3 Types of Sea Weed (Marine Algae)

Seaweed is the common name for marine algae—a group of species from the Protista kingdom, meaning they are not plants at all, even though they may look like underwater plants, growing to more than 150 feet in length. 

Algae are not plants, although they do use chlorophyll for photosynthesis, and they do have plant-like cell walls. Seaweeds have no root system or internal vascular systems; nor do they have seeds or flowers.

Marine algae are divided into three groups:

  • Brown Algae (Phaeophyta)
  • Green Algae (Chlorophyta)
  • Red Algae (Rhodophyta)


There is a fourth type of algae, the tuft-forming blue-green algae (Cyanobacteria) that is sometimes considered to be seaweed.

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Brown Algae: Phaeophyta

Kelp washed ashore
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Brown algae is the largest type of seaweed. It's in the phylum Phaeophyta, which means "dusky plants." Brown algae is brown or yellow-brown in color and found in temperate or arctic waters. Brown algae typically have a root-like structure called a "holdfast" to anchor the algae to a surface.

One type of brown algae forms the giant kelp forests near the California coat, while another forms the floating kelp beds in the Sargasso Sea. Many of the edible seaweeds are kelps. 

Examples of brown algae: kelp, rockweed (Fucus), Sargassum.

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Red Algae: Rhodophyta

Seaweed balls on the beach
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There are more than 6,000 species of red algae. Red algae has its often brilliant color due to the pigment phycoerythrin. This alga can live at greater depths than brown and green algae because it absorbs blue light. Coralline algae, a subgroup of red algae, is important in the formation of coral reefs.

Several types of red algae are used in food additives, and some are regular parts of Asian cuisine. 

Example of red algae: Irish moss, coralline algae, dulse (Palmaria palmata).

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Green Algae: Chlorophyta

Long underwater exposure of a mountain stream, with Green algae (Chlorophyceae sp.) moving in the current, River Ogwen, Snowdonia NP, Gwynedd, Wales, UK, October 2009
Graham Eaton/Nature Picture Library/Getty Images

There are more than 4,000 species of green algae. Green algae may be found in marine or freshwater habitats, and some even thrive in moist soils. These algae come in three forms: unicellular, colonial or multicellular.

Examples of green algae: sea lettuce (Ulva sp.), which is commonly found in tidal pools, ​and Codium sp., one species of which is commonly called "dead man's fingers."