Ecological Roles Photosynthetic protists play major ecological roles on Earth. The position of photosynthetic protists at the base of the food chain makes much of the diversity of aquatic life possible. Some examples of ecological roles played by photosynthetic protists are shown in Figure 21–8.

FIGURE 21–8 Ecological Roles of Protists Photosynthetic protists play many roles in the environment. Apply Concepts Which example is disruptive to ecosystem homeostasis?

▸ Feeding Fish and Whales Photosynthetic protists make up a large portion of phytoplankton, the small, free-floating photosynthetic organisms found near the surface of oceans and lakes. About half of the photosynthesis that takes place on Earth is carried out by phytoplankton, which provide a direct source of nourishment for organisms as diverse as shrimp and baleen whales. And they are an indirect source of nourishment for humans. When you eat tuna fish, you are eating fish that fed on smaller fish that fed on still smaller animals that fed on photosynthetic protists.

▸ Supporting Coral Reefs Coral reefs, which are found in warm ocean waters throughout the world, provide food and shelter to large numbers of fish and other organisms. Protist algae known as zooxanthellae provide most of the coral's energy needs by photosynthesis. By nourishing coral animals, these algae help maintain the equilibrium of the coral ecosystem. Coralline red algae also help to provide calcium carbonate to stabilize growing coral reefs.

▸ Providing Shelter The largest known protist is giant kelp, a brown alga that can grow to more than 60 meters in length. Kelp forests provide shelter for many marine species, and the kelp itself is a source of food for sea urchins. Another brown alga, called Sargassum, forms huge floating mats many kilometers long in an area of the Atlantic Ocean near Bermuda known as the Sargasso Sea.

▸ Recycling Wastes Many protists grow rapidly in regions where sewage is discharged, where they play a vital role in recycling waste materials. When the amount of waste is excessive, however, populations of protists like Euglena can grow to enormous numbers and create an algal bloom. Algal blooms can disrupt ecosystem homeostasis. For example, an algal bloom in a pond or lake depletes nutrients from the water, and the decomposition of the dead protists can rob water of its oxygen, causing fish and invertebrates to die. In another example, blooms of marine protists called dinoflagellates create what is known as a red tide. The buildup of toxins produced by these protists can poison fish and shellfish.

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