Members of the Kingdom Protista make up a good share of the phytoplankton that occupy the top few meters of the world's oceans. There they form the basis of the ocean's food chain, capturing sunlight and converting it into useable biological molecules. Another important ecological role of protists is in oxygen production; the ocean's phytoplankton create about half of all the oxygen in Earth's atmosphere.
Protists can be harmful becuase some of them can cause diseases. Malaria is caused by the protist Plasmodium falciparum. The sexually transmitted disease trichomoniasis is caused by Trichomoniasis vaginalis. Giardiasis, an uncomfortable intestinal condition, is caused by the protist Giardia lamblia.