the five-kingdom classification proposed by R.H. Whittaker placed all unicellular eukaryotic organisms under Kingdom Protista . This reclassification brought together Chlamydomonas and Chlorella—which were previously categorized as Algae within the plant kingdom because they possess cell walls—with other single-celled eukaryotes like Amoeba and Paramoecium . While they are functionally algae, their cellular organization as single-celled eukaryotes dictates their placement in Protista under this specific system .