Algae are found in natural waters in an impressive array of shapes, sizes, biochemical characteristics, and ecological roles. They constitute a group of life forms that may be unicellular or multicellular, mobile or immobile, of which practically all have photosynthetic pigments.
Since algae occur naturally in surface waters, they fall into the category of secondary pollutants. They can be beneficial to waters because they add oxygen, remove carbon dioxide, are significant in the self-purification processes and serve as food for certain aquatic fauna.
However, since algae require inorganic material such as phosphate, ammonia, nitrate and carbon dioxide which are then photosynthetically transformed into cellular organic materials and oxygen, the algae should be considered as producers, rather than destroyers of organic matter.
Classifications of algae is at present based upon the chemical composition of food storage substances and of the cell wall and upon the quality of the pigments present. The algae and the plant-like protists constitute the major segment of the aquatic pastures, the level in which radiant energy is fixed in protoplasm and transferred to non-autotrophic organisms ranging from zooplankton to fish.