from Wikipedia, the free encyclopedia
Batrachospermum moniliforme , a red alga that lives in freshwater
Colony of the green alga Pediastrum (light microscope image)
Some species of diatoms that vary in size, shape, and color (light microscope image)

The term alga ( lat. Alga = "sea grass", "tang") is applied to various eukaryotic organisms that live in water and carry out photosynthesis . This also includes numerous photosynthetic protists . Algae are not a monophyletic kinship group in the sense of the biological system . Nevertheless, the collective term alga is also used in biology .

Cyanobacteria are traditionally referred to as "blue-green algae" because they were initially assigned to algae due to their external similarities. As bacteria, however, they belong to the prokaryotes and are the subject of bacteriology , are only partially treated in botany for historical reasons .

Two equivalent foreign words are used to denote algae science : algology or phycology (Greek φῦκος phykos "tang"). The Phycology Section of the German Botanical Society chooses an alga of the year every year .

Diversity of algae

Macroalgae and microalgae

Algae can be divided into two groups based on their size. Microscopic species are grouped together as microalgae ; they include, in particular, single-celled forms. The macroalgae (large algae), on the other hand, can be seen with the naked eye, their length ranges from a few millimeters to 60 meters. Most of the large algae live in the sea ( seaweed ). In freshwater, for example, chandelier algae are macro algae.

Habitat and way of life

Trentepohlia aurea : a widespread, orange-red air algae

Marine algae and freshwater algae

Algae are mainly found in the light-penetrated layers of the seas and in all freshwater habitats . Algae floating freely in the water form phytoplankton , the photoautotrophic part of plankton . The phytobenthos , the "plants" of the water floor, is mainly formed by algae. As Tang refers to large macroalgae, some of which extended Tangwälder form in the coastal areas of the oceans.

The microalgae of the sea are mixotrophic in their ecosystem as a whole . Although they do photosynthesis , they get a quarter of their biomass from the consumption of bacterioplankton . Mixotrophy is also known from many protists that occur in freshwater, known as algae, such as the "eye animal" Euglena .

Algae in other habitats

A smaller part of the algae has specialized in habitats outside of water bodies by adapting to (temporary) drought:

  • Air algae (aerophytes) grow on exposed surfaces such as tree trunks or rocks. You can color the surface of these with bright colors. Optionally, they also occur as endosymbionts in lichens at similar locations . One example is the genus Trentepohlia , which is common in Central Europe .
  • Soil algae (terrestrial algae) live on or in soils . For example, the green alga Fritschiella is a representative of the Edaphon .
  • Snow algae have specialized in slowly thawing snow fields in mountains and polar regions, where in summer they form the phenomenon of blood snow .


Unicellular algae in particular also enter into symbiosis , for example as zooxanthellae in some marine animals, which thereby become independent of external food supplies or are simply camouflaged. The symbiosis between algae and fungi has grown most intensely in lichens . These represent true double beings that develop common reproductive organs.

Morphological levels of organization

Coccal green algae and rod bacteria (secondary electron microscope image)
Thallous giant kelp :
phylloid above, cauloid in the middle, rhizoid below

An organizational level comprises groups of algae with common morphological characteristics of the individuals (for example regarding the external cell structure or the cell arrangement in multicellular cells), regardless of their actual relationship. The organizational levels were used in the classic algae system to artificially subdivide the various classes into orders.

A distinction is made between the following levels (selection):

