Polycyclic aromatic hydrocarbon (PAH) pollutants originating from oil spills and wood and fuel combustion are pollutants which are among the major threats to mangrove ecosystems. In this study, the composition and relative abundance in the sediment bacterial communities of naphthalene dioxygenase (ndo) genes which are important for bacterial adaptation to environmental PAH contamination were investigated. Three urban mangrove sites which had characteristic compositions and levels of PAH compounds in the sediments were selected. The diversity and relative abundance of ndo genes in total community DNA were assessed by a newly developed ndo denaturing gradient gel electrophoresis (DGGE) approach and by PCR amplification with primers targeting ndo genes with subsequent Southern blot hybridization analyses. Bacterial populations inhabiting sediments of urban mangroves under the impact of different sources of PAH contamination harbor distinct ndo genotypes. Sequencing of cloned ndo amplicons comigrating with dominant DGGE bands revealed new ndo genotypes. PCR-Southern blot analysis and ndo DGGE showed that the frequently studied nah and phn genotypes were not detected as dominant ndo types in the mangrove sediments. However, ndo genotypes related to nagAc-like genes were detected, but only in oil-contaminated mangrove sediments. The long-term impact of PAH contamination, together with the specific environmental conditions at each site, may have affected the abundance and diversity of ndo genes in sediments of urban mangroves.

BackgroundMangrove forests are of global ecological and economic importance, but are also one of the world's most threatened ecosystems. Here we present a case study examining the influence of the rhizosphere on the structural composition and diversity of mangrove bacterial communities and the implications for mangrove reforestation approaches using nursery-raised plants.Methodology/Principal FindingsA barcoded pyrosequencing approach was used to assess bacterial diversity in the rhizosphere of plants in a nursery setting, nursery-raised transplants and native (non-transplanted) plants in the same mangrove habitat. In addition to this, we also assessed bacterial composition in the bulk sediment in order to ascertain if the roots of mangrove plants affect sediment bacterial composition. We found that mangrove roots appear to influence bacterial abundance and composition in the rhizosphere. Due to the sheer abundance of roots in mangrove habitat, such an effect can have an important impact on the maintenance of bacterial guilds involved in nutrient cycling and other key ecosystem functions. Surprisingly, we also noted a marked impact of initial nursery conditions on the rhizosphere bacterial composition of replanted mangrove trees. This result is intriguing because mangroves are periodically inundated with seawater and represent a highly dynamic environment compared to the more controlled nursery environment.Conclusions/SignificanceIn as far as microbial diversity and composition influences plant growth and health, this study indicates that nursery conditions and early microbial colonization patterns of the replants are key factors that should be considered during reforestation projects. In addition to this, our results provide information on the role of the mangrove rhizosphere as a habitat for bacteria from estuarine sediments.

Municipal sewage, urban runoff and accidental oil spills are common sources of pollutants in urban mangrove forests and may have drastic effects on the microbial communities inhabiting the sediment. However, studies on microbial communities in the sediment of urban mangroves are largely lacking. In this study, we explored the diversity of bacterial communities in the sediment of three urban mangroves located in Guanabara Bay (Rio de Janeiro, Brazil). Analysis of sediment samples by means of denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments suggested that the overall bacterial diversity was not significantly affected by the different levels of hydrocarbon pollution at each sampling site. However, DGGE and sequence analyses provided evidences that each mangrove sediment displayed a specific structure bacterial community. Although primer sets for Pseudomonas, alphaproteobacterial and actinobacterial groups also amplified ribotypes belonging to taxa not intended to be enriched, sequence analyses of dominant DGGE bands revealed ribotypes related to Alteromonadales, Burkholderiales, Pseudomonadales, Rhodobacterales and Rhodocyclales. Members of these groups were often shown to be involved in aerobic or anaerobic degradation of hydrocarbon pollutants. Many of these sequences were only detected in the sampling sites with high levels of anthropogenic inputs of hydrocarbons. Many dominant DGGE ribotypes showed low levels of sequence identity to known sequences, indicating a large untapped bacterial diversity in mangrove ecosystems.

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