Study On Transportation And Toxic Effects Of Typical PAHs In Mangrove Wetland Simulation System

The mangrove ecosystem,as one of the most important intertidal estuarinewetlands along tropical and subtropical coastlines,fulfills important ecosystemservice functions.With the increasing development of destructive activities in coastalecosystems,more and more industrial,agricultural wastewater and living sewage aredischarging directly into offshore marine areas.Human activities have seriously led tothe damage of many mangrove wetlands.The unique features of mangroves such ashigh primary productivity,rich organic carbon and anoxic conditions make them alocation for uptake and preservation of PAHs from anthropogenic inputs.It is veryimportant to investigate the behaviors and environmental effects of PAHs compoundsin mangrove swamps.Phenanthrene (PHE) and fluoranthene (FLA) were selected from 16 priorityPAHs (US EPA) because they were the main representatives of 3-ring and 4-ringPAHs,showing high concentration in mangrove sediments and water.Therefore,theintention of this study was to provide information of the transport and transformationof typical PAHs in simulation systems containing water,sandy soil,algae and plant.Simultaneously,toxicity of typical PAHs to plant and algae was studied.Accumulation and degradation of typical PAHs by the algae enriched from amangrove aquatic ecosystem were discussed.Some successful results have beenreceived.(1) Mangrove plant (Kandelia candel) propagules were obtained from JiulongRiver Estuary Mangrove Nature Reserve,China.K.candel seedlings were cultivatedin PHE or FLA contaminated sandy soil for determining the distribution andtransportation of PHE and FLA in a soil-plant system.The sandy soils withconcentrations of PHE (0,1,5,10,50 and 100μg·g^-1) and FLA (0,2,20,40,80 and100μg·g^-1) were prepared.The results showed that the translocation of PAHs fromroot to shoot was an important pathway for the PAHs intake by plant above-groundpart.The root,shoot and leaf accumulations of PHE and FLA were enhanced with theincrease of their concentrations in the sandy soil.As the exposing time passed,PHEand FLA concentrations of different plant tissues increased significantly in comparison with the control.PHE concentrations of root,shoot and leaf in 13 weeks'treatment were ranged between 0.28～4.36μg·g^-1,0.04～0.84μg·g^-1 and 0.03～0.19μg·g^-1,respectively.And FLA accumulations of root,shoot and leaf were in the rangeof 0.79～35.76μg·g^-1,0.07～1.49μg·g^-1 and 0.06～0.13μg·g^-1,respectively.Thiswork consistently showed that root accumulation of FLA was greater than that of PHE,as shown by root PAH concentrations or RCF.However,the shoot concentrationfactor (SCF) and the leaf concentration factor (LCF) of PHE were much higher thanthose of FLA.In this research,higher values of BCF were more often noted in thePHE treated pots than in the FLA treated pots.The percentage uptake of PHE andFLA by K.candel from sandy soil was 31.7% and 19.7% in average.(2) The influence of increasing concentrations of PHE and FLA on theeco-physiological responses of K.candel,including total biomass,chlorophyll content,photosynthesis and root activity,were investigated.Results were as follows:Afterexposure to PAHs,the roots of K.candel appeared swelling,and then turned blackand rotten.With increasing concentrations of PAHs,the root activity of K.candelseedlings increased gradually before 6 weeks and then decreased after 9 weeks.Withthe prolonging of culture time,the root activity of K.candel seedlings increased atfirst and then decreased;while water content and the root to shoot ratio of K.candelseedling showed no significant difference from the control group at variousconcentrations of PHE and FLA.However,the root biomass and above-groundbiomass of K.candel seedlings decreased significantly in comparison with the control,with an increase of PAHs concentration in the sandy soil.The chlorophyll a,bcontents in the leaves of K.candel decreased moderately with an increase of PAHsconcentration in the sandy soil.Moreover,decrease in stomatal conductance reducedthe input of CO₂,which may be one of the reasons for decreasing net photosyntheticrate of K.candel seedlings leaves.With increasing concentrations of PHE and FLA,decrease in the transpiration rate of K.candel seedling was observed.The effects ofPHE and FLA on the growth of K.candel seedling were similar.However,the toxiceffect of PHE on K.candel was more serious than that of FLA.The tolerance of K.candel to PHE and FLA depended on different physical and chemical properties of thePAHs.