|InterJournal Complex Systems, 2032
|Manuscript Number: |
Submission Date: 20080226
|Structural Changes And Adaptation Of Algal Population Under Different Regimens Of Toxic Exposure|
Modern chemical residues from water pollution are a challenge to survival of microagal populations. Cell populations of microalgae are complex systems with resistant and sensitive cells. When such toxicants are added to a dense microalgal culture, the cell density will be reduced after a few days due to the death of sensitive cells. However, after further incubations, the culture will sometimes increase in density again due to the growth of cell variant which is resistant to the contaminants. Heavy metals are extremely toxic to microalgae in both laboratory cultures and natural populations. The changes of population structure of freshwater green alga Scenedesmus quadricauda and marine diatom alga Thalassiosira weissflogii were studied under different regimens of heavy metal (chromium) exposure. The experiments were performed both with single chromium addition at the start of experiment and with multiple additions during exposure time. The periods between toxicant additions approximately corresponded to doubling time for algae so that the dose of the toxicant per one cell was particularly the same as that at the initial day of experiment. The final cell number was slightly decreased in the presence of 0.001 mg/L chromium and was reliably smaller in the presence of 0.01; 0.1 and 1 mg/L chromium during the multiple intoxication as compared with the single one. The share of dead and dying cells was higher at the multiple intoxication than at the single one during all experiments. We have analysed the spontaneous occurrence of chromium-resistant cells in cultures of chromium-sensitive cells of S. quadricauda and T. weissflogii. On the base of Luria-Delbrück fluctuation analysis modified to be used with microalgae growing on the liquid medium containing the selective agent, we have estimated the rates of spontaneous mutation in the analysed microalgae. These values are low (~10-6 mutants per cell division) in different species. Such mutation rate coupled with rapid growth rates, are presumably high enough to ensure the adaptation of microalgae to water contamination. Adaptation of algal populations to modern pollution-derived environmental hazards seems to be the result of a rare instantaneous events and the result of resistant cells selection within heterogeneous population.
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