Environmental Toxicology And Toxicity Mechanism Of Silver Nanoparticles On Marine Microalgae

Silver nanoparticles (AgNPs) have been widely used in medical, electronic, coating and other consumer products due to its unique antibacterial properties and optical conductivity. Large-scale commercial application of AgNPs lead to potential water pollution problems. AgNPs cannot degrade in the environment and could be persistent pollutants in the water and sediment. Estuary inshore is one of the important final destinations of AgNPs, which makes it important to study the impacts of AgNPs on estuarine ecosystem. This study aims to understand the environmental impacts of AgNPs and the associated mechanisms.Algae is the vital primary producers of aquatic ecosystems, and its health state has important ecological significance to maintain the stability of the aquatic ecosystem structure and function. Skeletonema costatum (S. costatum) and Prorocentrum donghaiense are two kinds of red tide microalgae, which often dominate in estuary and coastal areas. In this study, S. costatum was more sensitive in the acute exposure to AgNPs. Diatom S. costatum was further applied to investigate the acute toxicity effects and toxic mechanism of AgNPs.The size and surface coating are the two important physicochemical parameters of the studied AgNPs. The toxicity effects of three kinds of different size and surface coating of AgNPs have been studied and compared in the selected marine diatom S. costatum. The result showed that the exposure to AgNPs induced growth inhibition in algal cells in a dose response relationship, and toxicity order is presented as lOnm-OA coated AgNPs> 10nm-PVP coated AgNPs> 20nm-PVP coated AgNPs. Exposure to AgNPs altered the expression of photosystem Ⅱ (PSⅡ) reaction center protein (D1) genes and also inhibited photosynthesis process. Smaller sized AgNPs is more toxic to the algal cell. Dynamic light scattering data showed that the hydrodynamic diameter of 10nm-PVP coated AgNPs is slightly larger than that of 10nm-OA coated AgNPs, which indicate the surface coating indirectly affect the toxicity of AgNPs by acting on the stability of AgNPs in water environment. The difference in surface coating resulted in difference of toxicity of AgNPs in algal cells. Polyvinyl pyrrolidone (PVP) coated AgNPs is less toxic than oil amine (OA) coated AgNPs in algal cells. Reactive oxygen species (ROS) generation is higher in the algal cells exposed to 10nm-OA coated AgNPs than that of 10nm-PVP coated AgNPs.This study investigate the impacts of 10 nm oil amine coated AgNPs on marine diatom S. costatum with a special focus on their photosynthesis and associated mechanisms. The acute toxicity experiments show that AgNPs had an obvious growth inhibition effect on diatom S. costatum. The EC50 and EC10 of AgNPs are 25.77 mg/L and 0.048 mg/L respectively. Exposure to AgNPs at a concentration of 0.5 mg/L significantly induced excess intracellular ROS (122%) and reduced 28% of their cell viability. The algal cell viability is negatively associated with the accumulation of ROS, which indicates ROS mediated oxidative damage may be one of the toxicity pathway of AgNPs on microalgae. Exposure to AgNPs resulted in reduction of the algal chlorophyll-a content. Scanning electron microscopy (SEM) was conducted, and the results revealed that AgNPs obstructed the light absorption of algae because they adhere to their surface. The shading effect can further affect the content of biomacromolecule such as lipid and protein. Upon exposure to 0.5 mg/L AgNPs, the algal cell PSII chlorophyll fluorescence parameters Fv/Fm and NPQ decreased by 13% and 19.5% respectively, which indicates exposure to AgNPs inhibited the conversion of light energy into photosynthetic electron transport. Moreover, the genes of the PSII light harvesting protein 3HfcpB and reaction center protein Dl were significantly down-regulated (P<0.05) upon exposure to 5 mg/L AgNPs, which indicates the photosynthesis key gene regulation is a possible toxicity mechanism of AgNPs on microalgae. These results suggest that the physical adhesion and effects of shading of AgNPs on algae might affect their light energy delivery system and damage the crucial protein function of PSII. The photosynthesis inhibition effect " of AgNPs is largely different from Ag+, which remains to be further research. This study shows that AgNPs at higher concentrations might have negative effect on the succession of the phytoplankton communities and aquatic ecosystem equilibrium.The toxicity of AgNPs is not only affected by their physical and chemical parameters, but also affected by exposure conditions such as the ambient medium and the environmental conditions. In aquatic environment, AgNPs will coexist with other pollutants. Copper and tetracycline were chosen as two kinds of pollutants representing antibiotics and metals, which have the potential to have combined exposure potential with AgNPs. Under the joint exposure scenario, joint exposure of AgNPs and copper showed synergistic toxic effects in algal cells in the acute exposure condition, but changed to antagonistic effect with the extent of the exposure time. Joint exposure of AgNPs and tetracycline showed synergistic toxic effects in algal cells. Exposure to AgNPs changed the structure of the marine microalgae community which is determined by multiple factors. It has been shown that phosphorus-rich condition can mitigate the toxicity of AgNPs.This study showed that the toxicity of AgNPs is not only affected by the particle size and surface coating, but also affected by the ionic strength and phosphorus pollution level of water environment. Moreover, ROS mediated oxidative damage and photosynthesis key gene regulation are the possible toxic mechanism of AgNPs on algal cell. Our result also reveals that AgNPs have a multi-level toxicity effects on microalgae, which will finally exert a potential ecological risk to aquatic ecosystem. The results generated in the current study can be used in the ecological risk assessment of AgNPs.