Seaweed: About Seaweed
Seaweed is a loose colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae. Seaweeds can also be classified by use (as food, medicine, fertilizer, industrial, etc.). The term seaweed refers to the large marine algae that grow almost exclusively in the shallow waters at the edge of the world’s oceans. They provide home and food for many different sea animals, lend beauty to the underwater landscape, and are directly valuable to man as a food and industrial raw material.
Seaweeds are plants because they use the sun’s energy to produce carbohydrates from carbon dioxide and water (this is called photosynthesis). They are simpler than the land plants mainly because they absorb the nutrients that they require from the surrounding water and have no need for roots or complex conducting tissues. Some large seaweeds such as the kelps have root-like parts called holdfasts, but these only serve to attach them to the rock. Most seaweeds have to be attached to something in order to survive, and only a few will grow while drifting loose in the sea.
A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups are not thought to have a common multicellular ancestor, the seaweeds are a paraphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered as seaweeds—”seaweed” is a colloquial term and lacks a formal definition.
Three groups of seaweeds are recognised, according to their pigments that absorb light of particular wavelengths and give them their characteristic colours of green, brown or red. Because they need light to survive, seaweeds are found only in the relatively shallow parts of the oceans, which means around the shores. Here they occur in a variety of shapes and sizes, from the large kelps (certain brown seaweeds) that form forests on temperate (cooler) coasts, to the hard “encrusting corallines” that look like pink icing, but are so important in building and cementing coral reefs in the tropics. Some seaweeds, especially many of the larger reds, are showy and attractive, while others may be small and inconspicuous, and grow in a low “turf” on the rocks.
