Portal:Algae

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Introduction

A variety of algae growing on the sea bed in shallow waters

A variety of microscopic unicellular and colonial freshwater algae

Algae (UK: /ˈælɡiː/ AL-ghee, US: /ˈældʒiː/ AL-jee; sg.: alga /ˈælɡə/ AL-gə) is an informal term for any organisms of a large and diverse group of photosynthetic eukaryotes, which include species from multiple distinct clades. Such organisms range from unicellular microalgae such as Chlorella, Prototheca and the diatoms, to multicellular macroalgae such as the giant kelp, a large brown alga which may grow up to 50 metres (160 ft) in length. Most algae are aquatic organisms and lack many of the distinct cell and tissue types, such as stomata, xylem and phloem that are found in land plants. The largest and most complex marine algae are called seaweeds. In contrast, the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and stoneworts. Algae that are carried passively by water are plankton, specifically phytoplankton.

Algae constitute a polyphyletic group since they do not include a common ancestor, and although their chlorophyll-bearing plastids seem to have a single origin (from symbiogenesis with cyanobacteria), they were acquired in different ways. Green algae are a prominent examples of algae that have primary chloroplasts derived from endosymbiont cyanobacteria. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from endosymbiotic red algae, which they acquired via phagocytosis. Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction via spores.

Algae lack the various structures that characterize plants (which evolved from freshwater green algae), such as the phyllids (leaf-like structures) and rhizoids of bryophytes ( non-vascular plants), and the roots, leaves and other xylemic/phloemic organs found in tracheophytes (vascular plants). Most algae are autotrophic, although some are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy or phagotrophy. Some unicellular species of green algae, many golden algae, euglenids, dinoflagellates, and other algae have become heterotrophs (also called colorless or apochlorotic algae), sometimes parasitic, relying entirely on external energy sources and have limited or no photosynthetic apparatus. Some other heterotrophic organisms, such as the apicomplexans, are also derived from cells whose ancestors possessed chlorophyllic plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from cyanobacteria that produce oxygen as a byproduct of splitting water molecules, unlike other organisms that conduct anoxygenic photosynthesis such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated to 1.6 to 1.7 billion years ago. (Full article...)

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Monosiphonous algae are algae which consist of a single row of cells with, or without, cortication. (Full article...)

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Microalgae or microphytes are microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist individually, or in chains or groups. Depending on the species, their sizes can range from a few micrometers (μm) to a few hundred micrometers. Unlike higher plants, microalgae do not have roots, stems, or leaves. They are specially adapted to an environment dominated by viscous forces.

Microalgae, capable of performing photosynthesis, are important for life on earth; they produce approximately half of the atmospheric oxygen and use the greenhouse gas carbon dioxide to grow photoautotrophically. "Marine photosynthesis is dominated by microalgae, which together with cyanobacteria, are collectively called phytoplankton." Microalgae, together with bacteria, form the base of the food web and provide energy for all the trophic levels above them. Microalgae biomass is often measured with chlorophyll a concentrations and can provide a useful index of potential production. (Full article...)

Selected images

Image 1Micrasterias radiata / from Shishitsuka-Pond, Tsuchiura, Ibaraki Pref., Japan / Microscope: Leica DMRD (DIC)
Image 2Stigeoclonium, a chlorophyte green alga genus
Image 3Zygnema. 10× objective, 15× eyepiece; numbered ticks are 122 µM apart.
Image 4Eucheuma farming (underwater), Philippines
Image 5Aerial view of algae farm ponds for production of beta-carotin, Whyalla, South Australia.
Image 6Algae at Yellowstone Park

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WikiProject Algae is dedicated to focusing the efforts of Wikipedia contributors on algae-related articles. Find articles to work on on its taxon notes page.