The dissipation and risk alleviation mechanism of PAHs and nitrogen in constructed wetlands: The role of submerged macrophytes and their biofilms-leaves.

The role of submerged macrophytes (Vallisneria natans, Hydrilla verticillata and artificial plant) and their biofilms-leaves for the dissipation and risk alleviation mechanism of PAHs (phenanthrene and pyrene) and nitrogen in constructed wetland systems with PAH-polluted sediments were investigated. Biofilms-leaves/surface might contribute to PAHs degradation, which was positively correlated with PAHs degrading bacteria. Nitrogen-fixing bacteria in biofilms on surface might cause total nitrogen in sediment (TNs) increasing by 4% from 14th d to 28th d indirectly when suffering PAHs pollution. The relative abundance of nitrogen-fixing bacteria significantly increased with the increase of PAHs concentrations in early period (p < 0.01), which might lead to risk of nitrogen accumulation further. Heat maps showed that the relative abundance of functional bacteria were influenced in order of attached surface > incubation time > spiking concentration of PAHs. Interestingly, differences of deduced bacterial functions were affected in order of incubation time > attached surface > spiking concentration. Thus, submerged macrophytes and their biofilms on leaves not only played an important role in PAHs degradation, but also regulated the nitrogen cycling in constructed wetland systems, which could reduce these pollutants risk for natural environment, organisms and human health.

The responding and ecological contribution of biofilm-leaves of submerged macrophytes on phenanthrene dissipation in sediments.

The bacterial communities and ecological contribution of biofilm-leaves of the Vallisneria natans (VN), Hydrilla verticillata (HV) and artificial plant (AP) settled in sediments with different polluted levels of phenanthrene were investigated by high-throughput sequencing in different growth periods. There was no significant difference among the detected Alpha diversity indices based on three classification, attached surface, spiking concentration and incubation time. While Beta diversity analysis assessed by PCoA on operational taxonomic units (OTU) indicated that bacterial community structures were significantly influenced in order of attached surface > incubation time > spiking concentration of phenanthrene in sediment. Moreover, the results of hierarchical dendrograms and heat maps at genus level were consistent with PCoA analysis. We speculated that the weak influence of phenanthrene spiking concentration in sediment might be related to lower concentration and smaller concentration gradient of phenanthrene in leaves. Meanwhile, difference analysis suggested that attached surface was inclined to influence the rare genera up to significant level than incubation time. In general, the results proved that phenanthrene concentrations, submerged macrophytes categories and incubation time did influence the bacterial community of biofilm-leaves. In turn, results also showed a non-negligible ecological contribution of biofilm-leaves in dissipating the phenanthrene in sediments (>13.2%-17.1%) in contrast with rhizosphere remediation (2.5%-3.2% for HV and 9.9%-10.6% for VN).