Figure 3. Removal of DOC from algal cultures in stationary growth restores the toxicity of AgNPs.

AgNPs cause complete mortality of cells in stationary growth (red) after removal of organic material and resuspension of algal cells in media without nutrients. Control cultures (green), which were also centrifuged and resuspended in media without nutrients, persisted for at least 5 days. This pattern of toxicity is very similar to the rate of decline of cultures in fast growth phase exposed to 5 mg/L AgNP (Figure 1). This finding indicates that the difference in toxicity observed between growth stages of the algae is most likely due to differences in the extracellular environments of the growth stages. The data points are averages from three replicate cultures and the error bars reflect their standard error.