Effects And Mechanisms Of Microplastics On Microalgal Growth Inhibitions By Phenanthrene And Nano Zinc Oxide

Microplastics(MPs)have become emerging pollutant and are widely distributed in ecosystem of the world.The status and environmental risks of MPs in the environment have attracted more and more attention due to their wide distribution,difficult degradation,easy migration,easy accumulation of toxic substances,and easy ingestion by animals.The adsorption capacity of MPs makes them carriers of pollutants,which brings more complexity to the ecotoxicological study of MPs.Organic pollutants are the main adsorbents of MPs,among which hydrophobic organic pollutants(HOCs)are the majority,and heavy metal pollutants have also been reported.However,there are few studies on the larger size of MPs(>100 ?m)stress microalgae.In addition,compared with HOCs,the mechanism of adsorption of metal oxides is still unclear.Therefore,studying the adsorption capacity of MPs alone or in combination with organic pollutants and metal oxides to investigate the effects of MPs on the transport of pollutants to marine microalgae is of great importance.In this study,using marine diatom Phaeodactylum tricornutum as experimental object,to investigate the effects of polyethylene plastic(PEMP,150 ?m)and unplasticized Polyvinyl Chloride microplastics(u PVCMP,250 ?m),and two MPs respectively combined with Phenanthrene(Phe)inhibited diatom growth and induced neutral lipid change.Meanwhile,in order to further investigate the mechanism of adsorption,using marine microalgae Tetraselmis helgolandica var.Tsingtaoensis as experimental object,to explore the microalgal growth under nano zinc oxide(20-30 nm)nZnO alone and with polyethylene micro plastic joint exposure(PEMP,150 ?m).The scanning electron microscope(SEM)observation,Inductively Coupled Plasma Massspectrometry(ICP-MS)and X-ray diffraction(XRD)techniques were used to investigate the mechanism of adsorption.The main results of this study are as follows:(1)In short-term exposure experiment(4 days),the growth curve trends of the experimental group and the control group were similar.Only in 0.05 g/L PEMP group,it showed significant difference in specific growth rate SGR(0-4).Neither of the two MPs inhibited the growth of diatoms in the 4-day short-term experiment,nor had any effect on the accumulation of neutral lipids in diatoms.(2)In the long exposure experiments(9 days),the microalgal growth curve under two kinds of MPs exposure in all concentrations are showing more complex growth trend in the later term(5-9 days).The results of neutral lipid assay showed that the relative cell lipid concentration increased by 23.93% to 208.94%.Compared with the short-term experiment,the long-term experiment showed a certain inhibitory effect on the growth of microalgae,and a certain promotion effect on the accumulation of neutral lipids in cells.(3)When the two kinds of MPs were combined with Phe,the growth results of microalgae showed that the two kinds of MPs significantly alleviated the stress of Phe on the growth of microalgae.Relatively speaking,PEMP was more obvious than u PVCMP in alleviating the inhibition of Phe.(4)In the combined exposure test,PEMP reduced the inhibitory effect of nZnO on the growth of microalgae.After four days exposure the microalgal growth inhibition effects of nZnO,PEMP-nZnO,ZnSO4 and PEMP-ZnSO4 are 65.1%±2.3% [61.2-67.5%],55.7%±3.9% [51.2-61.0%],76.3%±3.3% [72.5-81.2%] and 66.0%±3.1%[60.0-68.7%],respectively.PEMP significantly alleviated the toxicity of nZnO,and the growth inhibition rate of microalgae decreased by 14.4% on average.(5)The surface morphology of PEMP isolated from nZnO exposed system was observed by SEM.The plain PEMP particles present rough and lacunose surface which provide high specific surface area for its adsorption.However,there were no differences in particle and particle surface morphology in the isolated nZnO exposed system compared with the control group.The concentration ofZnon the surface of PEMP isolated from the nZnO exposure system was measured by ICP-MS,and the results showed that the adsorption amount ofZnreached the peak at 6h,and theZncontent showed a decreasing trend within 4 days of exposure.In addition,the crystal morphology ofZnon the PEMP surface separated from the nZnO exposed system was determined by XRD technique.The results showed that the crystal form ofZnwas stillZnO,and there was no other compound form.In conclusion,the two kinds of MPs have no negative effects on the growth and lipid accumulation of P.trigonatum in the short-term exposure experiment,while in the long-term exposure experiment,both kinds of MPs inhibit the growth of diatoms and induce lipid accumulation at the same time.The adsorption of the two MPs significantly reduced the toxicity of combined exposure of Phe and MPs.The microalgal toxicity of nZnO was also significantly alleviated by the participation of PEMP.The experimental results of the adsorption mechanism of PEMP showed thatZnO was the only crystal form ofZnelement adsorbed on the surface of PEMP.