  • monadoid or monadal stage: Algae, which are counted here, are flagellated unicellular organisms. The monodal level is therefore to be equated with the flagellates . It is present in almost all groups of algae, it is only absent in red algae , ornamental algae and the Pennales (a subgroup of diatoms).
  • rhizopodial or amoeboid : These are uncultivated, amoeboid single-cell organisms that do not have a cell wall. The locomotion occurs crawling through pseudopodia , i.e. through protuberances of the cell plasma . Some genera of the golden algae can be given as examples.
  • Monadoid, colony-forming stage: These are flagellated unicellular organisms that are held together in a jelly and form a cell colony . There is already a tendency for cells to differentiate . While Gonium sacculiferum still consists of four identical "Chlamydomonas-like" single cells, colonies made up of several thousand cells of the Volvox genus are already vegetative and sex cells .
  • capsal (kapsal, tetrasporal or palmelloid): uncultivated single cells that are held together by a gelatinous shell after division. Coenobia , associations of independent individual cells, arise . One example is Tetraspora .
  • coccal: unicellular organisms without proper movement (without flagella) that have a thickened cell wall. The algae of the genus Chlorococcum (green algae) have no flagella in their vegetative state, the level of organization is coccal. Flagellated unicellular organisms, the zoospores , are only formed during reproduction . Almost all diatoms in which the cell wall consists of silicon dioxide ("silica") belong to this organizational level.
  • trichal: Algae of this level form multicellular, thread-like vegetation bodies. The individual cells are separated from one another by cell walls. The cell threads are created by cell division in only one plane (one-dimensional, so to speak). Branches can also be formed. The screw alga is an example.
  • thallous: A thallus is formed by cell divisions in different spatial directions (three-dimensional) . This can seemingly be divided into tissues. The thallus of many brown algae is divided into rhizoid (analogous to the root tissue), cauloid (corresponds to the stem axis) and phylloid (leaf-like). Thallous algae can form large bodies of vegetation. The giant kelp has a length of up to 60 meters.
  • siphonal: The body contains many cell nuclei , but is not divided into cells by cell walls ( coenoblast ). It is created by multiple core divisions in several room levels without the formation of dividing walls. The siphonal thallus can be tubular or bubble-shaped or, as with the umbrella algae , morphologically differentiated into different organs. The siphonal seaweed Caulerpa taxifolia, introduced in the Mediterranean, reaches a height of 60 cm. In the system, the algae of this organizational level were previously summarized as siphonales .

The main groups of algae

Since the algae are not a natural group, here is a list of taxa in which algae occur (incomplete):

  • Glaucophyta : unicellular or small, undifferentiated cell colonies, in fresh water
  • Haptophyta : live mainly marine
  • Maw Geissler (Cryptista): mostly unicellular marine and freshwater inhabitants
  • Euglenozoa (Euglenophyta): known is the "eye animal" Euglena
  • Dinozoa (see Dinoflagellates ), approx. 1100 species: brown (the green chlorophyll is covered by red fucoxanthines), single-celled, flagellated cells with a lateral and a basal (at the rear pole) flagella. They have a solid cellulose armor within the cell membrane. They live marine or limnic. Many have special levitation devices.
  • Raphidophyceae (Chloromonadophyceae): mostly found in fresh water
  • Chlorarachniophyta : marine, there are 6 known genera
  • Yellow-green algae (Xanthophyceae): only live in fresh water
  • Golden algae (Chrysophyta): rarely marine, mostly found in fresh water with one or two apical flagella (= at the tip). Many of them form colonies.
  • Diatoms (Bacillariophyta, also called diatoms): mainly living in the sea
  • Brown algae (Phaeophyta), approx. 1500 species: almost exclusively marine, small, delicately built, filamentous, to very large, extremely resistant, tough organisms.
  • Red algae (Rhodophyta): mainly in the littoral zone of the sea, also in cold, clean streams
  • Green algae (Chlorophyta), approx. 8000 species: sea (2/5 of all species), fresh water (3/5 of all species) and also land-living representatives

In the classic classification of algae, the chloromonadophyta, yellow-green algae, golden algae, diatoms and brown algae are placed as classes in the group Heterokontophyta .

Taxa of the phylogenetic system in which groups of algae occur:

  • Excavata : The Euglenozoa are placed next to them.
  • Stramenopile (also known as Chromista): This group includes the Haptophyta, Cryptophyta, Chlorarachniophyta and Heterokontophyta.
  • Alveolata : The dinoflagellata are added to the alveolata.
  • Archaeplastida : The green algae ( Chlorophyta and Charophyta ), the Glaucophyta and the red algae are grouped together to form the Archaeplastida. This family group also includes the ("higher") plants (Embryophyta).

Number of species

The number of algae species is unknown and can only be estimated. The individual estimates vary widely and range from 30,000 to more than a million species. Estimates of the number of species in certain regions or within individual groups of algae are also uncertain. For diatoms alone , several authors have estimated high numbers of more than 200,000 species.

The uncertainty of the estimates is based, among other things, on the fact that different views exist about which organisms belong to the algae and how algae species are to be distinguished from one another. A 2012 study, using a cautious, conservative approach, came up with an estimated total of 72,500 species of algae. By June 2012 around 44,000 names for algae species had been published and 33,248 had been recorded by AlgaeBase.

