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Phytoplankton: Definition, Types & Facts

Instructor: Thomas Higginbotham

Tom has taught math / science at secondary & post-secondary, and a K-12 school administrator. He has a B.S. in Biology and a PhD in Curriculum & Instruction.

Phytoplankton are single-celled organisms of lakes, streams and oceans that make their own food from sunlight through photosynthesis. In this lesson, learn about the different types of phytoplankton and how their impact on aquatic ecosystems and our daily lives ranges from beneficial to harmful to beautiful.

Types of Phytoplankton

Phytoplankton are single-celled organisms of lakes, streams and oceans that make their own food from sunlight through photosynthesis. Phytoplankton occur almost anywhere there is water and sunlight. While there are thousands of different types of phytoplankton, there are several main categories that make up the most commonly-occurring: cyanobacteria (aka blue-green algae or blue-green bacteria), dinoflagellates (responsible for many 'red tides'), and diatoms (one of nature's most beautiful microorganisms). The terms algae and phytoplankton are often confused. While not all algae are phytoplankton (e.g., seaweeds are algae, but are not phytoplankton), all phytoplankton are considered algae.

A dinoflagellate under a microscope.

Role as Primary Producers

It is commonly known that plants are the foundation for any kind of terrestrial ecosystem. As primary producers, plants use the process of photosynthesis to turn the energy of the sun into energy (i.e., food) that is usable to animals. Phytoplankton - one of the most common primary producers for aquatic ecosystems - are single-celled photosynthetic organisms of aquatic environments that form the base of many aquatic food chains. Phytoplankton are the primary food source for many aquatic species ranging from microscopic zooplankton to many types of invertebrates (e.g., some types of coral), to small fish. Phytoplankton are also eaten by the ocean's largest mammals, whales.

How Do They Move?

Phytoplankton are subject to the currents of the ocean, lakes and streams in which they live for their movement (i.e., mostly, they just float around). However, dinoflagellates are also able to actively control their motion using flagella. Phytoplankton are also able to move up and down vertically throughout the day, depending on the available amount of sunlight, so they balance their needs to absorb sunlight for photosynthesis and to avoid predation by zooplankton and other aquatic organisms.

Uses & Impacts

Humans encounter phytoplankton or their byproducts nearly every day in life, because they are farmed and used commercially around the world for many different purposes. Diatoms are the source for diatomaceous earth, which can be used for filters and as an abrasive that used to be commonly used in toothpaste. Spirulina and Chlorella are commonly used as nutritional supplements. Carrageenan is a derivative of phytoplankton that is used as a thickening agent in ice cream, sauces and puddings. Other phytoplankton are eaten as food or as nutritional supplements and as a fertilizer. Finally, phytoplankton are being investigated for several new uses, including as a means by which to reduce carbon dioxide levels in the atmosphere.

Micrograph of diatoms in diatomaceous earth.

In addition to being useful as both a an aquatic primary producer and industrially, an overgrowth of phytoplankton can cause several types of problems in ecosystems. A toxic algal bloom occurs when a type of phytoplankton metabolic byproduct is toxic to animals and/or plants. Often, dinoflagellates are responsible for these toxic algal blooms.

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