Examples of Diatom Protists

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Diatoms are unicellular algae and are some of the most common types of plant material found in water. Read this lesson to learn more about these diverse algae and check out a number of examples--they're beautiful under the microscope!

What Are Diatom Protists?

Protists are very small, eukaryotic organisms (meaning their cells are organized into specialized compartments) that are not plants, animals, or fungi. We can classify protists into four sub-groups: unicellular algae, protozoa, slime molds, and water molds. Algae are a diverse set of plant-like protists found in aquatic environments, and one type of algae are diatoms, which will be the focus of our lesson.

Diatoms are single-celled organisms but can often be found lumped together, and they live in both freshwater and marine water. We still aren't sure how many diatom species exist, but the number has been estimated to be between 20,000 and 2 million!

Diatoms are photosynthetic, meaning they need light to make their food, and they also have elaborate, transparent shells made out of a chemical compound called silica (sometimes called silicon dioxide) that resembles glass. They come in different shapes and sizes, so let's take a look at some examples.

An assortment of freshwater diatoms, as seen under a microscope.
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Examples of Diatom Protists

We can classify diatoms based on their morphology, or shape--specifically the shape of the silica shell. This shell is called the frustule, and the shape of the frustule varies with the species of diatom.

Within the diatom world, two main body types prevail:

  • Centric diatoms have radial symmetry and limited movement.
  • Pennate diatoms have bilateral symmetry and more movement abilities.

The left diatom is pennate and the right diatom is centric.
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Centric Diatoms

Below are some examples of centric diatoms. Take note of the beautiful radial symmetry found in centric diatoms.

This is a microscopic image of an Arachnoidiscus species, which is found in South America.
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Here are a mix of diatoms found in the marine environment of Puget Sound, Washington. The large one in the center is an example of a centric diatom.
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Here is an image of a Cyclotella species, which is found in Mongolia.
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