Silver nanoparticles as a cause of oxidative stress in microalgae

            Silver nanoparticles (AgNPs) are one of the most widely used nanomaterials in products across industries. They are often used for their antimicrobial activity in medicine and are also often found in detergents. Unfortunately these nanoparticles are highly mobile and easily transported into aquatic systems and although AgNPs are beneficial to us, in the products previously discussed, they have been shown to have a negative impact on marine ecosystems, the extent of which is not yet known. As the nanoparticles have such a large surface area to volume ratio it is thought that AgNP react strongly with compartments within and outside of the cell which may cause problems such as an increase in free radical production, causing oxidative stress, which may fatally damage the cells

            This paper investigates the detrimental effects of AgNPs on two species of microalgae; Chlorella vulgaris, a freshwater species, and Dunaliella tertiolecta, a marine species. The two species of algae were exposed to varying concentrations of AgNPs (0 mg/l, 0.01 mg/l, 0.1 mg/l, 1 mg/l and 10 mg/l) for 24 hours. They investigated the damage done to the different algae and different exposures using a number of methods. Any morphological changes between the different exposures were recorded. As well as this the total chlorophyll, amount of viable cells, amount of reactive oxygen species (ROS) formation and amount of lipid peroxidation were measured.

            It was found that both algal species showed cell aggregate formation (at 0.1 mg/l AgNPs) compared to the control (o mg/l) and it was also found that at 10 mg/l AgNPs C. vulgaris formed large aggregates. It was also found that the total chlorophyll count decreased with increasing concentrations of AgNPs in both species of algae. The total chlorophyll in D. tertiolecta was 75% (of the total chlorophyll in the control) at 10 mg/l in comparison to 50% in C. vulgaris. The number of viable cells also decreased with increasing AgNP concentration and there was a drastic decrease in viable cells at 10 mg/l with 4% (of the viable cells in the control) in D. tertiolecta and a 12% in C. vulgaris. The amount of lipid peroxidation and ROS levels (both per viable cells) increased with increasing concentration. It was also found that the amount of lipid peroxidation and ROS levels in D. tertiolecta were much higher than those in C. vulgaris.

            The results of this paper clearly indicate that both of the species of algae are badly affected by the presence of AgNPs at concentrations as low as 0.01 mg/l and that at 10 mg/l the production of ROS increases dramatically.  D. tertiolecta, the marine species, is more severely affected by AgNPs, It is however thought that this may be because more chloride is present in the seawater growth medium, in comparison to the freshwater growth medium, and that this chloride augments the toxic effects of silver. As algae play a vital role in oxygen production and are involved in the food chain the presence of AgNPs may have a huge effect on the wider aquatic community and it is therefore clear that something needs to be done to reduce the amount of AgNPs being leaked into the environment.

Reference: Oukarroum, A., Bras, S., Perreault, F., & Popovic, R. (2012). Inhibitory effects of silver nanoparticles in two green algae, Chlorella vulgaris and Dunaliella tertiolecta. Ecotoxicology and Environmental Safety, 78, 80-85.