Is hydrodynamic diameter the decisive factor? - Comparison of the toxic mechanism of nSiO and mPS on marine microalgae Heterosigma akashiwo.

To investigate the toxic mechanism of SiO nanoparticles (nSiO) and polystyrene microplastics (mPS) on microalgae Heterosigma akashiwo, growth inhibition tests were carried out. The growth and biological responses of the algae exposed to nSiO (0.5, 1, 1.5, 2, 5, 10 and 30 mg L) and mPS (1, 2, 5, 10, 30 and 75 mg L) were explored in f/2 media for 96 h. It was found that the hydrodynamic diameter of the particles seems to be one of the more important factors to influence the algae. nSiO and mPS with similar hydrodynamic diameters have the similar toxic mechanism on H. akashiwo, and the effects were dose- and time-dependent. High concentrations of micro-/nano-particles (MNPs) could inhibit the growth of algal cells, however, low concentrations of MNPs did not restrict or even promoted the growth of algae, known as "Hormesis" phenomenon. The 96 h-EC20 values of nSiO and mPS on H. akashiwo were 2.69 and 10.07 mg L, respectively, and chlorophyll fluorescence parameters indicated that the microalgal photosynthetic system were inhibited. The hydrophilic surface of nSiO increased the likelihood of nSiO binding to the hydrophilic functional group of microalgae, which may account for the slightly stronger toxic effect of nSiO than mPS. The algae continued to produce reactive oxygen species (ROS) under stress conditions. Total protein (TP) levels reduced, and superoxide dismutase (SOD) and catalase (CAT) levels increased to maintain ROS levels in the cells. The decrease in adenosine triphosphate (ATPase) indicated an impact on cellular energy metabolism. Cell membrane damage, cytoplasm and organelle efflux under stress were confirmed by scanning and transmission electron microscopy (SEM and TEM) images. This study contributes to the understanding of the size effect of MNPs on the growth of marine microalgae.