Responses of bacterial communities in seagrass sediments to polycyclic aromatic hydrocarbon-induced stress

The seagrass meadows represent one of the highest productive marine ecosystems, and have the great ecological and economic values. Bacteria play important roles in energy flow, nutrient biogeochemical cycle and organic matter turnover in marine ecosystems. The seagrass meadows are experiencing a world-wide decline, and the pollution is one of the main reasons. Polycyclic aromatic hydrocarbons (PAHs) are thought be the most common. Bacterial communities in the seagrass Enhalus acoroides sediments were analyzed for their responses to PAHs induced stress. Dynamics of the composition and abundance of bacterial communities during the incubation period were explored by polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) and quantitative PCR assay, respectively. Both the incubation time and the PAHs concentration played significant roles in determining the microbial diversity, as reflected by the detected DGGE bands. Analysis of sequencing results showed that the Gammaproteobacteria were dominant in the seagrass sediments, accounting for 61.29 % of all sequenced bands. As PAHs could be used as carbon source for microbes, the species and diversity of the PAH-added groups (group 1 and 2) presented higher Shannon Wiener index than the group CK, with the group 1 showing the highest values almost through the same incubation stage. Patterns of changes in abundance of the three groups over the experiment time were quite different. The bacterial abundance of the group CK and group 2 decreased sharply from 4.15 × 1011 and 6.37 × 1011 to 1.17 × 1010 and 1.07 × 1010 copies/g from day 2 to 35, respectively while bacterial abundance of group 1 increased significantly from 1.59 × 1011 copies/g at day 2 to 8.80 × 1011 copies/g at day 7, and then dropped from day 14 till the end of the incubation. Statistical analysis (UMPGA and PCA) results suggested that the bacterial community were more likely to be affected by the incubation time than the concentration of the PAHs. This study provided the important information about dynamics of bacterial community under the PAHs stress and revealed the high bacterial diversity in sediments of E. acoroides. Investigation results also indicated that microbial community structure in the seagrass sediment were sensible to the PAHs induced stress, and may be used as potential indicators for the PAHs contamination.