC input can provide carbon subsidy for bacterioplankton bacteria growth.The lower degree of humification of DOC(sucrose and plant sources)input would bring more unstable components(protein-like components)and enhanced the function of carbon metabolism in bacteria,and increase the use of different carbon composition "generalist" species;however,higher degree of humification of DOC(soil source)input would lead to more stable components(humus-like components)and more complex bacterial community structure,which is beneficial to increase the number of "specialist" species using only a specific carbon component.In addition,the effect of DOC concentration on the bacterioplankton community presented a nonlinear relationship,and there was a threshold effect(the threshold value was in the range of 15-20 mg/L).Below or above this concentration threshold range,the DOC metabolism of bacteria will be limited to a certain extent.Therefore,this study indicates that the concentration and source of DOC in the input lake will affect the response mechanism of the bacterioplankton community and the carbon transformation process of the lake,and then affect the carbon balance of the lake ecosystem.This study will deepen the understanding of the important role of the whole freshwater lake ecosystem in the process of processing and converting exogenous organic carbon,and improve the understanding of the macroregulation mechanism of global carbon cycle and carbon transformation. |
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