Some protists are food for marine animals, while others poison those animals. Some are packaged as nutritional supplements for humans, and some are human pathogens. The remains of some form the sands of many modern beaches, and others are a major source of the oil that sometimes fouls those beaches.
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Plasmodium is a specialized group of malaria-
The overproduction of some diatoms and dinoflagellates can result in deadly red tides.
Corals rely on dinoflagellate endosymbionts to obtain food.
Diatomaceous earth comes from the decomposition of diatoms.