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 MUTATIONS¹

Abstract

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.

Bioremediation of Al-Sayyadin Lagoon polluted water using wild and mutant strains of microalgae

“…By means of these mechanisms, microalgae achieve adaptation toxenobiotics (García-Villada et al, 2002). In addition, Chlorophytes species were more capable of rapid adaptation to extreme environments than other algae phyla (de-Godos et al, 2010).…”

Uptake and biotransformation of 2,4,6‐trinitrotoluene (TNT) by microplantlet suspension culture of the marine red macroalga Portieria hornemannii

“…Previous work has showed that TNT dissolved in water can be unhealthy to aquatic or submerged plants, green algae, and cyanobacteria, as evidenced by a variety of phytotoxicity indicators such as reduced growth or reproduction, cell viability, chlorosis, or browning (Best et al, 1999;Burken et al, 2000;French et al, 1999;Hannink et al, 2001;Lucero et al, 1999;Nipper et al, 2001;Pavlostathis et al, 1998;Pavlostathis and Jackson, 1999;Smock et al, 1976;Won et al, 1976). Inhibition of photosynthesis resulting from TNT uptake by vascular plants or marine macroalgae has not been described earlier, although one study (García-Villada et al, 2002) demonstrated that photosynthesis in the freshwater green microalga Dictyosphaerum chlorelloides decreased exponentially with increasing TNT concentration form 3 to 30 mg/L. Inhibition of photosynthesis by P. hornemannii microplantlets during TNT uptake may occur by a mechanism similar to nitroaromatic herbicide action on the photosynthetic apparatus of higher plants, where the nitro groups disrupt electron transport in photosystem I, photosystem II and form nitroxide radicals that damage chlorophyll (Dodge, 1989;Fedtke, 1982;Van Assche and Carles, 1982).…”

Adaptation of Phytoplankton to Novel Residual Materials of Water Pollution: An Experimental Model Analysing the Evolution of an Experimental Microalgal Population Under Formaldehyde Contamination

“…The mutation rate from formaldehyde susceptibility to formaldehyde resistance in D. chlorelloides (3.61 9 10 -6 divisions) was found to be on the same order of magnitude than mutation rates we have described for resistance to many other biocides in chlorophyta López-Rodas et al 2001;Baos et al 2002;García-Villada et al 2002), significantly higher than mutation rates for SET 1 (105 tubes) They were allowed to grow, before adding the substance in study, till they reached the number of 4.2 9 10 5 cells. Set 2 consists on 25 tubes control with 4.2 9 10 5 cells that directly incorporated the studied product.…”

“…The fluctuation test (Luria and Delbrück 1943) provides the appropriate procedure to discriminate between adaptation by selection of rare spontaneous mutations and other procedures. Recently, fluctuation test has been conducted entirely in liquid media, growing microalgae cultures first in a benign medium and then exposing them to contaminants , 2007Costas et al 2001Costas et al , 2007Baos et al 2002;García-Villada et al 2002;FloresMoya et al 2005).…”