| Phytoremediation is a strategy that use specific plants and their associated microbes for environmental cleanup. Although in soil habitat, many studies have indicated the ecological correlations and co-remediation mechanisms between the terrestrial plants and their rhizospheric microorganisms, so far in aquatic habitat, especially in seawater environment, few issues had been touched yet. Based on our previous work on the floating phytoremediation in seawater wetland system, we studied the role and the mechanisms of rhizosphere microbes in phytoremediation of seawater polycyclic aromatic hydrocarbons (PAHs) by halophytes. Both microbial and functional diversities in the rhizosphere of the typical remediating plant, Sesuvium portulacastrum were explored via metagenomic analysis. Under the direction of the results of molecular ecology, the typical PAHs-degradative bacteria from rhizosphere were isolated and their characteristics on degradation were analyzed. Moreover, the typical PAHs-degradative bacteria were re-inoculated to their host in microcosms, under which the symbiotic relation and co-remediation mechanisms were uncovered. The results of this study provide meaningful insights into the symbiosis of microbe-plant as well as the practical significance of PAHs bioremediation in seawater environments. The results were as follows:(1) High-throughput sequencing was conducted to study the bacterial community structure. The results showed that, there were44classes from26phyla in Yundang Lagoon. Proteobacteria was the dominant phylum which included4main classes, Gammaproteobacteria, Alphaproteobacteria, Epsilonproteobacteria and Deltaproteo-bacteria. Furthermore, Cyanobacteria and Bacteroides were also the common phyla. Bacterial community structures of different samples were complicated at genus level, which contained many reported functional bacteria. According to the diversity index from high to low, the order was root, rhizosphere, non-rhizosphere and non-floating bed. These results showed that the rhizosphere effect was significant in eco-floating bed system.In order to study the Beta diversity of bacterial community structure, sample distances, clustering and PCA were calculated using Weighted Unifarac, and the results showed that the bacterial diversity and community structure might be affected by the existence of root. The samples of both rhizosphere and non-rhizosphere from the same site were clusted together, while the samples from root and non-floating bed were different from other samples.(2) In all,4phyla,55genera and140species of culturable bacteria were obtained from the enrichment and domestication culture. Using modified sublimation method, three PAHs-degradative bacteria, identified as Celeribacter neptunius strain R1, Erythrobacter vulgaris strain R2and Novosphingobium resinovorum strain R3, were isolated from the rhizosphere of5. portulacastrum. They can utilize phenanthrene as the sole carbon. After7days incubation,72.17%,82.25%and82.94%of phenanthrene (initial concentration100ppm.) were degraded, respetctively. The results indicated that PAHs-degradative bacteria have high phenanthrene degradation capability.(3) After14days incubation, the phenanthrene removal rate of different groups presented the order of CK (18.9%), S+Phe (34.3%), S+Phe+T (41.7%), S+B+Phe (82.2%), NS+Phe (87.8%) and S+B+Phe+T (97.3%). By subtraction method, results showed that the contribution of phenanthrene removal caused by different factors presented the order of combined effect of root and original bacteria (78.5%), combined effect of sterilized root and strain R3(77.0%), R3(58.3%) and sterilized root (18.8%). Hence, it was suggested that the interrelationship between the root of S. portulacastrum and rhizosphere PAHs-degradative bacteria might play the most important role in the removal of phenanthrene.PCR-DGGE was adopted to investigate the bacterial compositon and community dynamic changes during the experiment. The results showed that, the addition of phenanthrene, PAHs-degradative bacteria and sterilized method might influence the bacterial community structure most. The structures were more complex in treatment of non-sterilized root, compared to treatment of sterilized root, i.e. the sterilized method inhibited the growth of bacteria effectively. Moreover, the band that represented the re-inoculated PAHs-degradative bacteria was persistently existed and it showed that strain R3might be the dominant bacteria during the rhizodegradation of phenanthrene. Hence, the re-inoculation in microcosms was successful. |
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