Marine Algae: The 3 Types of Seaweed

Seaweeds may look like plants but they're not plants

Seaweed is the common name for marine algae. Even though they may look like underwater plants—in some cases, growing in excess of more than 150 feet in length—seaweeds are not plants at all. Instead, marine algae are a group of species from the Protista kingdom that fall into three distinct groups:

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

Although algae are not plants, they do share some basic characteristics with them. Like plants, marine algae use chlorophyll for photosynthesis. Seaweeds also have plant-like cell walls. However, unlike plants, seaweeds have no root or internal vascular systems, nor do they produce seeds or flowers, both of which are required to be classified as plants.

Brown Algae: Phaeophyta

Kelp washed ashore

Darrell Gulin / Getty Images

Brown algae, from the phylum Phaeophyta (meaning "dusky plants"), is the most prevalent type of seaweed. Brown or yellow-brown in color, brown algae are found in the waters of both temperate or arctic climates. While not roots in the true sense, brown algae typically have root-like structures called "holdfasts" that are used to anchor the algae to a surface.

Seaweeds can thrive in both salt and freshwater, but the brown algae known as kelp grows only in saltwater, most often along rocky coastlines. There are about 30 kelp varieties. One of them forms the giant kelp forests near the California coast, while another makes up the floating kelp beds in the Sargasso Sea in the North Atlantic Ocean.

One of the most widely consumed seaweeds, kelp contains many important vitamins and minerals including vitamin K, vitamin A, vitamin C, folate, vitamin E, vitamin B12, vitamin B6, thiamin, riboflavin, niacin, pantothenic acid, iodine, calcium, magnesium, iron, sodium, phosphorus, as well as small amounts of zinc, copper, manganese, and selenium.

In addition to kelp, other examples of brown algae include rockweed (Ascophyllum nodosum) and Sargassum (Fucales).

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 thanks to the pigment phycoerythrin. The ability to absorb blue light allows red algae to live at greater depths than either brown or green algae.

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. Examples of red algae include Irish moss, coralline (Corallinales), 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 can be found in marine or freshwater habitats, and some even thrive in moist soils. These algae come in three forms: unicellular, colonial, or multicellular.

Sea lettuce (Ulva lactuca) is a type of green algae commonly found in tidal pools. Codium, another green algae variety, is the favored food of some sea slugs, while the species Codium fragile is commonly referred to as "dead man's fingers."

Aquarium Algae

Although not considered one of the major types of algae, tuft-forming blue-green algae (Cyanobacteria) is sometimes considered a form of seaweed. This type of algae (also called slime algae or smear algae) is routinely found in home aquariums.

While a little bit of algae is a normal facet of a healthy aquarium ecosystem, if left unchecked, it will cover pretty much every surface in an amazingly short span of time. While some aquarium owners use chemicals to keep the algae in check, most prefer to introduce one or more species of algae-eating catfish (sometimes referred to as "suckerfish") or snails into the environment to keep algae at a manageable level.