Algae (; singular alga ) is an informal term for a large, diverse group of photosynthetic organisms that are not necessarily closely related, thus polyphyletic. Included organisms range from unicellular microalgae genera, such as Chlorella and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 m in length. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata, xylem, and phloem, which are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and the stoneworts.
No definition of algae is generally accepted. Onedefinition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells". Some authors exclude all prokaryotes and thus do not consider cyanobacteria ( blue - green algae) as algae.
Algae constitute a polyphyletic group since they do not include a commonancestor, and although their plastids seem to have a single origin, from cyanobacteria, they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria. Diatoms and brownalgae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga.
