Diversity And Biogeographical Patterns Of Bacterioplankton Community In Three Subtropical Bays From China

Bacterioplankton are an important component of subtropical bay's ecosystem,having a high genetic diversity,and playing a key role in the global biogeochemical cyclesis.Thus,it is an important goal for understanding the diversity and biogeographical pattern of the marine bacterioplankton community in the study of ecology.However,unlike the general studies of animals and plants biogeography,little is known about the spatial distribution patterns and assembly mechanisms of bacterioplankton,especially that of abundant and rare subcommunities.Here,we used high-throughput sequencing to compare the diversity and explore their biogeography of bacterioplankton community(including entire,abundant and rare taxa)in the three subtropical bays(Shenhu Bay,Dongshan Bay and Beibu Gulf)from southern China.Study on bacterioplankton distribution pattern and potential driving mechanisms to reveal the importance of environmental selection and neutral processes in regulating microbial community structure.The main contents and results of the dissertation are as follows:1.Based on entire bacterial communities,our results indicated that Proteobacteria and Bacteroidetes were the most diverse bacterial phylum across all samples,of which the both classes(Alphaproteobacteria and Gammaproteobacteria)have high diversity and abundance in Proteobacteria.2.Based on abundant and rare bacterial subcommunities,our results indicated that the composition and genetic diversity of both subcommunities are significantly different.The richness(OTU number)of rare taxa was two orders of magnitude greater than that of abundant taxa,but the abundances of rare OTUs were about one tenth of the abundant ones.In addition,the most diverse and abundant groups in both abundant and rare subcommunities were assigned to Actinobacteria,Bacteroidetes,Cyanobacteria,Planctomycetes,Alphaproteobacteria and Gammaproteobacteria.We also found that Bacteroidetes,Alphaproteobacteria and Gammaproteobacteria exhibited significantly higher diversity(OTU number)and abundance(sequence number)in the abundant bacteria compared with the rare subcommunity.The network analysis showed that the Cyanobacteria,Actinobacteria,and Gammaproteobacteria were the most critical groups that caused diversity changes of abundant taxa.3.Analysis of biogeographical patterns of bacterial communities showed that bacterial assemblages(entire community,abundant and rare subcommunities)clearly separated the three subtropical bays,but the degree of community separation between the surface and bottom samples of each bay was small.All abundant OTUs(100%)were shared in the three bays,but the percentage of shared OTUs(6.20%)was much lower for the rare taxa.We found that entire,abundant and rare bacterial taxa have the similar biogeographical patterns and strong distance-decay relationships,but the dispersal of rare bacterial taxa was more limited than that of abundant taxa.4.Selective(environmental selection)and neutral processes(dispersal)are important determinants of community assembly and biogeographical patterns in three typical subtropical bays.Redundancy analysis(RDA)showed that six environmental variables(DO,Salinity,NH4-N,NO2-N,DSi,TN)and five spatial factors[principal coordinates of neighbor matrices(PCNMs)nos.1-4 and no.6]contributed significantly to explain the community composition of the entire bacterial community and the abundant bacterial subcommunity by forward selection(P<0.01).However,Canonical correspondence analysis(CCA)showed that four local environmental factors(DO,Salinity,NH4-N,NO2-N)and three spatial variables(PCNM no.1 and nos.3-4)exhibited significant effects on the variation of the rare bacterial subcommunity(P<0.01).5.Analysis of the relative contribution of selective and neutral processes to the biogeographical patterns of bacterial communities.Variation partitioning(based on adjusted R2 in redundancy analysis)showed that spatial factors exhibited a slightly greater influence on both abundant and rare subcommunities compared to environmental selection;however,the abundant subcommunity had a much stronger response to spatial factors(17.3%of pure variance was explained)than that shown by the rare bacteria(3.5%).The Mantel and partial Mantel tests further confirmed that the environmental selection and neutral processes explained the changes of the entire community,abundant and rare sucommunities.However,a large proportion of unexplained variation in the rare taxa(91.1%)implies that more complex assembly mechanisms may exist to shape the rare bacterial assemblages in the three subtropical bays.In addition,the results of the neutral model showed that the neutral processes affected the composition of the entire and abundant groups,but it had no effect on the rare taxa.The result of rare taxa is inconsistent with variation partitioning analysis,Mantel and partial Mantel test methods,which further indicates that the assembly and biogeography of rare bacterioplankton subcommunity may be affected by more complex ecological factors.We believe that the exploration of the biogeographical patterns of bacteria(entire community,abundant and rare subcommunities)and their ecological processes in the subtropical bay ecosystem will contribute to better maintain microbial diversity,thus promoting the implementation of sustainable development strategies for biodiversity conservation.