Common Freshwater Protists

Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

In bodies of fresh water, we find unicellular organisms known as protists. Some are easy to see, such as clumps of algae. However, many are microscopic. Learn more about common freshwater protists in this lesson.

Life in Fresh Water

If you were asked to list off all living things found in a lake or pond, what would you say? You would probably think of frogs, fish, turtles, and ducks. You might include some insects that are found in the water. And yes, you may even mention people that are swimming. However, if these are the only organisms that you can think of, then you are literally just skimming the surface.

A pond contains many life forms
Pond

There is an entire kingdom of unseen or simply unrecognized organisms lurking in bodies of water. Kingdom Protista represents an extremely diverse group of organisms, many of which are found in freshwater environments. A large number of protists are microscopic; however, various forms of algae are protists as well. These organisms play a very important role in the aqueous food chain. In this lesson, we will learn more about the protists that make their homes in fresh water.

Protists at a Glance

Protists are mainly unicellular organisms that are also eukaryotic; so although they are tiny, they are complex. Unicellular indicates that the organism is made of a single cell. That cell is eukaryotic, meaning that it contains a nucleus as well as a plethora of organelles with important jobs, helping the cell run smoothly.

Microscopic protists
Protists

Protists can be categorized by the way they gain their nutrition. Some are called animal-like, or heterotrophic, which means that they must find and ingest food like you. Those that are plant-like are autotrophic and are mainly forms of algae. These organisms make their own food, just like plants, through the process of photosynthesis.

Animal-Like Freshwater Protists

Let's first take a look at the freshwater protists that are animal-like. We begin with phylum Rhizopoda, which contains a familiar little blob: the amoeba. You may know an amoeba as a shapeless microorganism that engulfs its prey, ingesting its nutrients. Found in fresh water, one species can cause a nasty disease known as amoebic dysentery if it makes its way into a water supply or food.

Next we find phylum Actinopoda, which literally means 'ray feet.' Members of this phylum look like balls with slender projectiles radiating out like rays from the sun. An organism belonging to this phylum is aptly named heliozoan, meaning sun animal. The rays are used for feeding, as other microorganisms get stuck to these projectiles and they are ingested. Heliozoans are an important component of plankton, which you may know as the lowest members of the aqueous food chain.

Ciliophora is a phylum that includes those tiny protists that use cilia to move about. Known as ciliates, these microorganisms are covered in thousands of cilia, or minute hair-like projections. Like our eyelashes that move together when we blink, cilia beat in unison to help the organism move. Paramecium is a prime example of a ciliate.

Ciliate under scanning electron microscope
Ciliate

Algal Freshwater Protists

Now we will uncover the world of plant-like freshwater protists, otherwise known as algae. Did you ever brush past a slimy plant-like clump while swimming and thought you were being attacked by the Creature from the Black Lagoon? Then you are familiar with algae. Often seen in lakes and ponds, algae is made up mostly of colonized groups of unicellular protists. Multicellular protists that make up the forms of seaweed are found mostly in marine environments.

Let's look first at the lovely golden algae, found in phylum Chrysophyta. This freshwater algae is made up of conglomerations of single-celled algae. Although not quite as glamorous as its name suggests, it is named for its yellow and brown pigments that give it a golden appearance. Golden algae makes its home in lakes and ponds, creating its own food through photosynthesis.

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