Algae Algae A seaweed Laurencia up close the branches are multicellular and only about mm thick Much smaller algae are seen attached to the structure extending upwards near the lower right corner
 This article is about an organism See algae programming language for a programming language in computing.The algae singular alga comprise several different groups of living organisms usually found in wet places or water bodies and that capture light energy through photosynthesis converting inorganic substances into simple sugars with the captured energy Algae were traditionally regarded as simple plants and some are closely related to the higher plants Others appear to represent different protist groups alongside other organisms that are traditionally considered more animal like protozoa Thus algae do not represent a single evolutionary group but a level of organization that may have developed several times in the early evolutionary history of life on earth.
Algae range from single celled organisms to multi cellular organisms some with fairly complex differentiated form and called seaweeds All lack leaves roots flowers and other organ structures that characterize higher plants They are distinguished from other protozoa in that they are photoautotrophic although this is not a hard and fast distinction as some groups may contain members that are mixotrophic deriving energy both from photosynthesis as well as through the uptake of organic carbon either by osmotrophy myzotrophy or phagotrophy Some unicellular algae rely entirely on external energy sources and have reduced or lost their photosynthetic apparatus.
All algae have photosynthetic machinery ultimately derived from the cyanobacteria and so produce oxygen as a by product of photosynthesis unlike other non cyanobacterial photosynthetic bacteria.
Algae are common in terrestrial as well as aquatic environments but usually inconspicuous on the land and more common in moist tropical climates see however Lichens The various sorts of algae play significant roles in aquatic ecology Microscopic forms that live suspended in the water column called phytoplankton provide the food base for most marine food chains In very high densities so called algal blooms they may discolor the water and outcompete or poison other life forms The seaweeds grow mostly in shallow marine waters some are used as human food or are harvested for useful substances such as agar or fertilizer The study of algae is called phycology or algology.
 Relationships among algal groups. Prokaryotic algae.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 million years ago Precambrian when they may have played a major role in creating Earth's oxygen atmosphere They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis directly within the cytoplasm rather than in specialized organelles.
 Eukaryotic algae.All other algae are eukaryotes and conduct photosynthesis within membrane bound structures organelles called chloroplasts Chloroplasts contain DNA and are similar in structure to cyanobacteria presumably representing reduced cyanobacterial endosymbionts The exact nature of the chloroplasts is different among the different lines of algae possibly reflecting different endosymbiotic events There are three groups that have primary chloroplasts.
 Green algae together with higher plants
 Red algae
 Glaucophytes
In these groups the chloroplast is surrounded by two membranes both now thought to come from the chloroplast The chloroplasts of red algae have a more or less typical cyanobacterial pigmentation while the green algae and higher plants have chloroplasts with chlorophyll a and b the latter found in some cyanobacteria but not most.