Polystyrene nanoplastics (PS-NPs) can cause toxicity in aquatic organisms, but presence of natural organic matter (NOM) may alter toxicity of PS-NPs. To better understand effects of NOM on acute toxicity of PS-NPs, humic acid (HA) as a model of NOM was added to green microalga Chlorella vulgaris medium in the presence of amino-functionalized polystyrene nanoplastics (PS-NH2). Acute toxicity tests of PS-NH2 to C. vulgaris biomass and chlorophyll a content showed statistical differences between media treated with different concentrations of PS-NH2 and control groups (p<0.05). HA significantly mitigated PS-NH2 toxicity to C. vulgaris biomass and chlorophyll a end-points (p<0.05). Additionally, high HA concentration was more effective than low concentration (10 vs 5 mg/L), showing a greater ameliorative effect on PS-NH2 acute toxicity (p<0.05). Algae exposed to higher PS-NH2 concentrations showed greater morphological changes (i.e., diminution of photosynthetic pigments, reduction of algal size and formation of more cellular aggregates). Formation of high amounts of algal aggregates under influence of PS-NH2 was presumably related to the high electrostatic tendency of these particles (with positively charged surfaces) to C. vulgaris polysaccharide walls (having negative charge). Formation of aggregates was significantly reduced in the presence of HA. HA with dominant negatively charged functional groups (following sorption by PS-NH2 via reduction of PS-NH2 zeta potential), could decrease electrostatic attraction between PS-NH2 and algae, thereby substantially ameliorating cellular aggregation and cell size reduction.
Keywords: Acute toxicity; Chlorella vulgaris; Humic acid (HA); Natural organic matter (NOM); Polystyrene nanoplastics (PS-NPs).
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