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6.4: Plant-like Protists

Difficulty Level: At Grade Created by: CK-12
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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 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).

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 Number (approximate) Example
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


  • algae (singular, alga): Plant-like protists, such as diatoms and seaweeds.
  • autotroph: Organism that makes its own food.
  • kelp: Plant-like, multicellular protist that can grow an "ocean forest."


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


Use the resource below to answer the questions that follow.

  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?
  1. What is a red tide?
  2. How are algae helpful?
  3. How can some algae be harmful?
  4. Are all algal blooms harmful? Explain your answer as fully as you can.


  1. How are some protists similar to plants?
  2. What are some examples of plant-like protists?




(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:
Mar 23, 2016
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