| Polycyclic aromatic hydrocarbons（PAHs）are typical persistent organic pollutants which widely exist in the environment.PAHs exhibt teratogenic,carcinogenic and mutagenic properties and can bioaccumulate into human body through food chains after accumulating in sediments.Vallisneria natans（V.natans）is an adaptable submersed macrophyte with the potential of remediating PAHs-contaminated sediments.However,how different sediment types influence PAHs remediation by V.natans are still not well understood.The adaptive strategies and the remediation potential of V.natans in heterogeneous habitats still need to be investigated.The structure and composition of microbial community along the sediment-V.natans continuum and interaction between V.natans and microbial community to enhance removal of PAHs in sediments remain to be elucidated.To address above issues,this study selected V.natans and sediments from two typical different levels of PAHs pollution areas,which were named as Dashui Harbor and Dingxiang River.A total of four microcosms were set up for 40 days’phytoremediation experiments.Firstly,two microcosms simulated the growth of V.natans in Dashui Harbor（high PAHs contaminated sites）and Dingxiang River（low PAHs contaminated sites）under natural conditions,assessing the phytoremediation ability by V.natans in different contaminated sediments.Secondly,two sediment microcosms（（1）sediment and overlying water from Dashui Harbor,with planted or unplanted V.natans from Dingxiang River,（2）sediment and overlying water from Dingxiang River,with planted or unplanted V.natans from Dashui Harbor）were performed to explore the adaptive strategies and the phytoremediation ability of V.natans in heterogeneous habitats.Concurrently,this study analyzed microbial community in sediments（with or without V.natans）and different niches of V.natans,aiming to explore interactions among V.natans,microorganisms and environmental factors.Through these works,this study contributes to understand the mechanism of PAHs degradation in sediment-V.natans continuum and set up novel decontamination strategies based on rhizodegradation between V.natans and microbial community.Specifc results were shown as follows:（1）In experiments which sediments planted with local V.natans:The removal efficiency of PAHs in rhizosphere sediment of Dashui Harbor（Dingxiang River）was 38%（16%）,which was higher than those in non-rhizosphere and bulk sediments,indicating that V.natans presence was particularly useful for PAHs remediation in sediments,especially in the highly contaminated sites.In addition,the levels of ferric iron（74.30±10.68μM/g）in rhizosphere sediment of Dashui Harbor was significantly higher than those in initial sediments,non-rhizosphere sediment and bulk sediment,indicating that macrophyte root oxygenation occur in the rhizosphere zone of Dashui Harbor V.natans during phytoremediation,thus contributing to the removal of PAHs.（2）In heterogeneous habitats experiments:The removal efficiency of PAHs in rhizosphere sediment,non-rhizosphere sediment and bulk sediment were 23%,24%and 55%,respectively（sediment from Dashui Harbor,V.natans from Dingxiang River）.Concurrently,compared with initial sediment（sediment from Dingxiang River）,the concentrations of PAHs decreased in bulk sediment while increased in rhizosphere sediment and non-rhizosphere sediment compared with initial sediment（V.natans from Dashui Harbor）.The results indicated that V.natans’ability of promoting PAHs degradation in sediments was inhibited in heterogeneous habitats.It is worth nothing that V.natans still posses the potential of promoting some PAH monomers degradation in heterogeneous habitats.For example,the concentration of acenaphthene in rhizosphere sediment was lower than that in non-rhizosphere sediment in both Dashui Harbor and Dingxiang River sites.In addition,the removal efficiency of acenaphthene differed in rhizosphere sediment between Dashui Harbor and Dingxiang River,indicating that V.natans’remediation potential on PAH monomers is affected sampling sites of sediment and V.natans.（3）In experiments which sediments planted with local V.natans:Steroidobacteraceae,Burkholderiaceae and Rhodocyclaceae were dominant in rhizosphere sediment of Dashui Harbor and the relative abundance of some function groups such as aromatic hydrocarbon degradation,aromatic compound degradation,and aliphatic non methane hydrocarbon degradation in rhizosphere sediment were significantly higher（p<0.05）than those in other sediments,indicating different degradation patterns between rhizosphere and non-rhizosphere;Rhodocyclaceae,Anaerolineaceae and Burkholderiaceae were dominant in rhizosphere sediment of Dingxiang River and the relative abundance of some function groups such as aromatic hydrocarbon degradation in rhizosphere sediment were