Algae in the world's oceans

In the world's oceans, phytoplankton accumulates very frequently in the Arctic and in the coastal shelf seas . There is very little phytoplankton in the subtropical area . However, a research result from 2016 suggests that phytoplankton activity in the subtropical area is much higher than previously assumed. The proportion of plankton can be estimated from space using satellite images with special cameras. The algae of the phytoplankton are between a thousandth of a millimeter and half a millimeter in size. Tiny plankton animals (zooplankton) eat the algae in the world's oceans. A large part of the algae dies and sinks to the sea floor.

Marine algae are believed to have a very important influence on the binding of carbon dioxide from the atmosphere. It is estimated that 45 to 50 gigatons of carbon from carbon dioxide are bound in phytoplankton biomass annually . It is assumed that after the phytoplankton die, they sink into the depths of the sea and that the carbon dioxide produced by the microbial degradation remains bound. Around 15 percent of the carbon assimilated in phytoplankton - around 8 gigatons - sink into the depths. Without the phytoplankton of the oceans, the carbon dioxide concentration in the atmosphere would probably be 565 ppm instead of 365 ppm. The phytoplankton thus acts as a carbon pump by binding carbon dioxide from the air and from aqueous solution and transporting the carbon into the deep sea.

After many millennia, the dead phytoplankton that has sunk into the deep sea and is there under high pressure eventually turns into crude oil and natural gas .

Scientists have found that algae production in seawater increases significantly when iron ions are added. Such iron fertilization could lead to increased storage of carbon from atmospheric carbon dioxide in the sea floor by sinking algae.


The cultivation of algae in the sea, in aquaculture or in photobioreactors is gaining in importance.

In List on the island of Sylt, there is an experimental cultivation of red and brown algae, namely Palmaria and Laminaria, financed by the German Federal Environment Foundation and carried out under the direction of Klaus Lüning from the Alfred Wegener Institute for Polar and Marine Research .

A worldwide unique production facility for microalgae has existed in Klötze in Germany since 1999 . In this, under the direction of Steinberg, the green alga Chlorella vulgaris is cultivated in a 500 km long patented glass tube system.


Dried nori seaweed

Algae and their ingredients can be used for a variety of purposes. Sometimes it is extracted directly from the sea, and sometimes from facilities in which algae are cultivated. Only around 160 types of algae are used industrially, including as food.

Algae as food

Main article: Algae (food)

The focus of the use of algae as food is in Southeast Asia , where around 9 million tons are consumed annually. Different large types of algae (macro algae) are eaten raw as a salad or steamed as a vegetable . In countries like Japan, the cultivation of red algae (e.g. for sushi ) is an important industry.

Algae have a very high proportion of minerals and trace elements . A high proportion of carbohydrates , unsaturated fatty acids or beta-carotene are arguments for using other types of algae as food.

Since dried seaweed products from Asia in particular can contain a lot of iodine , caution is advised if they are consumed frequently. The German consumer advice centers warn against products that lack precise information on the iodine content and the maximum consumption.

Energetic use

Production of biohydrogen using algae on a laboratory scale

Various possibilities for energetic use of algae, e.g. B. as algae fuel ( biofuel ) are being investigated. In part, this is linked to environmental applications:

Other uses

In other, sometimes very special applications, products made from algae, their ingredients, their degradation capabilities or their degradation products are used:

  • For example, diatoms are rich in carbohydrates, fatty acids , steroids, and vitamins . These are used in a variety of ways, e.g. B. used as food supplements ("spirulettes"), thickeners ( agar ) in cosmetic products or in industry.
  • The pigments in the algae could be a more environmentally friendly alternative to ink in the future , as they are more biodegradable.
  • In the wastewater sector , algae can be used to bind fertilizers that have been washed out and used again as algae fertilizer . As with other plants, they can bind carbon dioxide (CO 2 ). In addition, pathogens are incorporated or they die in the environment that the algae produce during their growth, so that it is suitable for drinking water disinfection in the rural sector.
  • A coalition of scientists from the University of Bath , University of Bristol , Cardiff University and the University of Exeter is currently researching the potential of special algae to remove toxic heavy metals from wastewater, e.g. B. in connection with closed pits and mines to filter. These were discovered during a research project in the context of a decommissioned tin mine in Cornwall, Great Britain, on the reeds growing there, when the plants in the vicinity of the decommissioned mine were examined for the effects of the toxic mine waste water.
  • In addition, large parts of the fossil raw materials used today ( crude oil , natural gas ) were formed from deposits of dead algae .
  • Alginic acid can be obtained from brown algae , the salts (alginates) of which are used as thickening and gelling agents . Alginate is also used in biomedical engineering, for example to cover wounds.
  • In some medical and alternative medicine applications, products derived from algae are used.
  • A highly crystalline form of cellulose can also be obtained from algae , which can be used, for example, in the manufacture of tablets or as a reinforcement material for natural fiber composites .
  • Seaweed juice


