The Diversity And Distribution Characteristics Of Bacteria In Seawater And Sediments Of The South China Sea

The ocean is the largest and most extensive habitat for marine organisms in the world,in which diverse groups of microorganisms are present.Microorganisms largely contributes to energy cycle and material transportation in marine ecosystem,and play a vital role in the global biogeochemical cycle of carbon,nitrogen,phosphorus,and sulfur.Detailed description of microbial community composition and distribution characteristics is expected to enhance the understanding of the role of microbes in the biogeochemical cycle,as well as the utilization of microbial resources.The South China Sea is the largest marginal sea in China,and even in the western pacific.It is affected by multiple physical factors such as the Kuroshio,circulation,upwelling,eddy currents,fronts and the inputs from rivers.Highly diverse habitats in the South China Sea enrich microbial resources.It is of great significance for the study on bacterial communities and fauna characteristics in the seawater and sediments in the South China Sea.This paper mainly analyzed the bacterial community composition and distribution characteristics in the seawater in the northern part of South China Sea and the sediments in the eastern part.Fifty three seawater samples were collected in the Pearl River EstuaryWestern South China Sea shared voyage in June and September 2018.Eleven seawater samples were collected in Beibu Gulf shared voyage organized by Guangdong Ocean University in July in 2019.Eleven sediment samples were collected in the northeast South China Sea in November 2020.High-throughput sequencing technology was used to perform V3-V4 sequencing of bacterial 16 S r RNA gene for all samples.A total of 512,666 sequences and 15,119 OTUs(total)were obtained from 11 sediment samples in the northeast South China Sea.The number of OTUs obtained from sediments varied among the samples.The orders of the abundance and diversity indices of sediment bacterial community among sampling stations were as follows:S7>S6>S8>S3>S11>S2>S10>S9>S4>S1>S5,S11>S7>S2>S9>S4>S10>S3>S5>S1>S6>S8.Based on the similarity of 97%,a total of 38 bacterial phyla including Proteobacteria,Planctomycetes,Actinobacteria,Bacteroidetes,Acidobacteria,Chloroflexi,Nitrospirae and some unclassified bacterial phyla were determined.At the Class level,γ-Proteobacteria(72.65%),δ-Proteobacteria(4.29%)and α-Proteobacteria(3.53%)were the most numerically dominated bacteria of the Proteobacteria.According to NMDS analysis and hierarchical cluster analysis,the results showed that there were significant differences in the bacterial community structure in different sediment samples.Except for S7 and S11,the composition of the bacterial communities in the remaining stations is relatively similar and divided into the T1 group,while the composition of the bacterial communities in the S7 and S11 stations is relatively similar and divided into the T2 group.OTUs with significant differences between the two groups include γ-Proteobacteria,Vibrioles,Vibrioceae,Vibrio,Alteromonadales,Pseudoalteromonadaceae,and Pseudoalteromonadaceae,Planctomycetes and Chloroflexi,Acidobacteria,Deltaproteobacteria,Nitrospirae,Bacteroidetes,and Spirochaetes.The significant differences between the two groups were mainly due to the occurrence and the abundance of Planctomycetes and Chloroflexi,Acidobacteria,Deltaproteobacteria,Nitrospirae,Bacteroidetes,and Spirochaetes.Correlation analysis of environmental factors showed that the Hg content in sediment had the greatest impact on bacterial communities,and there was a significant negative correlation.For the prediction of bacterial functional genes in sediments,it was found that the main functional genes were metabolism-related,with a total of 37 level 2 pathway-related functional genes and a total of 279 level 3 pathway-related functional genes.A total of 3,041,767 high-quality sequences and 2,891 optimized OTUs(without duplication)were obtained from 64 seawater samples from the northern South China Sea based on the similarity of 97% OTU.Through the comparison and annotation of OTU representative sequences,the sequences belonged to 37 Phylum,101 Classes,243 Orders,404 Families,and 753 Genus.A number of unclassified taxa were present at each classification level,indicating that there are some unclassified taxa in the Northern South China Sea.Alpha diversity analysis revealed that the richness indices of bacterial community in bottom seawater samples were greater than those in surface seawater samples.In addition,the diversity indices of bacterial community in bottom seawater samples were higher than those of the surface seawater.The composition of bacterial communities varied among sampling stations.Proteobacteria,Cyanobacteria,Actinobacteria and Bacteroidetes were the four most numerical dominated bacteria in seawater samples.They accounted for 39.20%,29.33%,11.00% and 10.62% of total sequences,respectively.Proteobacteria,Cyanobacteria,Actinobacteria,Bacteroidetes,Planctomycetota,Verrucomicrobiota,Firmicutes had varied abundances in all samples.At the class level,Cyanobacteria(29.67%),α-Proteobacteria(22.76%),γ-Proteobacteria(16.47%),Bacteroidia(10.71%)and Acidimicrobiia(8.52%)were the five most numerically dominated bacterial classes.The results of NMDS and hierarchical cluster analysis showed that there were differences in bacterial community structure in different sampling areas.The community structure of the bottom seawater samples and the surface samples at stations 37,38 and 39 were similar to that of the surface layer,and the similarity of other bottom seawater samples is high.The similarity of the bacterial community structure among the 11-site surface and other samples was low.The differences in bacterial community composition among the three groups of Beibu Gulf(B1-B11),Qiongdong(B43-B50)and bottom(all bottom seawater samples)were mainly due to the frequency and quantity of Actinobacteria,γ-Proteobacteria,Rhodospirillales and SAR324 group.There were no significant differences between Eastern Guangdong(B12-B30)and the other three designated groups.Bacterial-related functional genes in seawater were mainly metabolism-related functions.A total of 40 functional genes related to seawater bacteria were obtained in the second(level 2)pathway,and 264 related functional genes of seawater bacteria were obtained in the third(level 3)pathway.The dominant groups in seawater and sediments were different,α-Proteobacteria was the dominant group in seawater,and γ-Proteobacteria was the dominant group in sediments.In addition,due to the different collection seasons of seawater(summer)and sediments(winter),the dominant groups in seawater and sediments might also differed among seasons.There were differences in the number of functional genes obtained in different pathways of different samples,and there was no difference in general.Overall,this study described the bacterial community structures in typical habitats in the South China Sea,as well as functional genes and species differences between groups.The results were considered to be helpful to understand the distribution characteristics of bacterial communities,and to expand the microbial database in the South China Sea.