Search

Several species of microalgae, closely related to mesophilic lineages, inhabit the extreme environment (pH 2.5, high levels of metals) of the Spain's Aguas Agrias Stream water (AASW). Consequently, AASW constitutes an interesting natural laboratory for analysis of adaptation by microalgae to extremely stressful conditions. To distinguish between the pre-selective or post-selective origin of adaptation processes allowing the existence of microalgae in AASW, a Luria-Delbrück fluctuation analysis was performed with the chlorophycean Dictyosphaerium chlorelloides isolated from non-acidic waters. In the analysis, AASW was used as selective factor. Preselective, resistant D. chlorelloides cells appeared with a frequency of 1.1 x 10(-6) per cell per generation. AASW-resistant mutants, with a diminished Malthusian fitness, are maintained in non-extreme waters as the result of a balance between new AASW-resistant cells arising by mutation and AASW-resistant mutants eliminated by natural selection (equilibrium at c. 12 AASW-resistants per 10(7) wild-type cells). We propose that the microalgae inhabiting this stressful environment could be the descendents of chance mutants that arrived in the past or are even arriving at the present.

Freshwater toxic cyanobacteria are an increasing problem to human and animal health. Control of cyanobacteria in water supply reservoirs involves the use of algaecides, such as copper sulphate, usually in a repetitive way. Repercussions of recurrent algaecide treatments on cyanobacteria population dynamics remain still unknown. We studied the adaptation of cyanobacteria to lethal doses of copper sulphate by using Microcystis aeruginosa as an experimental model. A fluctuation analysis demonstrated that copper-resistant cells arise by spontaneous mutations that occur randomly prior to exposition to copper sulphate. The rate of spontaneous mutation from copper sensitivity to resistance was 1.76 x 10(-6) mutants per cell division. Resistant mutants exhibited a diminished fitness in the absence of copper sulphate, but only these variants were able to grow at Cu(2+) concentrations higher than 5.8 microM. In addition, copper-resistant cells were significantly smaller than wild-type ones. Warnings on the long-term consequences of repetitive algaecide treatments in water supplies are suggested.

Abstract: 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 3.0×10−6 mutants per cell per generation in both strains of S. intermedius, and of 9.2×10−6 mutants per cell per generation in D. chlorelloides. Diquat-resistant cells in S. intermedius arose by pre-selective mutations with a rate of 17.9×10−6 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. [Copyright &y& Elsevier]

Metals are often spilled by industries into inland water environments, with adverse consequences. Numerous papers have reported that heavy metals produce massive destruction of algae. Nevertheless, algal populations seem to become tolerant when they have had previous exposures to heavy metals. Because the mechanisms allowing heavy metal tolerance of algae are not yet known, the present study analyzed the effect of hexavalent chromium on growth and photosynthetic performance of Dictyosphaerium chlorelloides, stressing on the adaptation mechanisms to chromium contamination. Growth and photosynthetic performance of algal cells were inhibited by Cr(VI) at 10 mg/L, and the 72-h median inhibition concentration was established as 1.64 and 1.54 mg/L, respectively. However, after further incubation for a three month period in an environment with 25 mg/L of chromium, some rare, chromiumresistant cells occasionally were found. A Luria-Delbrück fluctuation analysis was performed to distinguish between resistant algae arising from rare, spontaneous mutations and resistant algae arising from physiological adaptation and other adaptive mechanisms. Resistant cells arose only by spontaneous mutations before the addition of chromium, with a rate of 1.77 × 10-6 mutants per cell division. From a practical point of view, the use of both chromium-sensitive and chromium-resistant genotypes could make possible a specific algal biosensor for chromium. [ABSTRACT FROM AUTHOR]

