Resistance of microalgae to colloidal silver nanoparticles

Effect of colloidal silver nanoparticles on growth and structure of laboratory populations was investigated in chlorococcal algae Scenedesmus quadricauda (Turp.) Br,b. and Monoraphidium arcuatum (Korsch.) Hind. in the range of concentrations from 0.0001 to 1 mg/L. The toxicity of colloidal silver was manifested at a concentration of 0.1 mg/L and higher and was associated with an algostatic effect, which duration directly depended on silver concentration in the medium. After a period of growth suppression at a concentration of 0.1 mg/L, in both species cell population in the culture was gradually recovered and the structure of population (ratio between the fractions of dividing, quiescent, and dead cells) became the same as in control material. The effect of adaptation of S. quadricauda to colloidal silver was detected upon successive passages to the media with toxic concentrations (0.1 and 0.5 mg/L), which depended on the concentration and duration of initial intoxication of the culture. The toxicity of cell-free filtrates from the culture of S. quadricauda, which was in contact with colloidal silver for different time periods, was estimated using intact culture of the same species, whose population on cell-free filtrates was close to control level. Transfer of the cells that were exposed to silver to a pure medium led to a long lag phase in all the cases. It is assumed that the main mechanism of algal population adaptation to higher concentrations (0.1 and 0.5 mg/L) of this toxicant depends on the selection of resistant cells present in the initial heterogeneous population in the course of primary exposure to colloidal silver. These resistant cells accidentally emerge in the population by means of rare spontaneous mutations occurring in the course of replication even before the contact with silver and have more perfect mechanisms of its detoxification.