What is Algae? - Definition, Characteristics & Examples

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  • 0:05 Green Polar Bears
  • 1:33 Algae
  • 3:43 Algal Diversity
  • 4:47 Algal Examples
  • 6:31 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Plants are not the only organisms capable of carrying out photosynthesis. In this lesson we will look at algae and how an algal species can be differentiated from other organisms and microbes.

Green Polar Bears

I love going to the zoo. I like to see the exotic animals from all over the world all lined up, one after another. My favorite has always been the bear exhibits. Those huge predators in all different colors are the peak of North American predators. I like to start with the black bears. It is possible, however unlikely, that I could wake up in the morning, look out my window, and see a black bear strolling through my backyard. I love that connection.

Next are the brown bears. They are so huge and powerfully built. The last enclosure houses the polar bears - beautiful white beasts, perfectly evolved to the frozen Arctic landscape. But wait. That polar bear isn't white. It's green! Is this some sort of Irish leprechaun polar bear?

Would you believe that the green color is due to microscopic organisms living inside the individual hairs of the bear? Evolution produced a fur coat made of hollow hair shafts to help insulate the polar bear against the cold, harsh environment it lives in. Here in the more temperate United States, those hollow, transparent hairs have become an ecosystem of their own.

Algae have taken advantage of the clear, protected, and sunbathed coat of the zoo polar bears growing inside each hair shaft. The algae don't hurt the bear. They just make it appear more festive every St. Patrick's Day!


But what exactly are algae? Algae is the name given to a large and diverse group of oxygenic, phototrophic, eukaryotic microorganisms. Algae are eukaryotic, which means they have a nucleus. This differentiates them from bacteria and photosynthetic Cyanobacteria. They are oxygenic phototrophs, meaning they use light as their energy source for growth and produce oxygen as a byproduct, like plants.

But what distinguishes algae from plants is that algae do not have any tissue differentiation. Plants can differentiate their tissues into roots, trunks, and leaves, all very different tissue types. In contrast, algae are composed of cells that are generally all the same.

Despite the differences between plants and algae, many algal species are closely related to plants. But the algae are very diverse. Some algae, like Euglena, are closer evolutionarily to single-celled protozoa than to plants. The main unifying characteristic that determines if a species is considered an algae is that all algae contain or have a very recent ancestor that contained chlorophyll a.


As usual, in biology there are several exceptions to the algae rules. Remember those Euglena that are more related to protozoa than plants? Some Euglena species will spontaneously lose their chloroplasts and continue living as free-swimming algae that feed by absorbing nutrients from the environment.

Another genus, Prototheca, has also lost its chloroplasts and become a parasite. Prototheca has been found on the skin and in joints of dogs and, more rarely, in humans, causing inflammation. But these are exceptions, as the vast majority of algae are obligate phototrophs.

Algal Diversity

The diversity also extends to other aspects of algal life. Many algae live and grow as single, microscopic cells. Others form long chains of cells called filaments. Still other algae, like some seaweeds, can form large aggregates of cells up to 50 meters long.

Algae can be found in nearly any environment that is moist and receives enough sunlight for growth. Wet soils and both fresh and salt water are usually teeming with algae. They are one of the major members of the oceanic phytoplankton community. Phytoplankton are the large populations of free-floating microbes found throughout the oceans. It is estimated that these planktonic algae and Cyanobacteria are responsible for 80% of the total photosynthesis on Earth.

In addition to natural water sources, algae are a common inhabitant of pools, water storage tanks, and fish tanks. It is common to see fish scavenging and eating the algae that grows on the glass of aquariums, an ecofriendly way of dealing with this problem.

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