Study On The Ecotoxicity Mechanism Of Tire Microplastics On Microalgae And Coastal Sediments

Tire microplastics（TMPs）are common thermosetting polymers and are an important type of environmental microplastics（MPs）.The coastal zone is a huge reservoir of TMPs,and marine organisms may have been or are suffering from TMPs contaminations.However,there are few studies on the effects of TMPs on coastal marine organisms and sediments,and the potential ecological toxicity effects and mechanisms of TMPs in the coastal zone are still unclear.In the present study,TMPs were used as the research object to firstly investigate the effects of TMPs particles and their leachable additives at different concentrations and suffering from different aging treatments including UV irradiation and chemical oxidation on the growth and physiology of microalgae.Moreover,the effects of TMPs on the composition and function of microbial communities in coastal sediments were also explored.The details are as follows:We selected Phaeodactylum tricornutum（P.tricornutum）as the test organism and used TMPs produced in common car tires as the subject to study their growth and physiological response at different concentrations of TMPs over a 7-day period.The main findings were as follows:At day 7,the algal density of P.tricornutum increased by 15%compared to the control group at 0.6 mg·L^-1 TMPs,while the algal density decreased by 32%compared to the control group at 15 mg·L^-1 TMPs,resulting in a"low-promoting,high-suppressing"effect,as the concentrations of TMPs increased,the algal density showed a trend of increasing and then decreasing.The changes in chlorophyll a content,photosystem II maximum photochemical efficiency（Fv/Fm）and superoxide dismutase（SOD）activity of P.tricornutum were more obvious on day 4 after TMPs exposure.All physiological activities at low dose TMPs treatment basically recovered to the control level at day 7,but all physiological activities of microalgae at high dose TMPs treatment were still lower than those in the control group,indicating that high TMPs exposure may cause irreversible effects on microalgae.On this basis,we further investigated the growth and physiological response of P.tricornutum to the TMPs and their corresponding leachable additive suffering from different aging treatment including UV irradiation and chemical oxidation.The main results were as follows:the surface of TMPs became rougher,with more folds and larger pores after aging treatment.Compared with the original TMPs,the growth inhibition on P.tricornutum of aged TMPs was more significant,while TMPs suffering from different aging process had different effects on algal cells.The particles and their corresponding leachable additive of different aged TMPs had similar toxic effects,indicating that the effects of aged TMPs on microalgae may be related to their leachable additives.We selected coastal sediments as subjects to study the effects of different concentrations of TMPs and the leachable additives on their microbial community composition and function.The results were as follows:TMPs at environmental concentrations altered the microbial community structural,reduced community diversity,and inhibited functional genes associated with nutrient cycling such as carbon fixation and degradation,nitrification,denitrification,and sulfur cycling in sediments.The effects of TMPs particles and leachable additive on microbial community diversity,community structure,and function were similar at the same concentrations,suggesting that the response of microbial community structure,diversity and function is mainly attributed to leachable additives.A positive linear relationship was observed between the concentrations of five heavy metals（As,Cu,Co,Mn,Zn）and six organic compounds（acetophenone,4-methylaniline,benzothiazole,octadecyl acetate,N-cyclohexylfomamide and dicyclohexylamine）in sediment pore water and the exposure concentrations of TMPs,suggesting that these compounds can be released from TMPs particles into the sediment environment.The abundance of nitrification-and denitrification-related functional genes has been reduced,with the increase in the relative abundance of sulfate-reducing bacteria（such as Desulfofrigus and Desulfobacter）suggested that leachable additives could affect microbial community function through stimulating or inhibiting certain ecologically functional bacteria.All above suggested that TMPs could release a variety of leachable additives in the sediment environment,shifting the microbial community structure,losing the microbial diversity,and altering the functional genes related to nutrient cycling such as C,N,and S.