<p>Individuals within wild populations differ substantially in their fitness as a result of either genetic differences or acclimation. Within the Charophyte algae, the two taxa <i>Chara baltica</i> and <i>C</i>. <i>liljebladii</i> predominate at different water depths of the same habitat. The two taxa are mainly distinguished by quantitative characteristics, pointing to light acclimation. In particular, they differ by a length of the internode and the bract cells, as well as the cortication type. Genetic analyses revealed that individuals of both morphotypes are genetically identical, and hence may belong to the same species. In the present paper, we tested a hypothesis that <i>C</i>. <i>liljebladii</i> is a low-light phenotype of <i>C</i>. <i>baltica</i>. Can a <i>C</i>. <i>baltica</i> phenotype be transferred into a <i>C</i>. <i>liljebladii</i> (and vice versa) by manipulation of the environmental conditions such as irradiance? We observed significant changes in the morphology of <i>C</i>. <i>baltica</i>: decreasing the irradiance significantly increased the growth of the branchlets and internodal cells. Consequently, the plants grew larger, changing their morphology in the direction of the <i>C</i>. <i>liljebladii</i> morphotype. In the reverse experiment, subjecting <i>C</i>. <i>liljebladii</i> to increasing irradiances, the individuals had slightly better growth, but none of the analysed morphological characteristics changed significantly. Both taxa have thus shown different adaptations to light limitation. Their morphologies cannot be transferred into each other by environmental factors. Thus, the presence of the different morphologies reflects the ecological characteristics of their habitats, such as light availability or turbidity.</p
