The Accumulation And Detoxification Mechanisms In The Commonly Freshwater Algae In The Three Gorges Reservoir Area,exposed To Cadmium

After the construction of the Three Gorges reservoir,the reservoir area has adopted the operation mode of the storage clear water and releasing the muddy.This makes quite serious soil erosion.Soil erosion results in a situation in which some heavy metals in the soil enter into the water body of the reservoir area via surface runoff flow to occur or increase heavy metal pollution in the water area.At present,studies have showed that Cd,Hg,Pb and other heavy metal pollution is found in the reservoir water,and Cd is one of the most serious pollutions of heavy metals.The Cd in water body enters the food chain by the uptake and utilization of plankton.Algal is the main primary producer of marine and fresh water,and plays an important role in the aquatic ecosystem.Cd is a non essential element of aquatic organism,while it has high biological toxicity,and strong migration ability.After Cd is absorbed by algal,part of it will enter into the sediment after the death of algal,and the rest of it will transfer to zooplankton and finally arrive at the human body,resulting in great harm to human health.Therefore,it is very important to study the migration and transformation of Cd in algal.However,there has not been any report on the accumulation and transformation of Cd in the common algal of the Three Gorges Reservoir area yet.Therefore,this study selects Aphanizomenon flos-aquae,Pediastrum simplex and Synedra acus from the Three Gorges Reservoir area as test species,and discusses the response of the three species and the mechanism of detoxification to Cd.After analysis of their different concentrations of cadmium adsorption and accumulation,and their response to cadmium and the detoxification mechanism,our aims are to clarify the migration of the heavy metal cadmium in water of the Three Gorges Reservoir area,and reveal the three species of algal to cadmium response and detoxification mechanisms.Therefore,the study on common algal in the Three Gorges Reservoir Area on cadmium bioaccumulation and detoxification mechanism will be helpful to reveal the accumulation,migration and transformation rules of the heavy metal cadmium in the Three Gorges Reservoir Area’s algal,to provide new information for the protection of our country water,drinking water hygiene and safety and health,and to offer scientific basis for the bioremediation of algal in heavy metal cadmium,as well as to to monitorand evaluate heavy metal cadmium pollutionin the Three Gorges Reservoir area.The results are as follows:1.Exposure of A.flos-aquae,P.simplex and S.acus to a Cd concentration showed that the 96-h EC₅₀value was 1.18±0.044[0.955–1.452 CI_95%],4.32±0.068[3.54–5.232 CI_95%]and 3.7±0.055[2.748–5.052CI_95%]mg/L,respectively.The specific growth rate（μ）and OD significantly decreased as the Cd concentration increased in flos-aquae,P.simplex and S.acus.After exposure to 0.2 mg/L of Cd,the specific growth rate was significantly inhibited in A.flos-aquae.Significant decreases in the growth rate were found in P.simplex and S.acus only when they were exposed to Cd concentrations>5.0 mg/L.A similar result was also found for the chlorophyll a concentration.Compared with the controls,chlorophyll a concentrations significantly decreased with the increase of Cd concentration.Significant decrease in chlorophyll a concentration was displayed in P.simplex when Cd treatment level was greater than 0.5 mg/L.A similar trend was also discovered for carotenoids.The tolerance of the three species to cadmium was in the order green alga P.simplex>diatom S.acus>cyanobacterium A.flos-aquae.2.The chlorophyll a fluorescence induction curve（OJIP）can better reflect the structure and function of PSII.The chlorophyll a fluorescence transients became slightly lower at Cd concentrations 0.2,0.5,1mg/L.Phase I and phase P became obscure from the curves of chlorophyll a fluorescence transients when grown under the stress of 10 mg/L Cd,and disappeared at 20 mg/L Cd.An interesting result was that phase K occurred at 20 mg/L Cd only in S.acus.The F₀ values and RC/CSo of A.flos-aquae,P.simplex and S.acus decreased with the increase in Cd concentration.Compared with the controls,slight increases inφ_P0,ψ₀ andφ_E0 were found at Cd concentrations of 0.5 and 1.0 mg/L,but significant decreases were found at concentrations of 5.0,10.0 and 20.0 mg/L in the two species.3.The total protein contents increased generally with Cd concentration increase in the three species.Antioxidant enzyme（SOD and CAT）activities were differently affected by Cd among the three species.SOD activity was remarkably stimulated at concentrations of 0.2–5.0 mg/L in A.flos-aquae,0.5–10.0 mg/L in P.simplex and 1.0–20.0 mg/L in S.acus.However,CAT activity increased at concentrations of10.0–20.0 mg/L in A.flos-aquae,5.0–10.0 mg/L in P.simplex and 0.2–20.0 mg/L in S.acus.The peaks in CAT activity were 42,8 and 2 times higher than the controls at 10.0,20.0 and 5.0 mg/L in A.flos-aquae,P.simplex and S.acus,respectively.Lipid peroxidation,expressed as MDA concentration,the results showed that MDA levels increased generally with Cd concentration increase in the three species4.After 96h of exposure,surface-adsorbed–Cd increased as a response to cadmium increasing among the three freshwater phytoplanktons.Intracellular-Cd accumulation was exhibited a similar trend with surface-adsorbed-Cd.The results showed that the induction of PCs and glutathione showed a remarkable difference among the three species.Compared with the other two algae,synthetic phytochelatins of P.simplex had a lower.PC₃ was found at all treatments of A.flos-aquae and P.simplex.Additionally,glutathione（GSH）and PC₂ were also induced in high Cd concentrations in P.simplex.As exposed to Cd,glutathione was found at all treatments in S.acus,which decreased with the increase of Cd concentration.With the increase of Cd concentration,PC₂,PC₃ and PC₄ were induced and indicated a significant enhance PC₄ was only induced at the concentration of 5,10 mg/L at this algae.5.The levels of transcriptome of P.simplex were decreased after exposure to 5 mg/L Cd（Cd5）by contrast with the control（CK）.Annotation in all databases was mapped to over 1234 unigenes.There was only 47724 common unigenes between CK and Cd5.734 upregulated and 456 downregulated genes were observed when cells were exposed to Cd.Elevated levels of transcripts of several genes mainly comprised of proteasome,peroxisome and protein processing,suggesting that Cd5 caused damages to the cells under environmentally relevant conditions.Genes with photosynthesis-related functions were decreased obviously after exposed to Cd.This change suggested toxicological effects in the the photosynthetic stucture,including chloroplast,thylakoid,photosynthetic membrane,Photosystem II,light harvesting complex（LHC）proteins,Photosystem I,etc..This study showed that Cd could significantly inhibit the growth rate,damage the synthetic pigment（chlorophyll a,carotenoids）and the structure of PSII among three species of freshwater phytoplankton,A.flos-aquae,P.simplex and S.acus.The algal cells took two kinds of detoxification mechanisms to reduce the toxic effects of Cd²⁺.On the one hand,cells increased antioxidant enzyme systems（SOD,CAT）activity,to clear the intracellular excess ROS,and thus reducing the oxidative damage to cells.On the other hand,cells also synthesized GSH,PCs,or some unknown thiol compounds and chelated with Cd²⁺,and the free Cd²⁺into a non-toxic Cd-PC compounds,thereby reducing the toxicity of Cd²⁺on the cell.