Abstract: In order to compare the bacterial communities constitution and distribution in three sites of seagrass sediment in Xincun Bay, South China Sea, Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR-DGGE) analysis compounded with cloning and sequencing bands in DGGE profile was used. The results diverse bands of DGGE profile showed the most similarity of bacterial communities was found between the second and the third site in spring with an coefficient of 81.5%, whereas the least was 29.3% between the first and the third site in autumn. Shannon diversity indexes of the bacteria in spring and winter were higher than other seasons, which were of 3.03-3.07 and 3.23-3.25, respectively. Further, based on the 16S rDNA-V3 fragments from 42 individual DGGE bands and clone library sequence, the results showed most of the sequences belonged to Proteobactetia and Firmicutes. The most abundance Proteobactetia was Gammaproteobacteria with a percentage of 65.80%. Cyanobacteria were widely distributed in all samples. In addition, there were differences in bacterial communities among different seasons and sites. Psychrobacter and Pseudoalteromonas were advantage groups in the second and the third site during autumn, with a percentage of 31.61% and 19.41%, respectively.

Key words: seagrass sediment, bacterial communities constitution, South China Sea, Xincun Bay