• Adaptation ofSpirogyra insignis(Chlorophyceae) to growth and survival in an extreme natural environment (sulphureous waters from La Hedionda Spa, S. Spain) was analysed by using an experimental model.• Photosynthetis and growth of the alga were inhibited when it was cultured in La Hedionda Spa waters (LHW), but after further incubation for several weeks, the culture survived due to the growth of a variant that was resistant to LHW.• A Luria-Delbrück fluctuation analysis was carried out to distinguish between resistant filaments arising from rare spontaneous mutations and resistant filaments arising from other mechanisms of adaptation. It was demonstrated that the resistant filaments arose randomly by rare spontaneous mutations before the addition of LHW (preselective mutations). The rate of spontaneous mutation from sensitivity to resistance was 2.7 × 10−7 mutants per cell division.• Since LHWresistant mutants have a diminished growth rate, they are maintained in nonsulphureous natural waters as the result of a balance between new resistants arising from spontaneous mutation and resistants eliminated by natural selection. Thus, recurrence of rare spontaneous preselective mutations ensures the survival of the alga in sulphureous waters.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01325.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

The increasing rates of global extinction due to human activities necessitate studies of the ability of organisms to adapt to the new environmental conditions resulting from human disturbances. We investigated the evolutionary adaptation of a microalga to sudden environmental change resulting from exposure to novel toxic chemical residues. A laboratory strain of Dictyosphaerium chlorelloides (Naum.) Kom. and Perm. (Chlorophyceae) was exposed to increasing concentrations of the modern contaminant 2,4,6-trinitrotoluene (TNT). When algal cultures were exposed to 30 mg·L- 1 TNT, massive lysis of microalgal cells was observed. The key to understanding the evolution of microalgae in such a contaminated environment is to characterize the TNT-resistant variants that appear after the massive lysis of the TNT-sensitive cells. A fluctuation analysis demonstrated unequivocally that TNT did not facilitate the appearance of TNT-resistant cells; rather it was found that TNT-resistant cells appeared spontaneously by rare mutations under nonselective conditions, before exposure to TNT. The estimated mutation rate was 1.4 × 10- 5 mutants per cell division. Isolated resistant mutants exhibited a diminished fitness in the absence of TNT. Moreover, the gross photosynthetic rate of TNT-resistant mutants was significantly lower than that of wild-type cells. Competition experiments between resistant mutants and wild-type cells showed that in small populations, the resistant mutants were driven to extinction. The balance between mutation rate and the rate of selective elimination determines the occurrence of about 36 TNT-resistant mutants per million cells in each generation. These scarce resistant mutants are the guarantee of potential for adaptation. [ABSTRACT FROM AUTHOR]

The increasing rates of global extinction due to human activities necessitate studies of the ability of organisms to adapt to the new environmental conditions resulting from human disturbances. We investigated the evolutionary adaptation of a microalga to sudden environmental change resulting from exposure to novel toxic chemical residues. A laboratory strain of Dictyosphaerium chlorelloides (Naum.) Kom. and Perm. (Chlorophyceae) was exposed to increasing concentrations of the modern contaminant 2,4,6-trinitrotoluene (TNT). When algal cultures were exposed to 30 mg·L- TNT, massive lysis of microalgal cells was observed. The key to understanding the evolution of microalgae in such a contaminated environment is to characterize the TNT-resistant variants that appear after the massive lysis of the TNT-sensitive cells. A fluctuation analysis demonstrated unequivocally that TNT did not facilitate the appearance of TNT-resistant cells; rather it was found that TNT-resistant cells appeared spontaneously by rare mutations under nonselective conditions, before exposure to TNT. The estimated mutation rate was 1.4 × 101 mutants per cell division. Isolated resistant mutants exhibited a diminished fitness in the absence of TNT. Moreover, the gross photosynthetic rate of TNT-resistant mutants was significantly lower than that of wild-type cells. Competition experiments between resistant mutants and wild-type cells showed that in small populations, the resistant mutants were driven to extinction. The balance between mutation rate and the rate of selective elimination determines the occurrence of about 36 TNT-resistant mutants per million cells in each generation. These scarce resistant mutants are the guarantee of potential for adaptation. [ABSTRACT FROM AUTHOR]- 5 mutants per cell division. Isolated resistant mutants exhibited a diminished fitness in the absence of TNT. Moreover, the gross photosynthetic rate of TNT-resistant mutants was significantly lower than that of wild-type cells. Competition experiments between resistant mutants and wild-type cells showed that in small populations, the resistant mutants were driven to extinction. The balance between mutation rate and the rate of selective elimination determines the occurrence of about 36 TNT-resistant mutants per million cells in each generation. These scarce resistant mutants are the guarantee of potential for adaptation. [ABSTRACT FROM AUTHOR]