significantly higher（p<0.05）than those in other sediments.In heterogeneous habitats experiments:（1）V.natans from Dingxiang River inhibited the increasement of Acinetobacter in Dashui Harbor sediment and the relative abundance of dominant bacterias such as Bacillus and Planococcaceae in rhizosphere sediment were also different from the other sediments.Inconsistent with the removal efficiency of PAHs,the relative abundance of some function groups such as aromatic hydrocarbon degradation and aliphatic non methane hydrocarbon degradation in rhizosphere sediment were higher than those in other sediments,suggesting that the remediation potential of rhizosphere sediment enchanced after Dingxiang River V.natans planted in Dashui Harbor sediment.（2）V.natans from Dashui Harbor decreased the relative abundance of Rhodocyclaceae and inhibited the increasement of the relative abundance of aromatic hydrocarbon degradation in Dingxiang River rhizosphere sediment,indicating the ability of degrading PAHs of rhizosphere sediment was inhibited after Dashui Harbor V.natans planted in Dingxiang River sediment.The results above showed that the structural and functions of microbial communities in sediments was shaped predominantly by types of sediment and sampling sites of V.natans.（4）In experiments which sediments planted with local V.natans:Selective enrichment of Herbaspirillum（relative abundance=94.80%）was observed in endosphere of Dashui Harbor V.natans,which increased V.natans’tolerance against PAH toxicity and V.natans’ability of degrading aromatic hydrocarbon;selective enrichment of Pseudomonas was observed in endosphere of Dingxiang River V.natans,which selectively promoted the degradation of some PAHs monomers in rhizosphere sediment.In heterogeneous habitats experiments:Selective enrichment of Herbaspirillum（relative abundance=44.12%）was also observed in endosphere of Dingxiang River V.natans and relative abundance of some function groups such as hydrocarbon degradation significantly increased（p<0.05）in rhizoplane and endosphere microbial communities increased after planted in Dashui Harbor sediment for 40 days,indicating that although the ability of degrading PAHs of V.natans decreased in 40 days,the potential of degrading PAHs of V.natans enchanced;the structure of the microbial communities of V.natans from Dashui Harbor shifted dramatically after planted in Dingxiang River sediment for 40 days and and relative abundance of hydrocarbon degradation decreased during phytoremediation,corresponding to the outcome that the degradation of PAHs was inhibited in rhizosphere sediment.The results above showed that the microbial communities and functions of V.natans were significantly influenced by sampling sites of sediments and V.natans.（5）The microbial communities in sediments of different experiments were significantly associated with environmental factors and their different combinations.Especially,Fe3+was significantly correlated with microbial communities only in Dashui Harbor sediments（experiments planted with local V.natans）.V.natans can release compounds into the environment,thus influencing structural and functions of microbial communities.V.natans in Dashui Harbor siginificantly（p<0.05）increased relative abundance of microbial community for dark iron oxidation and nitrate reduction while decreased relative abundance for sulfate respiration and iron respiration in Dashui Harbor sediment,thus acclecrating PAHs degradation.V.natans in Dingxiang River increased relative abundance of microbial community for nitrate reduction and sulfate respiration in Dingxiang River sediment,thus selectively promoted the degradation of some PAHs monomers.V.natans from Dingxiang River shifted relative abundance of function groups such as nitrate reduction,iron respiration and sulfate respiration in Dashui Harbor sediment.Although PAHs degradation in rhizosphere sediment was inhibited in 40 days,PAHs degradation potential may be enhanced in the long term.V.natans from Dashui Harbor inhibited relative abundance of microbial community for iron respiration and sulfate respiration in Dingxiang River sediment,thus inhibiting degradation of PAHs.In summary,this study clarified the remediation potential of V.natans in different levels of PAHs-contaminated sediments and heterogeneous habitats;this study analyzed the response of microbial communities during PAHs remediation process and revealed the interaction among V.natans,microbial communities and environmental factors.The findings provided the theoretical foundation for rhizoremediation using V.natans and would be helpful to evaluate and optimize novel decontamination strategies based on the rhizosphere and root interior interactions between plants and their microbial associates. |
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