Autotrophic Protists

What is the ecological significance of photosynthetic protists?

If you've seen greenish scum growing along the banks of a pond or maybe even at the edges of a poorly maintained swimming pool, you might have called it “algae” without thinking. What you may not have realized at the time is that many of the organisms in that scum were, in fact, protists.

Diversity Biologists long ago realized that the organisms commonly called “algae” actually belong to many different groups. Some (the cyanobacteria) are prokaryotes, some (the green algae) belong to the plant kingdom, and some are protists. Photosynthetic protists include many phytoplankton species and the red and brown algae, as well as euglenas and dinoflagellates. These organisms share an autotrophic lifestyle, marked by the ability to use the energy from light to make a carbohydrate food source.

You might think that all photosynthetic protists are closely related to plants, but this is not the case. In fact, it is the red algae that are most closely related to plants. Many other photosynthetic protists are more closely related to nonphotosynthetic protists. In some cases certain species within a group have lost chloroplasts. In other cases endosymbiosis added a chloroplast to some species but not to their relatives.

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