Adaptation of green microalgae to the herbicides simazine and diquat as result of pre-selective mutations

Aquatic ecosystems located close to agricultural areas are increasingly polluted by herbicides. We evaluated the capacity for adaptation of green microalgae to lethal concentrations of the herbicide simazine in one strain of Dictyosphaerium chlorelloides and two strains of Scenedesmus intermedius, as well as adaptation to the herbicide diquat in one of the strains of S. intermedius. A Luria-Delbrück fluctuation analysis was carried out in order to distinguish between resistant cells arising from physiological adaptation (acclimatization) or post-adaptive mutation (both events occurring after the exposure to the herbicides), and adaptation due to mutations before the exposure to the herbicides. Simazine-resistant cells arose by rare spontaneous mutations before the exposure to simazine, with a rate of 3.0 x 10(-6) mutants per cell per generation in both strains of S. intermedius, and of 9.2 x 10(-6) mutants per cell per generation in D. chlorelloides. Diquat-resistant cells in S. intermedius arose by pre-selective mutations with a rate of 17.9 x 10(-6) per cell per generation. Rare, pre-selective mutations may allow the survival of green microalgae in simazine- or diquat-polluted waters, via herbicide-resistant selection. Therefore, human-synthesized pollutants, such as the herbicides simazine and diquat, could cause the emergence of evolutionary novelties in aquatic environments.