Bacterial diversity in the surface sediments of the hypoxic zone near the Changjiang Estuary and in the East China Sea.

Changjiang (Yangtze River) Estuary has experienced severe hypoxia since the 1950s. In order to investigate potential ecological functions of key microorganisms in relation to hypoxia, we performed 16S rRNA-based Illumina Miseq sequencing to explore the bacterial diversity in the surface sediments of the hypoxic zone near the Changjiang Estuary and in the East China Sea (ECS). The results showed that numerous Proteobacteria-affiliated sequences in the sediments of the inner continental shelf were related to both sulfate-reducing and sulfur-oxidizing bacteria, suggesting an active sulfur cycle in this area. Many sequences retrieved from the hypoxic zone were also related to Planctomycetes from two marine upwelling systems, which may be involved in the initial breakdown of sulfated heteropolysaccharides. Bacteroidetes, which is expected to degrade high-molecular-weight organic matter, was abundant in all the studied stations except for station A8, which was the deepest and possessed the largest grain size. In addition, dissolved organic carbon, water depth, percentage ratio of clay to silt, salinity, and sedimentary grain size were environmental effectors that shaped the sedimentary microbial community structure. Our results showed that putative Gammaproteobacteria-affiliated sulfur-oxidizing bacteria may not only detoxify hydrogen sulfide produced by sulfate-reducing prokaryotes, but also serve as the primary producers in the marine sediments. Specific groups of aerobic Bacteroidetes and Planctomycetes participated in degrading organic matter, which might contribute to the oxygen depletion in the hypoxic zones.