Influence Mechanism Of Microplastics On Bioavailability Of Arsenic In The Environment

Microplastics（MPs）are easy to accumulate and migrate in the soil due to the slow natural degradation rate.MPs are posing a significant threat to biosafety as it can serve as a pollutants carrier,which has been highly concerned by the society.The pollution of microplastics has become a hot spot in the field of ecological and environmental science.In China,heavy metals（such as arsenic（As））in a wide range have contaminated the large area of farmland soils,coexisting with microplastics and cause compound pollution.Therefore,it is important to explore the mechanisms of MPs effect on As uptake by organisms（such as plants and single-celled organisms）and the response of organisms to their toxicity under the combined contamination of MPs and As.In this study,the interaction and mechanisms of polystyrene microplastic（PSMP）,polytetrafluoroethylene（PTFE）microplastics,and As（Ⅲ）were studied by batch adsorption experiment,effects of combined contamination of MPs and As on plants and microalgae were discussed by hydroponic experiment（carrot,chlamydomonas reinhardtii and rice seedlings）,the effects and mechanisms of the bioavailability and yield of rice were examined by pot experiments and As volatilization experiments.The results are as follows:（1）The maximum adsorption capacity of PSMP and PTFE for As（III）is about 1 mg·g^-1,and the adsorption of As（III）decreases with the increase of PM particle size.The adsorption of As（III）by PSMP and PTFE inhibited by the increasing solution p H and concentration of NO₃?and PO₄^3-in the solution.Electrostatic and non-covalent interactions are the key mechanisms for the adsorption of As（III）on PSMP and PTFE.The adsorption of As（III）by PSMP and PTFE is an exothermic process,as the rise in temperature destroyed the hydrogen bond,which is not conducive to the adsorption.The positive electrostatic potential area of PSMP is higher than that of PTFE.Therefore,the adsorption of As（III）on PSMP is higher than that of PTFE.Humic acid has positive effect as it contains a large number of oxygen-containing functional groups that can formπcomplexes with MPs and improved the hydrophilicity of MPsand consequently,significantly increased the rate and capacity of MPs to adsorb As.（2）Large particle size（5μm）PSMP cannot enter cells,but can cause cytoplasmic wall separation,while small particle size（100 nm）PSMP can enter cells,interact with the phospholipid structure on the membrane through dispersion force,and affect the function of the cell membrane,therefore the uptake of As（III）by Chlamydomonas reinhardtii was Inhibited.The small particle size PSMP decreased more As（III）uptake rate of Chlamydomonas reinhardtii than the large particle size PSMP.PSMP and As（III）can cause intracellular"oxidative burst",increase the activity of superoxide dismutase（SOD）and catalase（CAT）,and then trigger lipid peroxidation of cell membranes,destroy the structure of chloroplasts,and inhibit the photosynthesis and growth of Chlamydomonas reinhardtii.In the combined pollution of PSMP and As（III）,large particle size PSMP can adsorb As（III）to reduce the impact of As（III）on Chlamydomonas reinhardtii.The small particle size PSMP increases the toxicity of As（III）to Chlamydomonas reinhardtii because of it’s nano-toxicity.（3）MPs reduced the quality of carrots and absorption of As by carrots.As（V）in the growth medium increased the negative charge on the surface of PSMP,and at the same time,distorted the cell wall and inhibit its function,resulting in more MPs entered the carrot and the cell.MPs after entering the edible part of carrots caused the oxidative stress in the carrot tissues,inhabited pectin methyl esterase activity,reduced the quality and lost fragility.MPs and As reduced the biomass of carrots by inhibiting the photosynthetic capacity and root vitality of carrots.（4）PSMP and PTFE reduced the As uptake of rice by directly adsorbing As and inhibited the activity of rice roots.As（III）triggered the"oxidative burst"of rice seedlings,destroyed the structure of antioxidant enzymes,lead to Reactive oxygen species（ROS）accumulation,damage rice chloroplasts,PhotosystemⅡ（PSII）reaction centers and Ribulose-1,5-bisphosphate carboxylase/oxygenase（Ru Bis CO）activities,thereby inhibited rice photosynthesis and biomass accumulation.PSMP and PTFE caused mechanical damage to the root system,produced a large amount of ROS,inhibited rice root vitality and transpiration,reduced the ability of plants to absorb nutrients and water,thereby reducing rice photosynthetic capacity and biomass accumulation.Compared with the control,0.2 g·L^-1 PSMP and PTFE reduced the root（leaf）biomass of rice seedlings by 21.4%（10.2%）and 25.4%（11.8%）,and 4mg·L^-1 As（III）reduced the root（leaf）biomass of rice seedlings by 26.2%（16.2%）.0.04 and 0.1 g·L^-1PSMP and PTFE can reduced the toxicity of As（III）to rice,while 0.2 g·L^-1 PSMP and PTFE increased the toxicity of As（III）to rice.（5）PSMP and PTFE in the soil reduced the As content in rice tissues and the proportion of methyl As in rice grains,and inhibited the As（V）-As（III）-Monomethylated arsenic（MMA）-Dimethyl arsenate（DMA）metabolic pathway in grains and the function of non-symbiotic hemoglobin.The As in the soil increased the iron-manganese membrane content on the root surface,while PSMP and PTFE reduced the iron-manganese membrane content on the root surface and root vigor.PSMP and PTFE also reduced soil p H and effective As content,increased the relative abundance of As-tolerant bacteria（delta proteobacteria and alpha proteobacteria）,deccreased the content of organic matter,available phosphorus and available nitrogen in the soil,inhibited the soil’s nitrogen fixation capacity and nutrient index.Therefore,PSMP and PTFE inhibited the biomass and yield of rice tissues by affecting soil nutrients and rice uptake function and non-symbiotic hemoglobin functions.（6）After adding PSMP,As volatilization reached the maximum in the fourth week.The addition of 0.8%PSMP with small particle size promoted Asvolatilization,while the addition of 0.8%PSMP with large particle size only increased As volatilization in 25.9 mg·kg^-1 As-contaminated soil.PSMP results in the decrease of water-soluble As and the increase of adsorbed As by adsorbing As in soil.PSMP had no significant effect on soil peroxidase（POD）activities,but increased the activities of CAT and urease（UE）in soil.The abundance of Proteobacteria and Firmicutes was opposite to the trend of As volatilization,while the abundance of Bacteroidetes and the gene expression of As methyltransferase was similar to the trend of As volatilization.Mechanisms of MPs promoting As volatilization from soil:1)PSMP affected the abundance of Proteobacteria,Firmicutes and Bacteroidetes in the soil,thus promoting the expression of Bacteroidetes arsenomethyltransferase gene;2)PSMP increased As volatilization by reducing soil nutrients and the proportion of water-soluble As in soil.MPs can directly or indirectly interact with As to promote the volatilization of As from soil,inhibit the absorption of As by plants.Microplastics and arsenic cause physiological toxicity and inhibit plant growth by triggering"oxidation bursts"in plants.Large-size microplastics（>10μm）can inhibit the effect of As on plants.However,due to the toxicity of MPs to plants,in the case of high concentration(>0.2 g·L^-1)and small particle size（<200 nm）,they tend to increase the toxicity of As to plants and inhibit the As metabolic pathway in plants.