What Are Red Algae?

They're not plants, though they use chlorophyll for photosynthesis

Germany, Schleswig-Holstein, Baltic Sea, Red seaweed
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Of the more than 6,000 species of red algae, most are, not surprisingly, red, reddish, or purplish in color. Red algae are protists or microscopic organisms in the phylum Rhodophyta, and range from simple one-celled organisms to complex, multi-celled organisms.

All algae get their energy from photosynthesis, but one thing that distinguishes red algae from other algae is that their cells lack flagella, the long, whiplike outgrowths from cells that are used to generate locomotion and sometimes serve a sensory function. Also surprisingly, they are not technically plants, although like plants they use chlorophyll for photosynthesis and they have plant-like cell walls.

How Red Algae Get Their Color

When you think of algae, you might think of organisms that are usually green or brown. So what is it that gives red algae their distinctive hue? Red algae contain a variety of pigments, including chlorophyll, red phycoerythrin, blue phycocyanin, carotenes, lutein, and zeaxanthin. The most important pigment is phycoerythrin, which provides these algae with their red pigmentation by reflecting red light and absorbing blue light.

Not all of these algae are a reddish color, though, as those with less phycoerythrin may appear more green or blue than red due to the abundance of the other pigments.

Habitat and Distribution

Red algae are found around the world, from polar waters to the tropics, and are commonly found in tide pools and in coral reefs. They also can survive at greater depths in the ocean than some other algae, because the phycoerythrin's absorption of blue light waves, which penetrate deeper than other light waves do, allows red algae to carry out photosynthesis at a greater depth.

Classification of Red Algae

  • Kingdom: Protista
  • Phylum: Rhodophyta

Red Algae Species

Some common examples of red algae include Irish moss, dulse, laver (nori), and coralline algae.

Coralline algae help to build tropical coral reefs. These algae secrete calcium carbonate to build a hard shell around their cell walls. There are upright forms of coralline algae, which look very similar to coral, as well as encrusting forms, which grow as a mat over hard structures such as rocks and the shells of organisms such as clams and snails. Coralline algae are often found deep in the ocean, at the maximum depth that light will penetrate the water.

Natural and Human Uses of Red Algae

Red algae are an important part of the world's ecosystem because they are eaten by fish, crustaceans, worms, and gastropods, but these algae are also eaten by humans.

Nori, for example, is used in sushi and for snacks; it becomes dark, almost black when it is dried and has a green hue when cooked. Irish moss, or carrageenan, is an additive used in foods including pudding and in the production of some beverages, such as nut milk and beer. Red algae are also used to produce agars, which are gelatinous substances used as a food additive and in science labs as a culture medium. Red algae are rich in calcium and sometimes are used in vitamin supplements.