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For the programming language, see algae (programming language) Algae are a large and diverse group of simple plant-like organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are considered "plant-like" because of their photosynthetic ability, and "simple" because they lack the distinct organs of higher plants such as leaves and vascular tissue. Though the prokaryotic Cyanobacteria (commonly referred to as Blue-green algae) were traditionally included as "algae" in older textbooks, many modern sources regard this as outdated and restrict the term algae to eukaryotic organisms. All true algae therefore have a nucleus enclosed within a membrane and chloroplasts bound in one or more membranes. Algae constitute a paraphyletic and polyphyletic group: they do not represent a single evolutionary direction or line, but a level or grade of organization that may have developed several times in the early history of life on Earth.
Algae lack leaves, roots, and other organs that characterize higher plants. They are distinguished from protozoa in that they are photosynthetic. Many are photoautotrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species 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 byproduct of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. It is estimated that algae produce about 73 to 87 percent of the net global production of oxygen that is available to humans and other animals for respiration.
Algae are most prominent in bodies of water, but are also common in terrestrial environments. However, terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adaptations to live on land. Algae are also found in other situations, such as on snow and on exposed rocks in symbiosis with a fungus as lichen.
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) these algae may discolor the water and outcompete, poison, or asphyxiate other life forms. Seaweeds grow mostly in shallow marine waters, however some have been recorded to a depth of 300 m.Some are used as human food or harvested for useful substances such as agar, carrageenan, or fertilizer.
Study of algae
The study of marine and freshwater algae is called phycology or algology.
The US Algal Collection is represented by almost 300,000 accessioned and inventoried herbarium specimens.
While Cyanobacteria have been traditionally included among the algae, referred to as the Cyanophytes or blue-green algae, recent works on algae usually exclude them due to large differences such as the lack of membrane-bound organelles, the presence of a single circular chromosome, the presence of peptidoglycan in the cell walls, and ribosomes different in size and content from eukaryotes . Rather than in chloroplasts, they conduct photosynthesis on specialized infolded cytoplasmic membranes called thylakoid membranes. Therefore, they differ significantly from the algae despite occupying similar ecological niches.
By modern definitions algae are eukaryotes and conduct photosynthesis within membrane-bound organelles called chloroplasts. Chloroplasts contain circular 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, reflecting different endosymbiotic events. The table below lists the three major groups of algae and their lineage relationship is shown in the figure on the left. Note many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost them entirely.
It was W.H.Harvey (1811 — 1866) who first divided the algae into four divisions based on their pigmentation. This is the first use of a biochemical criterion in plant systematics. Harvey's four divisions were: red algae (Rhodophyta), brown algae (Heteromontophyta), green algae (Chlorophyta) and Diatomaceae (Dixon, 1973 p.232).
Forms of algae
Most of the simpler algae are unicellular flagellates or amoeboids, but colonial and non-motile forms have developed independently among several of the groups. Some of the more common organizational levels, more than one of which may occur in the life cycle of a species, are
In three lines even higher levels of organization have been reached, with full tissue differentiation. These are the brown algae —some of which may reach 50 m in length (kelps)—the red algae , and the green algae . The most complex forms are found among the green algae (see Charales and Charophyta), in a lineage that eventually led to the higher land plants. The point where these non-algal plants begin and algae stop is usually taken to be the presence of reproductive organs with protective cell layers, a characteristic not found in the other alga groups.
The first plants on earth evolved from shallow freshwater algae much like Chara some 400 million years ago. These probably had an isomorphic alternation of generations and were probably heterotrichous. Fossils of isolated land plant spores suggest land plants may have been around as long as 475 million years ago.
Algae and symbioses
Some species of algae form symbiotic relationships with other organisms. In these symbioses, the algae supply photosynthates (organic substances) to the host organism providing protection to the algal cells. The host organism derives some or all of its energy requirements from the algae. Examples include
Rhodophyta, Chlorophyta and Heterokontophyta, the three main algal Phyla, have life-cycles which show tremendous variation with considerable complexity. In general there is an asexual phase where the seaweed's cells are diploid, a sexual phase where the cells are haploid followed by fusion of the male and female gametes. Asexual reproduction is advantageous in that it permits efficient population increases, but less variation is possible. Sexual reproduction allows more variation but is more costly because of the waste of gametes that fail to mate, among other things. Often there is no strict alternation between the sporophyte and gametophyte phases and also because there is often an asexual phase, which could include the fragmentation of the thallus.
Numbers and distribution
In the British Isles the UK Biodiversity Steering Group Report estimated there to be 20,000 algal species in the UK, freshwater and marine, about 650 of these are seaweeds. Another checklist of freshwater algae reported only about 5000 species. It seems therefore that the 20,000 is an overestimate or an error (John, 2002 p.1).
