The algae (singular is alga) comprise several different groups of living things that produce energy through photosynthesis. They are generally regarded as simple plants, and some are related to the higher plants. Others represent independent lines of evolutionary development, although they all appear to have acquired photosynthesis from cyanobacteria. All algae lack true leaves, roots, flowers, and other tissue structures found in higher plants. They are distinguished from bacteria and protozoa in that they are photoautotrophic. Thus, the algae are no longer considered a natural grouping, but the term is still used for convenience. The study of algae is called phycology or algology.
Traditionally the cyanobacteria have been included among the algae, referred to as the cyanophytes or Blue-green Algae, though some recent treatises on algae specifically exclude them. Cyanobacteria is one of the first groups of living things to appear in the fossil record, dating back some 3800 million years ago (Precambrian) when they may have played a major role in creating Earthâ€™s oxygen atmosphere. They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis directly within the cytoplasm, rather than in specialized organelles.