Toxic effect and adaptation in Scenedesmus intermedius to anthropogenic chloramphenicol contamination: genetic versus physiological mechanisms to rapid acquisition of xenobiotic resistance.

Anthropogenic water pollution is producing a challenge to the survival of phytoplankton populations. From an ecological point of view, the tolerance of these microorganisms to water pollution is of paramount importance since they are the principal primary producers of aquatic ecosystems. The adaptation of a common chlorophyta species (Scenedesmus intermedius) exposed to selected dose-response chloramphenicol (CAP) concentrations has been analyzed. A fluctuation analysis demonstrated that CAP-resistant cells arise due to spontaneous mutation which occurs randomly prior to the antibiotic exposure. CAP-inhibited growth and photosynthetic performance of algal cells at 0.28 mg/l, and the IC(50(72)) value was established in 0.10 mg/l for both parameters. The mutation rate from CAP sensitivity to resistance was 1.01 x 10(-5) mutations per cell division, while the frequency of CAP-resistant allele in non-polluted environment was estimated to be 5.5 CAP-resistant mutants per 10(3) sensitive-cells. These results demonstrate that resistant mutants exhibit a diminished fitness until 5 mg/l of CAP, thus enabling the survival of microalgae population.