The Smithsonian collection of algae has over 300,000 specimens 
World-wide it is thought that there are over 5,000 species of red algae, 1,500 — 2,000 of brown algae and 8,000 of green algae. In Australia it is estimated that there are over 1,300 species of red algae, 350 species of brown algae and approximately 2,000 species of green algae totalling 3,650 species of algae in Australia.
Around 400 species appear to be an average figure for the coastline of South African west coast.
669 marine species have been described from California (U.S.A.).
642 entities are listed in the check-list of Britain and Ireland (Hardy and Guiry, 2006).
No publication has been found which attempts to discuss the general distribution of algae in the seas world-wide. However, notes and comments have been made by some authors. The floristic discontinuities may appear to determined by geographical features such as Antarctica, long distances of ocean or general land masses. However, the distances between Norway, the Faroes and Iceland do not show great changes in distribution.
There has been dispersal in some species by ships, water currents and the like, further some algae drifting algae can quickly become entangled and easily drift. Two red species have been introduced from the Pacific to Europe and the Mediterranean: Bonnemaisonia hamifera Hariot and Asparagopsis armata Harvey, A. armata is a native of Australia.Colpomenia peregrina is a native of the Pacific but has also invaded Europe.
Britain and Ireland
Northumberland and Durham (England)
Ireland: County Donegal
Isle of Man
Uses of algae
For centuries seaweed has been used as a fertilizer; Orwell writing in the 16th Century referring to drift weed in South Wales: "This kind of ore they often gather and lay in heaps where it heats and rots, and will have a strong and loathsome smell; when being so rotten they cast it on the land, as they do their muck, and thereof springeth good corn, especially barley" and "After spring tides or great rigs of the sea, they fetch it in sacks on horse brackets, and carry the same three, four, or five miles, and cast it on the lande, which doth very much better the ground for corn and grass" (Chapman p.35).
Algae are used by humans in many ways. They are used as fertilizers, soil conditioners and are a source of livestock feed. Because many species are aquatic and microscopic, they are cultured in clear tanks or ponds and either harvested or used to treat effluents pumped through the ponds. Algaculture on a large scale is an important type of aquaculture in some places.
Maerl is commonly used as a soil conditioner, it is dredged from the sea floor and crushed to form a powder. It is still harvested around the coasts of Brittany in France and off Falmouth, Cornwall (also extensively in western Ireland) and is a popular fertilizer in these days of organic gardening investigated Falmouth maerl and found that L. corallioides predominated down to 6 m and P. calcareum from 6-10 m (Blunden et al., 1981).
Chemical analysis of maerl showed that it contained 32.1% CaCO3 and 3.1% MgCO3 (dry weight).
Chondrus crispus, (probably confused with Mastocarpus stellatus, common name: Irish moss), is also used as "carrageen". The name carrageenan comes from the Irish Gaelic for Chondrus crispus. It is an excellent stabiliser in milk products - it reacts with the milk protein caesin, other products include: petfoods, toothpaste, ice-creams and lotions etc. Alginates in creams and lotions are absorbable through the skin.
Seaweeds are an important source of food, especially in Asia; They are excellent sources of many vitamins including: A, B1, B2, B6, niacin and C. They are rich in iodine, potassium, iron, magnesium and calcium.
Algae is commercially cultivated as a nutritional supplement. One of the most popular microalgal species is Spirulina (Arthrospira platensis), which is a Cyanobacteria (known as blue-green algae), and has been hailed by some as a superfood. Other algal species cultivated for their nutritional value include; Chlorella (a green algae), and Dunaliella (Dunaliella salina), which is high in beta-carotene and is used in vitamin C supplements.
In China at least 70 species of algae are eaten as is the Chinese "vegetable" known as fat choy (which is actually a cyanobacterium). Roughly 20 species of algae are used in everyday cooking in Japan.
Certain species are edible; the best known, especially in Ireland is Palmaria palmata (Linnaeus) O. Kuntze (Rhodymenia palmata (Linnaeus) Kuntze, common name: dulse). This is a red alga which is dried and may be bought in the shops in Ireland. It is eaten raw, fresh or dried, or cooked like spinach. Similarly, Durvillaea Antarctica  is eaten in Chile, common name: cochayuyo. 
Porphyra (common name: purple laver), is also collected and used in a variety of ways (e.g. "laver bread" in the British Isles). In Ireland it is collected and made into a jelly by stewing or boiling. Preparation also involves frying with fat or converting to a pinkish jelly by heating the fronds in a saucepan with a little water and beating with a fork. It is also collected and used by people parts of Asia, specifically China and Japan as nori and along most of the coast from California to British Columbia. The Hawaiians and the Maoris of New Zealand also use it.
