They are associated with coral reefs and island atolls. A few saltwater species of green algae secrete limestone that contributes to reef formation. In the Southern Hemisphere, algae are commonly seen near the low-water mark and below the shoreline. In Arctic and Antarctic waters they extend to considerable depths. In fresh water, algae are not conspicuous unless the water is polluted.
All algae contain the green pigment chlorophyll.
This process is called photosynthesis. Other pigments also are present, giving the different algae the distinct colors that are used as a basis of classification. Algae are of special interest because they include the most primitive forms of plants. They have no true roots, stems, or leaves, and do not produce flowers or seeds, as higher plants do. Yet all other groups of plants have probably evolved from algae.