engleski

Ultrastructural changes in Chlorella vulgaris induced by silver nanoparticles and silver ions

Silver nanoparticles (AgNPs) have been used in a variety of industries including the medical, textile and agriculture sectors. Due to their tendency to agglomerate in various media, they are stabilised with various surface coatings like polyvinylpyrrolidone (PVP). As the use of AgNPs increases, so does the potential for their release into the environment. Since Chlorella vulgaris is one of the most ubiquitous microalgae inhabiting aquatic ecosystems, it is widely used as a model organism for assessing the impact of materials of anthropogenic origin, e.g. AgNPs, on aquatic habitats. To better evaluate the impact of AgNP- PVP on C. vulgaris, algae cultures were grown in a modified liquid BBM nutrient medium with and without the addition of 1% glucose. Once the algae reached the exponential growth phase, they were treated with 1.5 mg L-1 of either AgNP-PVP or ionic silver (AgNO3) (Fig.1). To evaluate the effects of AgNP-PVP and AgNO3, changes in the absorbance spectra (in the range 300-800 nm) of the algae cultures, and algae growth were analysed after 5 and 24 hours of exposure. Furthermore, to visualize ultrastructural changes after the treatments, algae cells were fixed with 1% glutaraldehyde in 50 mM cacodylate buffer (pH 7.2) for 1 h at +4 °C, washed twice and post-fixed with 1% osmium tetroxide in the same buffer for 1 h at +4 °C followed by 10 min wash in ice-cold water. After dehydration in a graded series of ethanol, the tissue was embedded in Spurr’s resin. Ultrathin sections were stained with 2% uranyl acetate and 2% lead citrate and examined using a FEI Morgagni 268D transmission electron microscope. After treatments with either AgNP-PVP or AgNO3, a significant dose-dependent decrease in cell number was observed when compared to the control sample. A significantly greater decrease was observed in cultures without added glucose compared to the cultures with added glucose. Similarly, a significant decrease was observed in the characteristic chlorophyll a and b absorbance ranges after treatments which is an indicator of toxic effect of the treatments on the algae. Again, a greater decrease was noted in the cultures without added glucose. Results from the TEM analyses (Fig.2.) showed that after 5 hours exposure in the nutrient medium without glucose, cells had undergone plasmolysis and had less starch, which can also be seen after 24 hours exposure (Fig. 2B-E). Furthermore, cell integrity was disturbed compared to the control sample, with greater disruption after exposure to AgNO3 (Fig. 2C and E). In comparison, after 5 hours of both treatments in a nutrient medium with 1% glucose, cells showed better integrity and contained more starch (Fig. 2G-J). However, after 5 hours of the same treatments supplemented with glucose, synthesis of lipid droplets had occurred, and which significantly progressed up to 24 hours (Fig. 2G-J). In conclusion, AgNP-PVP and AgNO3 showed toxic effects on the algae cultures, but with the addition of glucose to the nutrient medium, cells retained better integrity and suffered less ultrastructural damage. Decreased UV-vis absorbance and algae growth, combined with ultrastructural analyses, may indicate that AgNP- PVP and AgNO3 disrupt photosynthetic activity of the algae since the overall toxicity of both treatments was much lower in cultures grown in the nutrient medium with addition of glucose compared to those in the medium devoid of glucose.

Silver nanoparticles, Chlorella vulgaris, ultrastructure changes, silver

nije evidentirano