With the rise of nanotechnologies, the risk of contamination of aquatic ecosystems with nanoparticles is increasing. Glycodendrimer-coated gold nanoparticles have been developed for biomedical applications; however, their effect on microalgae has never been studied. In this report, their interactions with algae were investigated using two strains of Chlamydomonas reinhardtii, a wild type having cell wall and a cell wall-deficient mutant. Cultures were exposed 48 h to 6 and 12 ng ml⁻¹ of gold nanoparticles coated with mannose generation 0 polyamidoamine dendrimer. Culture aggregation was found only for wild type cells, probably because of interactions between mannose and cell wall glycoproteins. Nanoparticles penetrated cytoplasm in both strains; however, inhibition of algal growth and photosynthetic activity was found only in the wild type. We conclude that nanoparticles' deteriorating effect in algae is caused by interactions with the cell wall, causing an aggregation of cell culture, and not by nanoparticle penetration inside the cytoplasm.