Study On Structure And Influence Factors Of Bacterial And Archaeal Community In The Hypoxic Area Of Changjiang River Estuary

The Changjiang River Estuary(CRE)is a rich and sandy delta estuary with many tributaries in China.It is strongly influenced by ocean currents and water masses all year round.Under the combined influence of huge amount of runoff,complex watersheds and special regions,it presents a unique estuarine ecosystem.Seasonal hypoxia occurs at the bottom seawater of the CRE and its adjacent sea area in summer,and the existence of hypoxic zones and the entire East China Sea ecosystem will have a serious impact.Microorganisms play an indispensable role in marine ecosystem and biogeochemical cycle,and are important producers and decomposers.This thesis focuses on the ecosystem of the hypoxic zone of the CRE,combining microbiology,molecular biology,biostatistics and ecology and other research methods to study the distribution of bacterial and archaeal communities in the sea area on a short time scale and spatial scale and the relationship between microbial communities and environmental factors,and the representative of the microbial groups are studied in detail.Firstly,16 S rRNA gene analysis was used to explore the changes of bacterial communities in the oxic surface and hypoxic middle seawater layers at D3 site during24 hours in the CRE.Significant differences between the hypoxic and oxic layers were observed: the phyla Cyanobacteria,Bacteroidetes and Verrucomicrobia were enriched in the oxic layer,whereas the phylum SAR406 and the Verrucomicrobia were more abundant in the hypoxic layer.In addition,some subtle diurnal variations of the bacterial relative abundance were found in both two layers.The relative abundance of Synechococcus increased at night,and this change was more obvious in the hypoxic layer.The similar trend was also found in some phototrophic and several heterotrophic bacteria,such as Rhodobacteraceae,OM60 and Flavobacteriaceae.Their relative abundances peaked at 16:00 in the oxic layer,while the relative abundances peaked at around 7:00 and decreased until 13:00 in the hypoxic layer.Together,the results of the present study suggest that some photosynthetic bacteria and several heterotrophic bacteria have similar diurnal variations implying the light and physicochemical heterogeneity in the course of a day are important for bacterial diurnal changes in the CRE.Secondly,16 S rRNA gene analysis was used to investigate the archaeal communities in the surface and bottom seawaters of the CRE.Significant differences in the archaeal community composition were found between the two layers(p < 0.001).Marine-Group-II was dominant in the surface layers,while Nitrosopumilales and Marine-Group-III were enriched in the bottom(p < 0.001).Instead of dissolved oxygen,temperature and salinity were the most vital environmental variations in the distribution of archaeal communities.Some predicted functional subcategories were enriched in the hypoxic condition,like replication and repair,membrane transport,glycan biosynthesis and metabolism(p < 0.001),indicating the strong adaptability of archaea to harsh bottom seawaters.These findings expand the understanding on archaeal structure and functions,and contribute to further works of the archaeal community in the CRE.The study supplemented the distribution of microbial community structure in the hypoxic zones of the CRE,and analyzed the variation of microbial community combined with environmental factors,which has certain reference value for further study of the distribution pattern of microbial community in the CRE and its adjacent waters.