Aquatic toxicity of iron-oxide-doped microplastics to Chlorella pyrenoidosa and Daphnia magna

Novel metal-oxide-doped microplastic particles (MMPs) have become emerging particulate plastics. The toxicity of MMPs in the aquatic environment remains unknown yet. In this study, toxicological effects of a representative MMP (iron-oxide core) with the nominal diameter of 1 μm and two different surface functional groups, amine-modified (MPS (Fe)-NH) and carboxyl-modified (MPS (Fe)-COOH) polystyrene, were investigated by toxicity testing using Chlorella pyrenoidosa and Daphnia magna. The aquatic toxicity of two microplastic particles (MPS-NH and MPS-COOH) with the same particle size and surface modification in the absence of iron-oxide core was also observed and compared. Results show that the toxicity of MPS (Fe)-NH to the studied organisms (in terms of median effective concentration) was greater than the MPS (Fe)-COOH and non-iron-oxide-doped microplastic particles. Moreover, the MMPs mainly contributed to the toxicity rather than their dissolved fraction. The accumulation (based on total Fe) of MPS (Fe)-NH in C. pyrenoidosa was higher than the MPS (Fe)-COOH at low effect concentrations. The observations by optical microscopy indicated that the MPS (Fe)-NH was heavily adsorbed on the surface and distributed over the antennae, carapace, and apical spine of D. magna body whereas the MPS (Fe)-COOH was mainly accumulated inside the digestive tract of the daphnia. Furthermore, the analysis of intracellular reactive oxygen species level and antioxidant capacity confirmed that the intensities of the toxic effects cannot be linked to oxidative stress induced by the particles in the algae and daphnids. This work provides valuable insights into the ecological effects of MMPs, which is helpful for the quantitative assessment of food chain transfer of microplastics.