Research has already demonstrated the toxic effects of microplastics (MPs) on different biota. However, the underlying toxic mechanism of MPs remains to be elucidated, especially the effect of particle size and the presence of dissolved organic matter in water. This study investigated the impact on Scenedesmus obliquus exposed in five types of polystyrene particle suspensions with different sizes and surface charges, in the presence and absence of humic acid (HA). Results indicated that the 50% growth inhibition rate of S. obliquus showed no significant difference between the five types of MPs, but the toxic mechanism varied with particle size. Larger size MPs caused adverse effects by blocking the light transport and affecting photosynthesis, while smaller ones destroyed the cell wall by adsorbing onto the algae surface. Also, the addition of HA significantly alleviated the toxicity of smaller size MPs, but not of the larger ones. Scanning electron microscopy images and the reactive oxygen species assay demonstrated that the HA could form a corona on the surface of MPs, reduce the affinity to microalgae, and minimize the adverse effect. Together, these findings identified important factors in determining the toxicity of MPs, providing valuable data for risk assessment of MPs.

中文翻译：

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微塑料对微藻生长的影响：大小和腐殖酸的影响。

研究已经证明了微塑料（MPs）对不同生物群的毒性作用。但是，MPs的潜在毒性机制仍有待阐明，尤其是颗粒大小和水中溶解有机物的存在的影响。这项研究调查了在存在和不存在腐殖酸（HA）的情况下，五种不同尺寸和表面电荷的聚苯乙烯颗粒悬浮液对斜生鹿角藻的影响。结果表明，斜纹链球菌的50％生长抑制率在5种MPs之间没有显着差异，但其毒性机理随粒径的变化而变化。较大的MPs会阻止光传输并影响光合作用，从而产生不利影响，而较小的MPs则通过吸附到藻类表面上而破坏了细胞壁。还，HA的添加显着减轻了较小MP的毒性，但不能减轻较大MP的毒性。扫描电子显微镜图像和活性氧分析表明，HA可以在MP的表面形成电晕，降低对微藻的亲和力并使不良影响最小化。这些发现共同确定了确定MP毒性的重要因素，为MP的风险评估提供了有价值的数据。