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Autotrophic Protists: Definition, Characteristics & Examples

Instructor: Danielle Haak

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

An autotrophic protist is a type of single-celled organism that can create its own food. The best-known group is the algae. Read this lesson to learn more about autotrophic protists and see some examples!

What Are Autotrophic Protists?

To answer this question, let's break it down into two parts. First, we have the term autotrophic. What does this mean? Well, an autotroph is an organism that can make its own energy, or food, typically by converting sunlight into usable components. The most common way this is done in nature is through photosynthesis. Organisms that cannot make their own energy, called heterotrophs, have to acquire energy by consuming other things.

Now we know what autotrophs are, but what are protists? A protist is a eukaryotic, microscopic organism. The term eukaryotic means the cells are organized into specific structures. A protist may be single-celled or multi-celled. Protists usually require an aquatic environment to thrive, though this may be salt or fresh water.

Characteristics and Examples of Autotrophic Protists

By knowing what each part of the name means, we already know a lot about autotrophic protists. They make their own food through the process of photosynthesis, and they are very small, eukaryotic organisms that usually live in some type of water. What else do we need to know? The largest group of autotrophic protists is collectively called algae. You have probably heard of algae before, even if you didn't know that they were classified as autotrophic protists! We can break the algae down into more specialized groups based on the shape of their cells and the type of photosynthetic pigments they use: the most common examples are green algae, red algae, brown algae, and golden algae.

Let's take a look at each of these. First, we have green algae. This type uses the pigments chlorophyll a and chlorophyll b to aid in the process of photosynthesis, resulting in the organism having a green tint in color. Green algae may be unicellular, multicellular, or live in colonies, and species use both asexual and sexual reproduction techniques.

The green algae, Micrasterias, as seen under a microscope.
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Next, we have red algae. Unlike green algae, which comes in a variety of cellular forms, most red algae are multicellular. Like the name suggests, red algae lack certain types of chlorophyll and rely on red light wavelengths to create energy. As a result, they can live in deeper waters than the green algae (red light penetrates deeper into water than other wavelengths). Some red algae reproduce asexually by releasing spores, while others may utilize sexual reproduction.

Red algae, as seen under a microscope.
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Brown algae are different from the previous two types - they are all multicellular and can be larger in size, even visible to the naked eye. Brown algae rely on the pigment fucoxanthin in photosynthesis, and this pigment gives them their brown color. (Isn't it convenient how algae are often classified by their color?)

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