The structure of sedimentary bacterial communities in mangroves depends on environmental factors such as pH, salinity, organic matter content, and metal pollution. To investigate the effect of heavy metal pollution on such communities, core samples of sediments from four sites in three distinct mangrove reserves (Golden Bay Mangrove Reserve in Beihai, Guangxi province (GXJHW), Shankou Mangrove Reserve in Hepu, Guangxi province (GXSK), and MaiPo mangrove in Hong Kong (MPCT and MPFQ)) in South China were analyzed for physicochemical properties, multiple chemical forms of metals, and vertical bacterial diversity. Sedimentary bacterial communities varied greatly among the different sampling sites, with biodiversity decreasing in the order of GXSK, GXJHW, MPFQ, and MPCT. Proteobacteria was the dominant phylum, followed by Chloroflexi, across all four sampling sites. Multivariate statistical analysis of the effect of environmental factors on the sedimentary bacterial communities found that total carbon was the only physicochemical factor with a significant influence at all four sites. The correlations between environmental factors and bacterial structure were weak for the two sites in Guangxi province, but strong at MPCT in Hong Kong where environmental factors were almost all significantly negatively correlated with bacterial diversity. Variance partitioning analysis revealed that physicochemical properties and chemical forms of metals could explain most of the changes in bacterial diversity. Overall, we observed that heavy metal forms were more important than total metal content in influencing the sedimentary bacterial diversity in mangroves, consistent with the more bioavailable metal species having the greatest effect.

In order to study the spatial variability and influencing factors of heavy metals contents in five mangroves in South China, surface sediments and plant canopy new leaves of mangroves were sampled to determine Cr, Ni, Cu, Zn, Pb contents. Moreover, the corresponding pollution load index of sediments and bioconcentration factors of leaves were evaluated. Meanwhile, the physical and chemical properties, organic carbon and nitrogen of surface sediments and new leaves were measured. It was found that the average content of heavy metals in the surface sediments of Futian Reserve was the highest with a pollution load index of 2.61, classified as strongly polluted. The particle size distribution of the five sites was mainly silt-sand, and the pH was slightly acidic. Multivariate analysis of variance indicated that both sampling locations and vegetation types had significant influences on heavy metal contents in surface sediments and leaves as well as bioconcentration factors of leaves. In sum, natural factors such as plant species and organic matter contents significantly influence the metal contents and translocation in mangrove forests. However, in strongly polluted areas anthropogenic factors compromise the natural effects and play the major role in metal distribution.

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