As primary producers, they provide major support for food chains in aquatic habitats. In addition to other eukaryotic organelles, they contain chloroplasts where photosynthesis occurs. Storage lipids of algae form the basis of the world's petroleum reserves. So we know what they are not. Most protozoans are aquatic chemoorganotrophs.
Many protozoans are obligate parasites with complex life cycles involving several organisms.
Its life cycle requires passage through the gut and salivary gland of mosquitoes, and through the liver and red blood cells of humans. Our response to this parasite provides the most clear-cut and recent example of evolutionary pressure being brought to bear on humans. The sickled cells clog up capillaries, cause great pain and poor distribution of oxygen, leading to death. Carriers of HbS have a selective advantage because they are not as easily killed by malaria before they reach reproductive age. Many fungi infect plants and degrade plant material.