Photosynthetic Protists: Definition & Examples

Instructor: Nicholas Williams

Nick has taught various subjects internationally and holds an M.Ed.

This lesson defines protist and photosynthetic protists and gives examples of some relevant species. We'll also discuss mixotrophs and explains how they obtain nutrients in both plant-like and animal-like ways.

What Is a Protist?

Most things are categorized by what they have or what they are like. But sometimes there are leftovers - like that stuff in your junk drawer in the kitchen. Biologically speaking, protists are similar to this sort of miscellaneous category.

A protist is a eukaryotic organism (meaning it has a membrane-bound nucleus) that is usually unicellular, and cannot be classified as belonging to either the plant, animal, fungus, or bacteria kingdoms. I know it sounds weird that an organism would be defined by what it isn't, but protists are misfits: some of them are more like plants, and others more like animals or fungi.

Photosynthetic Protists

Now that we know what a protist is (or at least what it isn't) let's add on the other part of this lesson - photosynthesis. Because we know that photosynthesis is a process that plants use, we can define photosynthetic protists as 'plant-like' protists that get their nutrients by converting sunlight into energy using photosynthesis.

Some examples of exclusively photosynthetic protists include some phytoplankton and unicellular algae.


The word 'phytoplankton' comes from the Greek 'phyto-' meaning plant and '-plankton' meaning 'wanderer'. Combine the two and you get plant-like things that wander, or drift, in the ocean. For example, diatoms are a branch of phytoplankton that can't swim against a current, they just drift around. They make a silica-based shell and make for a beautiful array of unique organisms when seen under the microscope.


Phytoplankton provide the basis of the aquatic food web. They not only provide food to slightly larger, more animal-like types of plankton called zooplankton, but also dissolve billions of tons of carbon dioxide per year from the atmosphere. That's important because we pump a lot of carbon dioxide out into the atmosphere yearly as a byproduct of industry and other pollutants.


That brings us to algae. Now 'algae' is probably a word that you've seen thrown around to mean goopy or leafy green stuff in the oceam which isn't totally wrong. But, in reality, an alga (singular) may refer to any simple, nonflowering plant that doesn't have real vascular tissue. They can be microscopically small, or form together in huge systems like the famous kelp forests of the Pacific.

There are many types, but what you've probably seen most are the large seaweed algaes. If you've ever seen a beach warning sign that says something like 'Do not enter: Biohazard', then you've probably seen the more dangerous red kind that actually produce biotoxins. These huge 'blooms', as they're called, can be quite deadly and are visible from satellites.

The most famous case of deadly blooms is called the Gulf of Mexico dead zone. Agricultural runoff accumulates at the mouth of the Mississippi River and the excess nutrients generate an algal bloom, resulting in an anoxic dead zone where almost nothing can live.

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