Despite the fact, that microcistyns belongs to the most often studied cyanobacterial metabolites, there are relatively little knowledge about their effects on the natural "neighborhood" of cyanobacteria: other algae. The lack of knowledge is especially true regarding those algae, which can be characterized serious life cycle changes due to certain environmental conditions. Among the freshwater green algae, H. lacustris is the best known cyst forming species, because this species has been considered as one of the most important natural sources of astaxanthin under stressful environmental conditions. Astaxanthin has a significant economic value within different industries, therefore there are a lot of studies focused on the optimization of astaxanthin production. Previous studies have shown that a lot of stressful conditions may increase the rate of astaxanthin production. Moreover, it was demonstrated that different cell types are formed during the life cycle of H. lacustris according to the surrounding conditions and growth stage. Because of these, H. lacustris is an excellent model organism for studying cyst formation under different environmental stress conditions. The main aim of this thesis work was to study the growth, life cycle and cysts formation of H. lacustris under the presence of a cyanotoxin, MC-LR, to evaluate physiological changes. The results showed that vegetative cell number was lower in MC-LR treated cultures, in contrast, cyst number changes suggest the induction of encystment, seemingly proving our hypothesis. Evaluating the results of cell type proportion changes, our hypothesis was only partially proved: Indeed, the proportion of flagellated vegetative cells decreased, as a result palmella cells appeared so cyst formation was induced. However, the maturation of palmella did not continue, i.e. the accumulation of red pigment did not differed from control and was significantly lower than in induced culture. This shows that H. lacustris cells tried to adapt according to the surrounding environment and cyst formation was induced as its defense mechanism to survive under extreme conditions. However, the maturation of the formed palmella cells (accumulation of red pigments) missed. These preliminary results suggest that MC-LR could have a long adverse effect on the diversity of algal communities, but there are necessary more detailed studies to analyze this phenomenon.