Effects Of Microplastics And Their Adsorption Of Heavy Metals As Vectors On The Chlamydomonas Reinhardtii

With the continuous accumulation of plastic waste in the environment,the potential ecological risk of microplastics（MPs,<5 mm）has become one of the hot environmental issues worldwide.However,there are limited studies on the effects of microplastics and their continued fragmentation of smaller nanoplastics（NPs,<100 nm）on the freshwater environment.In addition,the widely distributed MPs has strong hydrophobic and larger specific surface area,so it is easy to act as important carriers of pollutants such as heavy metals in the aquatic environment,thus affecting the migration and transformation of pollutants in the environment and causing potential risks to water ecological health.Therefore,in this study,we compared the toxic effects of micro-and nano-sized polystyrene microbeads（PS-MPs and PS-NPs）on a widespread freshwater algal species,Chlamydomonas reinhardtii（C.reinhardtii）.Meanwhile,the adsorption/desorption behavior and adsorption mechanism of MPs on typical heavy metals Ag⁺and Cu²⁺were studied,and the influence of MPs as heavy metals vectors on the toxicity effect of microalgae and the bioavailability of heavy metals was further explored.The aim is to reveal the ecological and environmental risks of MPs as vectors and to provide data support and theoretical basis for the migration and transformation of heavy metals.The main results are as follows:（1）Both PS-MPs and PS-NPs had negative effects on the growth of C.reinhardtii in the range of 50 to 500 mg/L,reducing its biomass and photosynthetic pigment.The growth inhibitory rate（IR）of C.reinhardtii was 23.3%and 61.7%when exposed to 500 mg/L PS-MPs and PS-NPs for 96 h,respectively,demonstrating a higher toxic effect of the NPs than the MPs.After PS-NPs stress treatment,the levels of reactive oxygen species（ROS）and malondialdehyde（MDA）in algae cells increased,enhancing cell membrane permeability and then accelerating PS-NPs internalization.Compared with the PS-MPs test group,PS-NPs exposure significantly promoted the secretion of C.reinhardtii extracellular polymeric substances（EPS）and induced increased activity of the antioxidant enzymatic（superoxide dismutase（SOD）,peroxidase（POD）,and catalase（CAT））,which were associated with microalgae defense mechanisms.In addition,the transmission electron microscopy（TEM）showed that autophagic vacuole encapsulated internalized NPs,which accumulated into vacuoles in large quantities through transport,reducing intracellular dispersion of NPs and thus reducing its toxic effect.（2）The adsorption of Ag⁺and Cu²⁺on MPs showed that the adsorption performance of Ag⁺on MPs was higher than that of Cu²⁺,and the equilibrium adsorption amount was 364.25μg/g and 271.47μg/g,respectively.The adsorption kinetics of Ag⁺and Cu²⁺on MPs was in accordance with the pseudo-second-order kinetic model,and the adsorption speed was fast at first,then slow and finally tends to be flat.The adsorption isotherms were more consistent with Langmuir adsorption model(R~2 _Ag+=0.989;R~2 _Cu2+=0.967).There were complex adsorption processes dominated by functional active sites on the surface of MPs,including uniform adsorption of active sites at single molecular layers,chemical adsorption at multiple molecular layers and electrostatic adsorption,etc.After the rapid desorption rate of heavy metals on the surface of microplastic in the early stage,the equilibrium desorption rate of Ag⁺was between25%-30%,while that of Cu²⁺was between 14%-17%.（3）The effects of MPs as heavy metals vectors on the C.reinhardtii were showed that the existence of MPs alleviated the toxicity of heavy metals.The main reason is that MPs and microalgae’s competitive adsorption of heavy metals has reduced microalgae and heavy metal contact,thus reducing the internalized content of Ag⁺and Cu²⁺and reducing the bioavailability of heavy metals.The toxicity effect of microalgae and Ag⁺or Cu²⁺adsorption loaded was lower than the combined toxicity of the two simultaneously,indicating that MPs adsorption of Ag⁺or Cu²⁺has a strong stability,and the internalization content of Ag⁺or Cu²⁺in the combined group was significantly higher than that in the loaded group.In addition,because the adsorption performance of MPs on Ag⁺was higher than that of Cu²⁺,MPs and Ag⁺treatment groups showed less toxic effect on microalgae than Cu²⁺.The combined and loaded exposure of MPs and heavy metals significantly reduced the photosynthetic fluorescence parameters(α,r ETR_maxand I_k)of microalgae,resulting in the decrease of photosynthetic pigment content,which may be related to the inhibition of microalgae photosynthesis by MPs masking effect.