Toxicity of mercuric oxide nanoparticles on freshwater microalgae: Comprehensive analysis on their interactive effects and detoxification pathways

Abstract

So far, there is limited research on the potential risks and environmental consequences of mercuric oxide nanoparticles (HgO-NPs) on phytoplankton. To this end, we investigated the moderate and acute concentrations of HgO-NPs (0.8 and 1.6 mg/L, respectively) on the growth, photosynthesis, and detoxification processes in two microalgae, namely Scenedesmus obliquus and Nostoc muscorum. Compared to N. muscorum, S. obliquus had greater accumulation and removal efficiencies of HgO-NPs (92.8 %, 91.4 %) at moderate and acute levels. HgO-NPs accumulation inhibited the growth and decreased photosynthesis, RuBisCo activity and chlorophyll a content in a dose-dependent manner. At oxidative damage level, acute level of HgO-NPs evoked severe lipid and protein peroxidation, and increased NADPH oxidase activity particularly in N. muscorum. Exposure to HgO-NPs also resulted in distinct metabolic perturbations in both microalgal species. Notably, there was a dose-dependent increase in the activity of antioxidant enzymes (e.g., peroxidase and superoxide dismutase), as well as metabolites like tocopherols and flavonoids in S. obliquus. However, the activity of catalase, ascorbate peroxidase and dehydroascorbate reductase enzymes, and ascorbate levels were significantly increased in N. muscorum. At HgO-NPs detoxification level, S. obliquus increased glutathione level and glutathione-S-transferases activity. However, N. muscorum showed high levels of metal chelator (phytochelatins and metalothionine). Investigating the HgO-NPs bioremediation capacity and stress tolerance mechanisms opens possibilities to introduce S. obliquus as a promising bioremediator to relieve ecological risk of HgO-NPs.