Dissolved organic matter mitigates the toxicity of silver nanoparticles to freshwater algae: Effects on oxidative stress and membrane function.

The mechanisms underlying the destruction of cell membranes by silver nanoparticles (AgNPs) in the presence of dissolved organic matter (DOM) remain unclear despite extensive studies on the toxicity of AgNPs to the cell membranes of aquatic organisms. This study investigated the impact of dissolved humic acid (DHA) on the toxicity of AgNPs and silver ions (Ag) to Chlorella vulgaris cell membranes. Specifically, this research examined the alterations in membrane lipid peroxidation, cell membrane permeability, fatty acid (FA) profiles, membrane fluidity, and ultrastructural changes in algal cells resulting from Ag and AgNPs, both in the absence and the presence of DHA, as well as the potential cytoprotective effect of DHA. The results revealed that DHA promoted C. vulgaris growth, which increased the EC of the AgNPs and Ag from 180 μg L to 722 μg L and 25 μg L to 193 μg L, respectively. DHA addition significantly reduced algal membrane lipid peroxidation caused by Ag and AgNPs. Furthermore, DHA increased the permeability and potential of the algal cellular membranes, particularly in the presence of Ag and AgNPs. In the absence of DHA, both Ag and AgNPs significantly increased the relative content of saturated fatty acids (SFA), while decreasing the unsaturated fatty acids (UFA) levels (p < 0.05). Consequently, both forms of silver decreased the membrane fluidity of algal cells, which increased after adding DHA. Transmission electron microscopy (TEM) showed that the presence of DHA reduced cytoplasmic disintegration and vacuolization during exposure to Ag and AgNPs. These results suggest that DHA may regulate the FA composition of algal cell membranes, aiding C. vulgaris in maintaining membrane structure and function under the toxic stress induced by AgNPs and Ag.