(3) In the algae culture system,this study focused on the toxic effect and accumulation and degradation of the two typical PAHs on diatoms found in mangrovearea in Jiulongjiang river estuary,Fujian.After separated,purified and cultured,Skeletonema costatum and Nitzschia sp.were exposed to different concentrations ofPHE and FLA,respectively.The results showed that the values of EC₅₀ of 72 h forPHE to Skeletonema costatum and Nitzschia sp.were 0.95 mg·L^-1 and 0.32 mg·L^-1,respectively.While those of 72h-EC₅₀ of FLA on inhibition of the growths ofSkeletonema costatum and Nitzschia sp.were calculated as 0.17 mg·L^-1 and 0.09mg·L^-1,respectively.The tolerance of Skeletonema costatum to PHE and FLA wasbetter than that ofNitzschia sp..The toxic effect of FLA on Skeletonema costatum andNitzschia sp.was higher than that of PHE.As for accumulation and degradation ofPAHs by diatoms,the results were as follows:After being added into the media for 6h,PAHs were found in both microalgal species,suggesting that Skeletonema costatumand Nitzschia sp.removed PAHs from the media through rapid adsorption andabsorption.For Skeletonema costatum and Nitzschia sp.,the percentage degradationof PHE reached 16.4% and 54.8% after 168 h incubation,and the degradation of FLAwas only up to 6.9% and 2.7%.The results showed that the degradation of PHE wasfaster than that of FLA.In this study,simultaneous degradation of a mixture of PHEand FLA in f/2 medium by Skeletonema costatum was observed,and a synergisticeffect was found.The percentages of PAHs remaining in the media were significantlydifferent at different incubation time,while the differences also found in the rates ofaccumulation and degradation of PAHs by the two microalgal species.(4) Dissipation of PAHs in water in the presence of mangrove plant and algaewas studied.The results indicated an initial rapid uptake of the PAHs by root of K.candel seedlings and Nitzschia sp.,and the corresponding water concentrations oftested PAHs decreased constantly,along with the uptake time.The decrease amplitudeof PAHs in water phase followed the order:plant-algae system＞plant system＞algaesystem.For example,at about 24 h,the concentrations of PHE in water phase ofplant-algae,plant,algae system decreased the amplitude of 78.5%,66.7% and 12.1%,respectively.While those of FLA decreased the amplitude of 88.8%,84.3% and18.2%,respectively.The results suggested that the dissipation of FLA was faster thanPHE in water of different culture systems.The effects of different PAHs andincubation time on cell densities ofNitzschia sp.were studied.The cells in the treated sample did not show any multiplication in 96 h,whereas those in the control hadsignificant division.As the culture time prolonged,low concentration of PAHs inwater was benefit to recovering the viability of Nitzschia sp..The density of cellstreated with single PAH was higher than that treated with mixed PAHs.The resultssuggested that the joint toxicity of FLA and PHE on Nitzschia sp.exhibited synergism.With the prolonging of culture time,accumulation of single PAH by Nitzschia sp.gradually increased.And FLA accumulated in Nitzschia sp.was higher than that ofPHE.With the treatment of PAH mixture,accumulation of PAHs by Nitzschia sp.increased at first and then decreased in the plant-algae system.And the half-lives (T_1/2)for PHE and FLA degradation at the presence ofNitzschia sp.were 105.5 h and 120.7h,larger than those in PAH mixture (74.1 h and 108.2 h),respectively.After 30 dtreatment,root accumulation of PAH increased at first and then decreased,and higherPAH accumulations of root were more often noted in the plant-algae system than inthe plant culture systems.Distribution of PAH in different plant tissues wasinvestigated.The results showed that the accumulation of PAH in xylem and leaf wasobviously lower than that in root.Moreover,PHE accumulation in xylem and leaf waslarger than FLA.In mangrove wetland ecosystem,root of K.candel absorbed PAHs fromsediment and transmitted to shoot and leaf by transpiration of plant.And dominantmicroalgal species removed PAHs in the water environment through rapid adsorptionand absorption.On the other hand,after exposure to PAHs in sediment,the roots of K.candel appeared swelling,and then turned black and rotten.With increasingconcentrations of PAHs,the root activity,chlorophyll contents and net photosyntheticrate of K.candel decreased.Moreover,the root and above-ground biomass of K.candel decreased significantly in comparison with the control.The toxic effect ofPAHs on microalgal species was observed.The results suggested that microalgaecould be indicator organisms to PAHs pollution in marine environment.The researchprovides reliable information for environmental risk assessment of mangrove wetlandand a scientific support for protection of mangrove resources.