The algae comprise a huge biodiversity, of which relatively little is known so far. The search for unknown species and the exploration of possible usability in various branches of industry are all the more interesting.

The algae collection of the University of Göttingen is currently one of the most comprehensive in the world with around 2200 strains.

Algae cause diseases

A few unicellular algae from the genera Prototheca and Helicosporidium can cause infectious diseases in mammals (including humans), see protothecosis .

Some types of algae produce toxic metabolic products. These algae toxins ( algal toxins ) can check out food fish, shellfish and crustaceans in the food chain accumulate. The consumption of mussels in the tissues of which algae toxins have accumulated is particularly dangerous for humans. Eating contaminated mussels can cause symptoms of poisoning such as diarrhea or paralysis and, in extreme cases, lead to death.

See also



  • Forest of the Seas. Seaweed in Brittany. Documentary film, 2006, 45 min., A film by Rüdiger Mörsdorf, production: Rüdiger Mörsdorf-Produktion, Saarländischer Rundfunk , summary by arte

Web links

Commons : Alge  - collection of images, videos and audio files
Wiktionary: Alge  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Duden online: Alge
  2. ^ Duden online: Algologie und Phykologie
  3. Alga of the Year , Phycology Section of the German Botanical Society (DBG)
  4. Wolfram Braune: Marine algae. A color guide to the common benthic green, brown and red algae of the world's oceans . Ruggell: Gantner, 2008, ISBN 978-3-906166-69-8 , pp. 11-13.
  5. Zubkov MV, Tarran GA: High bacterivory by the smallest phytoplankton in the North Atlantic Ocean . In: Nature 455 (2008): 224-226
  6. a b Michael D. Guiry: How many species of algae are there? , in: Journal of Phycology 48 (5), October 2012, pp. 1057-1063, doi: 10.1111 / j.1529-8817.2012.01222.x , PMID 27011267 .
  7. François Dufois, Nick J. Hardman-Mountford, Jim Greenwood, Anthony J. Richardson, Ming Feng, Richard J. Matear: anticyclonic eddies are more productive than cyclonic eddies in subtropical gyres Because of winter mixing . In: Science Advances . tape 2 , no. 5 , May 20, 2016, doi : 10.1126 / sciadv.1600282 (English).
  8. Volker Mrasek: Warm "Eddys" stimulate algae growth. In: Deutschlandfunk. May 23, 2016, accessed July 12, 2019 .
  9. Victor Smetacek: The primary production of marine plankton algae , Spectrum of Sciences, Issue 12/1991, p. 52
  10. Paul G. Falkowski: The invisible forest in the sea , spectrum of sciences, issue 6/2003, p. 56 ff.
  11. Green light for marine fertilization
  12. Often too much iodine in seaweed consumer, March 7, 2017.
  13. Near-natural wastewater disinfection by downstream algae ponds, patent DE102006020917 .
  14. Researchers to use algae to clean up mine water
  15. ^ Willi Paul and Chandra P. Sharma: Chitosan and Alginate Wound Dressings: A Short Review , Trends Biomater. Artif. Organs, 2004, issue 18, pp. 18-23
  16. Werner-Christian Simonis: The lower medicinal plants. Mushrooms - algae - lichens. Heidelberg 1970.
  17. Maria Strømme, Albert Mihranyan, Ragnar Ek: What to do with all thesis algae? , Materials Letters, 2002, Issue 57, pp. 569-572
  18. Min Woo Lee, Seong Ok Han, Yung Bum Seo: Red algae fiber / poly (butylene succinate) biocomposites: The effect of fiber content on their mechanical and thermal properties. Composites Science and Technology, 2010, issue 68, pp. 1266-1272
  19. Lexicon of Biology: (1999)
  20. ^ Federal Institute for Risk Assessment : Assessment of marine biotoxins in food