ALGAE
Alga, unlike the protozoa, are unicellular or multicellular autotrophic organisms. All algae possess two types of light-absorbing pigments, chlorophylls and carotenoids, which function in photosynthesis. In addition, pigments specific to the type of alga may be present.

The most spectacular of the algal species are the brown algae. These algae are found in cooler marine environments, along rocky coastlines. They form extensive underwater forests known as kelp beds (see photograph). These kelp bed ecosystems provide food and habitat for many other organisms.

Another group of algae, the green algae, are thought to have given rise to plants. The rationale for this theory is that the green algae possess the same types of chlorophylls and carotenoids that are found in plants, and they have plant-like cell walls and energy storage molecules.

Other algae, such as the golden algae and diatoms, are important components of plankton, floating, microscopic organisms that are the base of the food web in most aquatic ecosystems.