Resistance of microalgae to modern water contaminants as the result of rare spontaneous mutations

Modern chemical residues from water pollution (such as herbicides, antibiot ics and even chemical substances of military use) are a challenge to surviv al of microalgal populations. Adaptation of three microalgal species - Pseu danabaena planctonica (Cyanophyceae), Dunaliella tertiolecta (Polyblepharid aceae, Chlorophyceae) and Scenedesmus sp. (Chlorophyceae) - to growth and s urvival in the presence of erythromycin, 3-(3,4-dichlorophenyl)- 1,1-dimeth ylurea, and 2,4,6-trinitrotoluene was analysed as an experimental model. Gr owth of these species was restricted even by micromolar concentrations of s uch xenobiotics. When such contaminants are added to a dense microalgal cul ture, the cell density will be reduced after a few days due to the death of sensitive cells. However, after further incubation for several days, the c ulture will sometimes increase in density again due to the growth of a cell variant which is resistant to the contaminants. A Luria-Delbruck fluctuati on analysis was carried out to distinguish between resistant cells arising from rare spontaneous mutation and resistant cells arising from other mecha nisms of adaptation. In all cases, the contaminant-resistant cells arise ra ndomly by rare spontaneous mutation during replication of cells prior to th e addition of the contaminant (pre-selective mutations). Since wild-type mi croalgal genotypes are unable to survive in the presence of such contaminan ts, spontaneous pre-selective mutation (i.e. mutation from herbicide sensit ivity to herbicide resistance) offers insights into the evolutionary capabi lities of microalgal populations in contaminated environments. The rate of spontaneous mutation from sensitivity to resistance ranged from 2.0 x 10(-6 ) mutants per cell division for erythromycin (sensitive)--> erythromycin(re sistant) P. planctonica to 8.2 x 10(-6) mutants per cell division for TNTse nsitive--> TNTresistant Scenedesmus sp. Since contaminant-resistant mutants have a diminished fitness, the resistant variants are maintained in the ab sence of the contaminants as the result of balance between new resistant ce lls arising from spontaneous mutation and resistant cells eliminated by nat ural selection. Thus recurrence of rare spontaneous pre-selective mutations ensures the survival of microalgal population in suddenly polluted environ ments.