Prototheca is a genus of algae in the family Chlorellaceae. All the species within this genus, even though classified as green algae, have forfeited their photosynthetic ability and have switched to parasitism.
Some species can cause the disease protothecosis. Symptoms include: Cutaneous lesions, Olecranon bursitis.
Protothecosis is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as Prototheca that lacks chlorophyll.
It and its close relative Helicosporidium are unusual in that they are actually green algae that have become parasites. The two most common species are Prototheca wickerhamii and Prototheca zopfii. Both are known to cause disease in dogs, while most human cases are caused by P. wickerhami. Prototheca is found worldwide in sewage and soil. Infection is rare despite high exposure, and can be related to a defective immune system. In dogs, females and Collies are most commonly affected. The first human case was identified in 1964 in Sierra Leone.
Further reading at National Institutes for Health. Human Protothecosis.