Patterns And Successional Mechanisms Of The Bacterial Community In A Subtropical Riverine Ecosystem Of Bahe River

Nowadays,water pollution and water resource shortage are getting more and more serious,however,there are still many gaps in the research of aquatic ecosystems,especially,the investigation on the microbial community of the main water resource—freshwater river ecosystems is relatively weak.Thus,this data-intensive study investigated the spatial and temporal dynamics of bacterial communities from both water body and sediment in various microhabitats of 9 transects in a subtropical freshwater ecosystem of Bahe River(including its upper tributaries of Shengli River and Yishui River),and their response mechanisms to different pollution stresses,via comprehensive application of high-throughput sequencing,bioinformation technology,and modern theories of community ecology.The major contents and results are as follows:1.In both water body and sediment,Proteobacteria,Bacteroidetes,Actinobacteria,Cyanobacteria,Firmicutes and Verrucomicrobia were the major groups in the bacterial communities.Comamonadaceae was the most dominant taxa(15.12%±10.16%)in both ecosystems.The second dominant taxon was Flavobacteriaceae(9.13%±6.31%)in water body,while Chitinophagaceae(5.82%±2.94%)in sediment.Besides,the seasonal and spatial environmental factors posed cross effects on the operational taxonomic unit(OTU)numbers,Shannon diversity,Simpson evenness,phylogenetic species variability(PSV)and phylogenetic species evenness(PSE)of microbial communities in water body and sediment,meaning that the bacterial communities in different seasons showed different spatial variations,and counterparts from different transects also displayed different seasonal variations.2.The species abundance distribution(SAD)of microbial communities in both water and sediment were fitted to seven classical theoretical models,including broken stick,log-normal,log-series,geometric series,metacommunity zero-sum Multinomial distribution model(MZSM),Volkov and Zipf.It showed that the niche-based log-normal and neutral Volkov models could separately explain 45.1%and 41.17%of the water samples,and for sediment counterparts,the proportions were 55.55%and 13.33%respectively.Simultaneously,Sloan's neutral community model(NCM)fitted the metacommunity in water better than that in sediment(R²_water=0.83,R²_sediment=0.72),and the migration rate m of the water metacommunity was 0.33,higher than that in sediment,which was 0.19.These results indicated consistently that,deterministic processes played a great role in both water and sediment ecosystems,whereas neutrality was much more involved in water assemblages than in non-fluidic sediment ecosystems.3.156 water samples from 5 transects(W5-W9)under different pollution gradients along Yishui River and Bahe River were also studied through NCM and co-occurrence network analyses.It was found that in the clean upstream river(W5),NCM fitted the metacommunity quite well(R²=0.87,m=0.65),and the co-occurrence network contained fewer nodes(170)and edges(539),also showed a lower average clustering coefficient(0.41)as well as a higher modularity(0.51).When the pollution stress was elevated,for example at the sewage outlet(W9),the goodness-of-fit to the NCM declined(R²=0.76,m=0.40),and more nodes(283)and edges(3453)were involved into the co-occurrence network,resulting in a higher average clustering coefficient(0.52)together with a lower modularity(0.34).These results demonstrated that in clean,mild,and appropriate conditions,bacterial assemblages were more affected by stochastic processes,and organized via relatively loosely linked taxa,which could be divided into more modules.Yet,when conditions became degraded under heavy pollution,the influence of neutrality diminished,and niche-based selection imposed more constraints on communities,inter-species connections were strengthened and many small modules merged into fewer larger ones.These changes made communities more sensitive to fluctuations in order to cope with the harsh conditions efficiently.4.Furthermore,Guimera et als'node division method was applied in this study to probe into the characteristics and functions of keystone taxa in the bacterial communities from Bahe River catchment.It was showed that,module hubs were mostly neutrally distributed(83.33%)generalists with higher abundances(0.91%),more positively correlated to the rest species(63.49%).Whereas connectors were less abundant(0.41%),their distributions were more subjected to the environments(32.43%),and among their correlations with other taxa,the proportion of negative ones was higher(44.51%)than module hubs(32.43%).Thus,it was deduced that,connectors were probably responsible for the information transmission between microbial communities and environments,as well as between different modules,thus guided the direction of community succession.Therefore,this study systematically described the spatial and temporal patterns of bacterial communities in a typical subtropical freshwater continuum,revealed the mechanisms of niche/neutrality balancing throughout the successions of communities in water body and sediment,and its effects on community structures and diversities.Furthermore,the associated mode of“various pollution gradients—niche/neutrality balancing—community structure—key taxa”was proposed,which could deepen the understanding of community patterns and succession processes,and provide scientific theoretical basis for human to appropriately develop microbial resources,as well as to cope with water pollutions.