Evolution of microalgae in highly stressing environments:: An experimental model analyzing the rapid adaptation of Dictyosphaerium chlorelloides (Chlorophyceae) from sensitivity to resistance against 2,4,6-trinitrotoluene by rare preselective mutations

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 x 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.