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6.4: Plant-like Protists

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What is pond scum?

Polluted water can form a frothy green scum on the surface. This "pond scum" is actually a living organism, algae. The algae are plant-like protists. Usually the algae are too small to notice, but sometimes algae grow in excess because of the excess nutrients in polluted water.

Plant-like Protists

Plant-like protists are known as algae ( Figure below ). They are a large and diverse group. Plant-like protists are autotrophs. This means that they produce their own food. They perform photosynthesis to produce sugar by using carbon dioxide and water, and the energy from sunlight, just like plants. Unlike plants, however, plant-like protists do not have true stems, roots, or leaves.

Red algae are a group of protists

Red algae are a very large group of protists making up about 5,000–6,000 species. They are mostly multicellular and live in the ocean. Many red algae are seaweeds and help create coral reefs.

Most plant-like protists live in oceans, ponds, or lakes. Protists can be unicellular (single-celled) or multicellular (many-celled). Seaweed and kelp are examples of multicellular, plant-like protists. Kelp can be as large as trees and form a "forest" in the ocean ( Figure below ).

Giant kelp is a type of multicellular, plant-like protist

Macrocystis pyrifera (giant kelp) is a type of multicellular, plant-like protist.

Plant-like protists are essential to the ecosystem. They are the base of the marine food chain, and they produce oxygen through photosynthesis for animals to breathe. They are classified into a number of basic groups ( Table below ).

Phylum Description Approximate Number of Species Examples
Chlorophyta Green algae (related to higher plants) 7,500 Chlamydomnas, Ulva, Volvox
Rhodophyta Red algae 5,000 Porphyra
Phaeophyta Brown algae 1,500 Macrocystis
Chrysophyta Diatoms, golden-brown algae, yellow-green algae 12,000 Cyclotella
Pyrrophyta Dinoflagellates 4,000 Gonyaulax
Euglenophyta Euglenoids 1,000 Euglena


  • Plant-like protists are autotrophs, meaning they make their own food.
  • Plant-like protists include algae, kelp, and seaweed.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

  1. How much of the total photosynthesis on the globe is carried out by phytoplankton? What does this mean for aquatic ecosystems?
  2. How are the flagella of dinoflagellates arranged?
  3. What organisms have dinoflagellates as symbiotes? Why are dinoflagellates important to these organisms?
  4. How do bivalves respond to red tides? What does this mean for humans?

Explore More II

  1. What is a red tide?
  2. Are all algal blooms harmful? Explain your answer.
  3. How can some algae be harmful?


  1. How are some protists similar to plants?
  2. How are plant-like protists different from plants?
  3. What are some examples of multicellular plant-like protists?
  4. How are plant-like protists essential to their ecosystem?




(singular, alga): Plant-like protists, such as diatoms and seaweeds.


Organism that makes its own food.


Plant-like, multicellular protist that can grow an ocean forest.

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Difficulty Level:

At Grade


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Date Created:

Nov 29, 2012

Last Modified:

Dec 17, 2014
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