Study On The Influence Of Microplastics-Pb Coexistence On The Environmental Behavior Of Chlorella Vulgari

Microplastics（MPs）have strong hydrophobicity and large specific surface area,which can easily serve as carriers of heavy metals and other pollutants in the water environment,thus affecting the migration and transformation of pollutants in the environment and posing potential risks to water ecological health.Eutrophication of water bodies and heavy metal pollution are also important environmental problems facing China’s water environment,which can produce corresponding harm to human life,lakes and reservoirs and phytoplankton.In recent years,the study of microplastics and heavy metals has gradually evolved into the study of the compound pollution of both,but the current research in this area is shallow.Few studies on the environmental behavior of microplastics and heavy metals on Chlorella have been reported.Therefore,in this study,Chlorella vulgaris,the dominant algal species in the Three Gorges reservoir area,was selected as the object of study,and Pb²⁺,NO₃^--N and PO₄^3--P were used as the condition parameters for indoor simulation experiments to investigate the effects of microplastic-Pb coexistence on the environmental behavior of Chlorella vulgaris.This study was divided into three parts:1.The toxicity of Polyethylene（PE）,Polypropylene（PP）and Pb to Chlorella was investigated based on the growth inhibition rates of PE,PP and Pb,respectively;2.The toxicity of Pb²⁺to Chlorella was investigated by varying the concentration of Pb²⁺（0,0.05,2,5 mg/L）.NO₃^--N and PO₄^3--P uptake by Chlorella;3.Probe the change of Pb²⁺concentration in the culture solution by diluting the nitrogen and phosphorus concentration(BG-11(340 mg/L NO₃^--N,7 mg/L PO₄^3--P),1/2BG-11,1/5 BG-11,1/10 BG-11)in the culture solution.The effect of microplastics on the environmental behavior of Chlorella under coexistence with Pb was investigated by using the number of algal cells,chlorophyll a,soluble protein,enzyme activity,orthophosphate and nitrate related indicators in the algal solution.The main findings are as follows:（1）The EC₅₀ values of PE and PP were 334 mg/L and 295.29 mg/L respectively after96h of action on Chlorella,with the toxicity magnitude of PP>PE and the toxicity intensity being low.The toxicity of Pb²⁺on Chlorella was related to its autochthonous concentration,and the degree of inhibition of algal cell growth was gradually strengthened with the increase of Pb²⁺concentration.（2）Pb²⁺concentration change on day 7,PE/PP and Pb²⁺mono-and co-cultures significantly promoted nutrient salt uptake by Chlorella,mainly for NO₃^--N.Under PE/PP and Pb combined stress,5 mg/L Pb²⁺and PE/PP had an antagonistic effect on PO₄^3--P uptake by Chlorella,except for the other concentrations of Pb²⁺and PE/PP,which significantly inhibited NO₃^--N and PO₄^3--P uptake by Chlorella.The other concentrations of Pb²⁺and PE/PP significantly promoted the uptake of NO₃^--N and PO₄^3--P.Both PE/PP and Pb²⁺concentrations significantly inhibited the growth of Chlorella biomass,produced some membrane lipid peroxidation in algal cells,and stimulated NR and AKP activity by repair of the algal organism.The Chlorella sp.showed an increase in NO₃^--N and PO₄^3--P uptake.However,under 5mg/L Pb²⁺and PE/PP co-culture of Chlorella,algal cell growth was the worst,and significant membrane lipid peroxidation occurred in algal cells,and algal cell damage was severe.In the later stages of the experiment,the uptake of NO₃^--N and PO₄^3--P by Chlorella was significantly inhibited by PE/PP and Pb²⁺at single and combined concentrations,and the inhibition was stronger for PO₄^3--P.The uptake of NO₃^--N and PO₄^3--P by Chlorella was synergistically inhibited by co-culture of Pb²⁺with PE/PP,and the inhibition increased with increasing Pb²⁺concentration.The long-term exposure of Chlorella to PE/PP and Pb²⁺resulted in severe membrane lipid peroxidation of algal cells,overloading of algal cells with their own repair,and poor growth of Chlorella,so even though NR and AKP activity increased significantly,the ability of Chlorella to absorb NO₃^--N and PO₄^3--P was significantly reduced.（3）On the 7th day of nutrient salt alteration,the greater the dilution of the culture solution the poorer the uptake of Pb²⁺by Chlorella except for the 1/2BG-11 group.Microplastics and different nutrient salt levels had an antagonistic effect on the ability of algal cells to take up Pb²⁺.This was due to the fact that high nutrient salt levels significantly promoted the increase of NR and AKP content in the algal cells,which significantly increased the number of algal cells and thus led to a significant decrease of Pb²⁺concentration in the culture medium.At the end of the experiment,the overall difference in Pb²⁺content in the culture medium was not significant except for the 1/2BG-11+PP treatment group,which was mainly due to the reduction in culture medium Pb²⁺concentration including four aspects of microplastics,adsorption of Pb²⁺by living algal cell membranes and dead algal cells,and uptake of Pb²⁺by Chlorella cells.In contrast,the1/2BG-11+PP treatment group had the highest biomass,chlorophyll content and a larger proportion of live algal cells in the total number of algal cells,while the other groups had a larger proportion of dead algal cells resulting in a higher Pb²⁺content in the culture medium than the other groups.The optimum nutrient salt level for Chlorella was 1/2BG-11for the whole experimental cycle;the Pb²⁺content in the culture medium was significantly lower in all experimental groups,indicating that algae and microplastics are better materials for heavy metal treatment applications,which will provide some theoretical basis for heavy metal pollution treatment.In summary,the conclusions obtained from this study will provide an important theoretical basis for water environmental protection.It is also an important guidance for the management of compound pollution in the water environment.