The Impacts Of Macrophytes Rhizosphere Effects On HCB Degradation In Constructed Wetlands

Hexachlorobenzene(HCB)has been listed among the twelve disabled persistent organic pollutants(POPs)by the Stockholm Convention.Owing to its lipophilic nature,HCB gets preferentially distributed in soil from air and ground water,then enters the food chain and accumulates in living organisms through lipid-rich cell membranes.It has chronic toxicity and "three-induced" effects,namely carcinogenic,teratogenic,and mutagenic effects,which seriously endangers the health of humans and ecosystems.Therefore,how to effectively remove HCB in the environment has been a hot topic.Although constructed wetlands(CWs)are considered to be effective for treating organic pollutants from environment,little is known about the mechanism of POPs biodegradation in this system.The study focused on the biodegradation characteristics of HCB in constructed wetlands and the impacts of macrophytes rhizosphere effect on HCB biodegradation.We simulated the constructed wetland microcosm system planted with Phragmites australis(P.australis)or Typha latifolia(T.angustifolia),in which the number of soil microorganism,the root exudates and the soil enzyme activity during the remove of HCB were measured.The main objectives of this study were to(1)compare the HCB degradation efficiency in water and soil of different constructed wetland treatments;(2)analyze the variation among the root exudates contents,soil enzyme activity and rhizosphere microorganisms in space-time scale,and the their interaction with responsing to HCB contamination and plant species;(3)identify common rhizosphere bacterial inhabitants and HCB-degrading bacteria.The main conclusions were shown as follows.(1)We used T.angustifolia grown in sterile Hoagland nutrient solution to determine the rhizosphere effects on the ability of bacteria in water to reduce HCB levels.The results revealed that T.angustifolia could facilitate HCB degradation and that the initial HCB concentration was the major factor responsible for HCB degradation in nutrient solution.Furthermore,HCB biodegradation in low-HCB nutrient solution with T.angustifolia fitted the first-order kinetics,owing to the high concentration of total organic carbon,low HCB toxicity,and unique bacterial community in the T.angustifolia rhizosphere.Denaturing gradient gel electrophoresis indicated that the rhizosphere effects and different dosages of HCB have significant effects on the bacterial community by repressing and favoring certain populations.The most successful bacteria to adapt to HCB contamination was Bacillus sp.,while the dominant bacterial phyla in HCB-polluted water were Proteobacteria and Firmicutes.(2)In constructed wetland,there was no correlation between HCB contents and root exudates,but the total sugar content in soil,was positively related to protein and amino acid contents(P<0.05)among three kinds of secretions.The three kinds of soil enzymes increased with HCB stress,but the reaction time was different.Soil dehydrogenase was most sensitive to HCB,next was polyphenol oxidase,followed by catalase.The HCB concentration have a notable negative correlation with dehydrogenase and polyphenol oxidase activities(P<0.05).(3)In order to understand the effects of rhizosphere distance on HCB biodegradation,we constructed the CWs,in which formed the rhizosphere and far from rhizosphere region by inserting nylon net,and analyzed the contents of HCB,the enzyme activity(dehydrogenase,catalase,and polyphenol oxidase),the root exudates(total suger,protein and amino acid),and the number of microorganism(bacteria,fungi,and actinomycetes).The results showed that after 91 days,the HCB concentrations decreased by 30.9%in the control unvegetated soil,40%,46.5%in the rhizosphere soil and 35.5%,31.2%in the non-rhizosphere soil vegetated with P.australis and T.angustifolia respectively,indicating that the degradation of HCB was:the rhizosphere>far from the rhizosphere>non-rhizosphere.There is a significant rhizosphere effects(R/S>1)in root exudates,soil enzyme activities and rhizosphere bacterial communites.The ratio of total suger,protein and amino acid contents was:2.9:3.9:3.3 in rhizosphere soil,moreover,the contents of root exudates from T.angustifolia were significantly higher than that from P.australis(P<0.05).The soil enzyme activity in T.angustifolia is also higher than that in P.australis,but the difference was not significant(P>0.05).The amount of soil microorganisms exhibited:bacteria>actinomycetes>fungi,and the number of bacteria accounted for 98%of the microbes.(4)The structure of the rhizosphere bacterial communities in different CWs was assessed by denaturing gradient gel electrophoresis(DGGE)and clone library based on PCR-amplified 16S rRNA gene.As expected,the rhizosphere bacterial communities also remained insensitive to HCB exposure in wetlands soil.The diversity of these microbes presented two stages,from the varied up and down to equilibrium during the entire experimental period.Molecular analysis revealed that the phylum Firmicutes dominated the bacterial communities.The genera that increased under HCB stress included the well-known HCB-degrading bacteria(Pseudomonas and Bacillus cereus)and other common bacteria found in contaminated soil but with lesser known practical functions(Burkholderia,Lysinibacillus fusiformis,and Achromobacter xylosoxidans subsp.)Furthermore,there was a certain difference in the relative abundances of the bacterial phyla and HCB removal efficiency among different CW treatments.The degradation of HCB in T.angustifolia microcosms was faster than that in P.australis and unvegetated wetlands,and the highest bacterial diversity and richness was found in the CWs comprising T.angustifolia.