One particular use is in "instant" puddings, sauces and creams. Ulva lactuca (common name: sea lettuce), is used locally in Scotland where it is added to soups or used in salads. Alaria esculenta (common name: badderlocks or dabberlocks), is used either fresh or cooked, in Greenland, Iceland, Scotland and Ireland.
The oil from some algae have high levels of unsaturated fatty acids. Arachidonic acid (a polyunsaturated fatty acid), is very high in Parietochloris incisa, (a green alga) where it reaches up to 47% of the triglyceride pool (Bigogno C et al. Phytochemistry 2002, 60, 497).  
It is a known fact that fish oil contains the omega-3 fatty acids docosahexaenoic acid, commonly known as DHA and eicosapentaenoic acid, or EPA; but The Martek Biosciences Corporation who discovered the source of DHA to be from algae manufactures DHA from algae, which is where fish get their DHA, explains J. Casey Lippmeier, Martek's senior scientist.
The algae are eaten by smaller marine life such as copepods, "and those are eaten by slightly larger fish," says Lippmeier. The DHA gets passed along the food chain, all the way up to the biggest fish, but the original source is the algae.
You can refer to the following npr.org link for an article on algae and omega-3 fatty acids; http://www.npr.org/templates/story/story.php?storyId=15823852.
There are also commercial uses of algae as agar.
The natural pigments produced by algae can be used as an alternative to chemical dyes and coloring agents. Many of the paper products used today are not recyclable because of the chemical inks that they use, paper recyclers have found that inks made from algae are much easier to break down. There is also much interest in the food industry into replacing the coloring agents that are currently used with coloring derived from algal pigments.Algea can be used to make pharmaceuticalsSewage can be treated with algae as well Some Cosmetics can come from microalgae as well. In Israel, a species of green algae is grown in water tanks, then exposed to direct sunlight and heat which causes it to become bright red in color. It is then harvested and used as a natural pigment for foods such as Salmon.
Between 100,000 and 170,000 wet tons of Macrocystis are harvested annually in California for alginate extraction and abalone feed. 
Further references to the uses
History of Phycology
Collecting and preserving specimens
Seaweed specimens can be collected and preserved for research. Such preserved specimens will keep for two or three hundred years. Those of Carl von Linné (1707 — 1778) are still available for reference, and are used. Specimens may be collected from the shore; those below low tide must be collected by diving or dredging. The whole algal specimen should be collected, that is the holdfast, stipe and lamina. Specimens of algae reproducing will be the more useful for identification and research. When collected the details of the location and site should be noted. They can then be preserved pressed on paper or in a preserving liquid such as alcohol or solution of 5 per cent formalin/seawater. However, formalin is reported to be carcinogenic.
Biological Exposure Scale
The ecology of the shores of the British Isles, including a discussion of the different shores from sheltered to exposed along with an exposure scale, is given by Lewis (1964). An exposure scale of five stages is given:- Very Exposed Shores; Exposed Shores; Semi-exposed Shores; Sheltered Shores and Very Sheltered Shores. Factors indicating the differences between these exposure scales are detailed. Very Exposed Shores have a wide Verrucaria zone entirely above the upper tide level, a Porphyra zone above the barnacle level and Lichina pygmaea is locally abundant. The eulittoral zone is dominated by barnacles and limpets with a coralline belt in the very low littoral along with other Rhodophyta and Alaria in the upper sublittoral. Exposed shores show a Verrucaria belt mainly above the high tide, with Porphyra and Lichina pygmaea. The mid shore is dominated by barnacles, limpets and some Fucus. Some Rhodophyta. Himanthalia and some Rhodophyta such as Mastocarpus and Corallina are found in the low littorral with Himanthalia, Alaria and Laminaria digitata dominant in the upper sublittoral. The semi-exposed shores show a Verrucaria belt just above high tide with clear Pelvetia in the upper-littoral and Fucus serratus in the lower-littoral. Limpets, barnacles and short Fucus vesiculosus midshore. Fucus serratus with Rhodophyta, (Laurencia, Mastocarpus stellatus, Rhodymenia and Lomentaria). Laminaria and Saccorhiza polyschides and small algae common in the sublittoral. The sheltered shores show a narrow Verrucaria zone at high water and a full sequence of fucoids: Pelvetia, Fucus spiralis, Fucus vesiculosus, Fucus serratus, Ascophyllum nodosum. Laminaria digitata is dominant the upper sublittoral. The very sheltered shores show a very narrow zone of Verrucaria, the dominance of the littoral by a full sequence of the fucoids and Ascophyllum covering the rocks. Laminaria saccharina, Halidrys, Chondrus and or Furcellaria.
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