Combined Toxicity of Silver Nanoparticles with Hematite or Plastic Nanoparticles toward Two Freshwater Algae

In the natural environment, the interactions of different types of nanoparticles (NPs) may alter their toxicity, thus masking their true environmental effects. This study investigated the toxicity of silver NPs (AgNPs) combined with hematite (HemNPs) or polystyrene (PsNPs) NPs toward the freshwater algae Chlamydomonas reinhardtii and Ochromonas danica. The former has a cell wall and cannot internalize these NPs, while the latter without a cell wall can. Therefore, the toxicity of AgNPs toward C. reinhardtii was attributed to the released Ag ions, while AgNPs had direct toxic effects on O. danica. Moreover, nontoxic HemNPs ameliorated AgNP toxicity toward C. reinhardtii, by decreasing the bioavailability of Ag ions through adsorption. Despite their role as Ag-ion carriers, HemNPs still reduced the toxicity of AgNPs toward O. danica by competitively inhibiting AgNP uptake. In both algae, Ag accumulation fully accounted for the combined toxicity of AgNPs and HemNPs. However, the combined toxicity of AgNPs and PsNPs was complicated by their significant individual toxicities and the synergistic interactions of these particles with the algae, regardless of differences in Ag accumulation. Overall, in environmental assessments, considerations of the combined toxicity of dissimilar NPs will allow more accurate assessments of their environmental risks.