World Builders™
World Builders™
Session Five  --  Seaweeds
Session Five  --  Seaweeds

The Colors of Seaweeds
The Colors of Seaweeds

There are three great families of seaweeds, the red, the brown, and the green.

     When we look at seaweeds, one of the first things that we notice is their color.  Some are a bright, light green.  Some are a dark brown. Some are even red.  Why is this?  What does it mean?

     There are many groups of protists.  Scientists are trying to sort them out and to find out what their relationships are.  They still have a lot of work to do on this subject, but when it comes to seaweeds, a few things are clear.

     The three colors of seaweeds represent three quite different families  They may have similar shapes, but biochemically they are different.  Let's look at some of the important differences.



Green Algae
Chlorophyll a (found in land plants)
Chlorophyll  b

These algae store extra energy as starch.
The cell wall contains cellulose, also found in land plants

Charophyte group are the ancestors of land plants
Brown Algae
Chlorophyll a (found in land plants)
Chlorophyll c
Chlorophyll d

Brown Algae do not produce starch


Red Algae
Chlorophyll a (found in land plants)
phycoerythrin (absorbs blue light)
absorb blue light and reflect red
Adjoining cells are connected by thin protoplasmic connections "

   When you go to the beach, especially at low tide. look for the green seaweeds.  Each of these families is represented by species that are unicellular, colonial, or. multicellular.  For example, the brown seaweed family includes tiny diatoms, unicellular organisms that protect themselves with a silica covering shaped like two little parts of a box.  They swim freely in the ocean and are an important food source for ocean dwellers.  The brown seaweeds also include the sargossa seaweeds, which float in the open ocean in the Sargosso Sea.  They are branching, seaweeds with many tiny floats so that they do not sink in the water.  They are not attached to anything, and form a large mass of  tangled weeds, which provides a home for many other free swimming organisms.  The giant kelp that form underwater forests in the deeper offshore waters also belong to the brown seaweed family.  Brown seaweeds can be found in many ocean environments.  Some of them even live in the intertidal zone where they are out of water twice a day.  Even some of the lower fungi belong to the brown seaweed family.

 Green algae have the greatest need for light, so they grow where the water is shallow, or even in the intertidal zone. They are a bright, vivid green.  Our land plants are descended from a group of them.

     Brown algae grow where the water is salty or brackish. They come in many different colors of brown. They are tough, and slippery so that debris does not stick to them. Some of them have little swim bladders, gas-filled pouches that help their fronds to float up close to the surface of the water where the light is strongest. Brown algae can get along with less light than the green algae species, so they can grow in deeper water.

    Kelp are some of the best known brown seaweeds.  There are many species of kelp, living in a variety of niches in the off-shore waters.  They are adapted to different depths and degrees of water turbulence, and are found on continental shelves all over the world except in polar regions. 

    Red algae can survive with the least light, and some of them even grow 80 -100 meters below the surface of the water if the water is clear. However, they can be found at shallower depths and even in the inter-tidal zone.  They belong to a family that emerged very early in the story of the Protist kingdom, and have developed ways of making use of blue light through the use of special pigments.

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(Nov. 5, 2002)  Reference: Photosynthesis.  Retrieved Dec 7, 2003 from
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