Toxic effects of polystyrene nanoplastics on microalgae Chlorella vulgaris: Changes in biomass, photosynthetic pigments and morphology

Presence of nanoplastics within aqueous media has raised concerns about their adverse impacts on aquatic organisms. This study evaluated toxic effects of amino-functionalized polystyrene nanoplastics (PS-NH) with diameters of 90 (PS-NH-90), 200 (PS-NH-200) and 300 (PS-NH-300) nm on green microalgae Chlorella vulgaris. A dose-dependent toxicity response by PS-NH-90 and/or PS-NH-200 on biomass and photosynthetic pigment (chlorophyll a) end-points of C. vulgaris was observed. Whereas varied concentrations of PS-NH-300 had no significant toxic effect on biomass and chlorophyll a end-points compared to control groups (p > 0.05). A comparison of toxicity of similar concentrations of PS-NH-90, PS-NH-200 and PS-NH-300 showed small-sized PS-NH were more toxic than large-sized PS-NH (toxicity of PS-NH increased in the order PS-NH-300 < PS-NH-200 < PS-NH-90). With decreasing PS-NH size, greater morphological changes and loss of original shape were observed, so that algal density/size reduced, and cell aggregations increased. Since PS-NH have high affinity to C. vulgaris due to electrostatic interaction with polysaccharide wall of algae, this could be as the main reason for formation of large aggregates at high concentrations of PS-NH compared to low concentrations of PS-NH used in algae medium. At high concentrations, PS-NH may act as intermediaries for connection of algal cells and therefore formation of aggregates. Field emission scanning electron microscopy images confirmed that high amounts of PS-NH-90 were found to be embedded and adsorbed on algal cells, thereby limiting transfer of materials, gas exchange and energy between the aqueous medium and algal cells. These data may have serious ecological health implications, as C. vulgaris are important primary producers responsible for producing oxygen in aquatic environments.