Copper is an important metal that is increasingly present in everyday life in the form of nanomaterials. The intensive production and use of materials containing copper nanoparticles leads to the release of these particles into the environment, posing a potential risk to living organisms due to their high reactivity and toxicity. The green alga Chlorella vulgaris is an established model organism in ecotoxicological research. In my master thesis, I investigated the influence of CuO and Cu2O nanoparticles and copper ions (CuSO4) in concentrations that enable 75% of the algal cells of C. vulgaris to survive after 72 hours. The results show that copper nanoparticles and ions significantly increase the level of reactive oxygen species, that damage the integrity of biomembranes, while CuO nanoparticles additionally cause protein damage in the cells. Copper nanoparticles had no significant effect changing the activity of antioxidant enzymes, while ionic copper significantly inhibited their activity. Reduced glutathione levels increased, and proline levels decreased after all treatments, indicating improved priming but also continuous defense of cells against oxidative stress. After all treatments, the expression of the Hsp70 protein did not change, while the expression of the Hsp90 protein decreased significantly. Pronounced plasmolysis and destabilization of the cell wall and membrane were observed after all treatments. CuO and Cu2O nanoparticles accumulate on algal cells, in the vacuole and on thylakoid membranes. Copper nanoparticle treatments have a more toxic effect on algal cells compared to copper ions. Cu2O nanoparticles were more phytotoxic to algal cells compared to CuO nanoparticles.