ИСТИНА

We have applied the Luria-Delbrück fluctuation analysis as a combined experimental and statistical method to distinguish between cells in which spontaneous mutations have occurred, and estimate the rate of occurrence of resistant cells. This method is widely used to distinguish between spontaneous pre-selective mutations, physiological adaptation and post-selective mutations and to assess the mutation rate in unicellular organisms ranging from bacteria to human cell cultures. In the present study we have analysed the spontaneous occurrence of chromium-resistant cells in cultures of chromium-sensitive (wild-tipe) cells of Scenedesmus quadricauda (Turp.) Breb. and Thalassiosira weissflogii (Grunow) Fryxell et Hastle and streptomycin-resistant cells in culture streptomycin-sensitive (wild-tipe) cells of Scenedesmus quadricauda. Fluctuation analysis showed that streptomycin-resistant cells and chromium- resistant cells appeared by rare spontaneous pre-selective mutations at a rate ~10-7 and ~10-6 mutants per cell division (respectively) rather than through a direct or specific adaptation in response to the presence of toxic substances. The increased the rate of spontaneous mutations in microalgae compared with the bacteria seems to be sufficient to enable them to survive in polluted conditions when the population size is large enough.