Single and combined nanotoxicity of ZnO nanoparticles and graphene quantum dots against the microalga Heterosigma akashiwo

Abstract

In order to investigate the combined toxicities of zinc oxide nanoparticles (nZnO) with graphene quantum dots (GQDs) on microalgae Heterosigma akashiwo, growth inhibition tests were carried out. The growth and biological responses of the algae exposed to nZnO (1, 3, 5, 10 mg L⁻¹) and GQDs (1, 5, 10, 20 mg L⁻¹) were explored in f/2 media. The density of algae cells and the endpoints of reactive oxygen species (ROS), total protein (TP), superoxide dismutase (SOD), malondialdehyde (MDA) and adenosine triphosphate (ATP) were used to explore the toxicity mechanism of the nanoparticles to the microalgae. Both nZnO and GQDs inhibited the growth of H. akashiwo and the toxic effect increased with the increase of particle concentrations and incubation time. The 4 d-EC₅₀ values of nZnO and GQDs to H. akashiwo were 4.9 and 8.8 mg L⁻¹, respectively. Nanoparticles caused oxidative stress to H. akashiwo with increased relative levels of ROS, which inhibited protein synthesis, enhanced SOD and ATP activity as well as MDA content significantly to resist oxidative damage to the cells and maintain cellular energy metabolism. nZnO and GQD aggregation and coverage on the cells led to the cellular damage observed by SEM. At low concentrations, the combined toxicity of the two nanoparticles was lower than that under the corresponding concentration of single nanoparticle exposure. The antagonistic effect was mainly attributed to the adsorption of Zn²⁺ released from nZnO on GQDs and interactions between particles such as heterogeneous aggregation and precipitation. At high concentrations, the combined toxicity showed a synergistic effect, which might be related to the “Trojan-Horse effect”. This study is helpful to understand the effects of different surface characteristic nanoparticles on the growth of marine algae.