Ecology of Protists

Primary Producers/Food Sources

Protists are essential sources of food and provide nutrition for many other organisms. In some cases, as with zooplankton, protists are consumed directly. Alternatively, photosynthetic protists serve as producers of nutrition for other organisms. Paramecium bursaria and several other species of ciliates are mixotrophic due to a symbiotic relationship with green algae. This is a temporary version of the secondarily endosymbiotic chloroplast found in Euglena. But these symbiotic associations are not restricted to protists. For instance, photosynthetic dinoflagellates called zooxanthellae provide nutrients for the coral polyps (Figure) that house them, giving corals a boost of energy to secrete their calcium carbonate skeleton. In turn, the corals provide the protist with a protected environment and the compounds needed for photosynthesis. This type of symbiotic relationship is important in nutrient-poor environments. Without dinoflagellate symbionts, corals lose algal pigments in a process called coral bleaching, and they eventually die. This explains why reef-building corals typically do not reside in waters deeper than 20 meters: insufficient light reaches those depths for dinoflagellates to photosynthesize.

This underwater photo shows coral polyps. Polyps are cup-shaped and have tentacles extending from the edge of the cup.
Coral with symbiotic dinoflagellates. Coral polyps obtain nutrition through a symbiotic relationship with dinoflagellates.

The protists and their products of photosynthesis are essential—directly or indirectly—to the survival of organisms ranging from bacteria to mammals (Figure). As primary producers, protists feed a large proportion of the world’s aquatic species. (On land, terrestrial plants serve as primary producers.) In fact, approximately 25 percent of the world’s photosynthesis is conducted by photosynthetic protists, particularly dinoflagellates, diatoms, and multicellular algae.

The photo collage shows mollusks, a crab, a dolphin, a penguin, and a school of fish.
Protists contribute to the food chain. Virtually all aquatic organisms depend directly or indirectly on protists for food. (credit “mollusks”: modification of work by Craig Stihler, USFWS; credit “crab”: modification of work by David Berkowitz; credit “dolphin”: modification of work by Mike Baird; credit “fish”: modification of work by Tim Sheerman-Chase; credit “penguin”: modification of work by Aaron Logan)

Protists do not create food sources only for sea-dwelling organisms. Recall that certain anaerobic parabasalid species exist in the digestive tracts of termites and wood-eating cockroaches, where they contribute an essential step in the digestion of cellulose ingested by these insects as they consume wood.

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