3 Types of Seaweed (Marine Algae)

Interesting Facts About This Non-Plant Ocean Life-Form

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. 

Although algae are not plants, they do use chlorophyll for photosynthesis, and they do have plant-like cell walls. Seaweeds, however, have no root or internal vascular systems, nor do they produce seeds or flowers, things that are required components to fall under the definition of plants.

Additionally, marine algae fall into three distinct groups:

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

Cyanobacteria: The Aquarium Algae

  • There is a fourth type of algae, the tuft-forming blue-green algae (Cyanobacteria) that are sometimes considered to be seaweed.
  • This type of algae is often found in home aquariums where it will cover all surfaces in a short time.
  • It is also called slime algae or smear algae.

Brown Algae: Phaeophyta

Kelp washed ashore

Darrell Gulin / Getty Images

Brown algae make up the largest type of seaweed. It's in the phylum Phaeophyta, which means "dusky plants." Brown alga 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 alga forms the giant kelp forests near the California coast, while another forms the floating kelp beds in the Sargasso Sea, a region of the North Atlantic Ocean. Many of the edible seaweeds are kelps. 

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

Red Algae: Rhodophyta

Seaweed balls on the beach

D E N N I S A X E R Photograph / Getty Images

There are more than 6,000 species of red algae. Red algae gain their often brilliant colors 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 include Irish moss, coralline algae and dulse (Palmaria palmata).

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 / Getty Images

More than 4,000 species of green algae exist on the planet. 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 include 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."