There are about 10 000 kinds of seaweeds in the world. They fall into three main groups: green, brown and red.
One big difference between the groups is the pigments that they have in their cells.
- Green seaweeds have mainly chlorophyll. This green pigment captures sunlight for photosynthesis (a process that uses sunlight to produce food).
- Brown and red seaweeds also contain chlorophyll, but their cells also have other pigments that mask the green.
Most of the seaweeds that you see on the beach are brown seaweed. The group includes the typical seaweeds that most people think of—brown seaweeds can look very plant-like.
Brown seaweeds live at fairly deep depths. They are usually larger than the other kinds of seaweeds. One well-known example is the bull kelp that grows on the west coast of North America. There, it grows so large and thick that it forms forests (along with some other kinds of kelps). See a video of a kelp forest.
Brown seaweeds have gold or brown pigments in their cells, which mask the green of its chlorophyll. This makes the seaweeds look olive-green, gold, or even dark brown.
Red seaweeds are not as big as the largest brown seaweed, but there many, many species.
They have pigments that hide the green chlorophyll and make them look pink, red, purple, or brown.
Many grow in the deepest and coldest water of all, although they can also live in shallow water.
Some resemble branches. Others are hard, crust-like organisms that cling to rocks and were once thought to be a kind of coral.
Most green algae (which includes seaweeds) live in fresh water, so the saltwater-dwelling seaweeds make up a small portion of this group. Green seaweeds represent only about 10% of all green algae.
Green seaweeds live mostly in the shallowest seawater, including intertidal pools that fill and drain with the tides. Some can live where salt water and fresh water mix, where rivers empty into the ocean.
Green seaweeds often prefer warmer, tropical waters over cooler, temperate ones.
Yummy, Yummy Seaweed!
Seaweeds have many uses. Some can be eaten. Seaweed farmers grow nori (Porphyra) for use in sushi wraps and other dishes. A red seaweed called pepper dulse (Laurencia pinnatifida) is also eaten.
Some seaweeds contain substances that are used in processed foods. Red seaweeds contain carrageenans, which are used to thicken and stabilize foods such as ice cream and chocolate milk.
But food isn't the only use for seaweeds.
- Some seaweed substances are used in makeup and medicine.
- Seaweeds can also be used for fertilizer, helping to grow plants on farms, or in gardens.
Seaweed: What's It Good For?
Seaweeds and other algae form the base of the ocean's food web. They are eaten by many organisms that in turn become food for other creatures, including humans.
For example, seaweeds are an important food source for marine animals such as sea urchins and fish.
Seaweeds also provide homes for invertebrates such as crabs, molluscs and worms, as well as for larger creatures, including many species of fish.
Many animals lay their eggs and raise their young in seaweed nurseries. Even diving birds and mammals may use seaweeds to hide from predators.