Nano-sized polystyrene plastics toxicity to microalgae Chlorella vulgaris: Toxicity mitigation using humic acid

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.