Algae

Algae
An informal term for a diverse group of photosynthetic eukaryotes
Temporal range: Mesoproterozoic–present[1]
A variety of algae growing on the sea bed in shallow waters
A variety of algae growing on the sea bed in shallow waters
Scientific classification
Groups included
Typically excluded:

Algae (/ˈældʒi, ˈælɡi/; singular alga /ˈælɡə/) is an informal term for a large, diverse group of photosynthetic eukaryotic organisms that are not necessarily closely related, and is thus polyphyletic. Included organisms range from unicellular microalgae, 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 stoneworts.

No definition of algae is generally accepted. One definition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells".[2] Although cyanobacteria are often referred to as "blue-green algae", most authorities exclude all prokaryotes from the definition of algae.[3][4]

Algae constitute a polyphyletic group[3] since they do not include a common ancestor, and although their plastids seem to have a single origin, from cyanobacteria,[5] they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga.[6]

Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction.[7]

Algae lack the various structures that characterize land plants, such as the phyllids (leaf-like structures) of bryophytes, rhizoids in nonvascular plants, and the roots, leaves, and other organs found in tracheophytes (vascular plants). Most are phototrophic, 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.[8][9][10] Some other heterotrophic organisms, such as the apicomplexans, are also derived from cells whose ancestors possessed plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from cyanobacteria that produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago.[11]

Etymology and study

The singular alga is the Latin word for "seaweed" and retains that meaning in English.[12] The etymology is obscure. Although some speculate that it is related to Latin algēre, "be cold",[13] no reason is known to associate seaweed with temperature. A more likely source is alliga, "binding, entwining".[14]

The Ancient Greek word for seaweed was φῦκος (phŷcos), which could mean either the seaweed (probably red algae) or a red dye derived from it. The Latinization, fūcus, meant primarily the cosmetic rouge. The etymology is uncertain, but a strong candidate has long been some word related to the Biblical פוך (pūk), "paint" (if not that word itself), a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, or blue.[15]

Accordingly, the modern study of marine and freshwater algae is called either phycology or algology, depending on whether the Greek or Latin root is used. The name Fucus appears in a number of taxa.

Classification

The committee on the International Code of Botanical Nomenclature has recommended certain suffixes for use in the classification of Algae. These are Phyta for division, Phyceae for class, Phycideae for subclass, ales for order, inales for sub order, aceae for family, oidease for sub family Greek name for genus and Latin name for a species.

Algal Characteristics basic to primary classification

The primary classification of algae is based on certain morphological features. The chief among these are (a) pigment constitution of the cell, (b) chemical nature of stored food materials, (c) kind, number, point of insertion and relative length of the flagella on the motile cell, (d) chemical composition of cell wall and (e) presence or absence of a definitely organized nucleus in the cell or any other significant details of cell structure.

History of Classification of Algae

Although Corolus Linnaeus (1754) included Algae along with Lichens in his 25th class Cryptogamia, he did not elaborate further on the classification of Algae.

Vaucher (1803) was perhaps the first to propose a system of classification of Algae and he recognized three groups, Conferves, Ulves and Tremelles. While Link (1820) classified Algae on the basis of the colour of the pigment and structure, Harvey (1836) proposed a system of classification on the basis of the habitat and the pigment. J. G. Agardh (1849 -1898) divided Algae into six orders: Diatomaceae, Nostochineae, Confervoideae, Ulvaceae, Floriadeae and Fucoideae. Around 1880, Algae along with Fungi were grouped under THALLOPHYTA, a division created by Eichler (1836). Encouraged by this, Engler and Prantle (1912) proposed a revised scheme of classification of algae and included fungi in algae as they were of opinion that fungi have been derived from algae. The scheme proposed by Engler and Prantle is summarised as follows:-

  1. Schizophyta
  2. Phytosarcodina
  3. Flagellata
  4. Dinoflagellata
  5. Bacillariophyta
  6. Conjugatae
  7. Chlorophyceae
  8. Charophyta
  9. Phaeophyceae
  10. Rhodophyceae
  11. Eumycetes (Fungi)[16]
    Gephyrocapsa oceanica color
    False-color scanning electron micrograph of the unicellular coccolithophore Gephyrocapsa oceanica
Gephyrocapsa oceanica color
False-color scanning electron micrograph of the unicellular coccolithophore Gephyrocapsa oceanica

The algae contain chloroplasts that are similar in structure to cyanobacteria. Chloroplasts contain circular DNA like that in cyanobacteria and are interpreted as representing reduced endosymbiotic cyanobacteria. However, the exact origin of the chloroplasts is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events. The table below describes the composition of the three major groups of algae. Their lineage relationships are shown in the figure in the upper right. Many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost plastids entirely.

Phylogeny based on plastid[17] not nucleocytoplasmic genealogy:

Cyanobacteria

Glaucophytes

rhodoplasts

Rhodophytes

Heterokonts

Cryptophytes

Haptophytes

chloroplasts

Euglenophytes

Chlorophytes

Charophytes

Land plants (Embryophyta)

Chlorarachniophytes

Supergroup affiliation Members Endosymbiont Summary
Primoplantae/
Archaeplastida
Cyanobacteria These algae have 'primary' chloroplasts, i.e. the chloroplasts are surrounded by two membranes and probably developed through a single endosymbiotic event. The chloroplasts of red algae have chlorophylls a and c (often), and phycobilins, while those of green algae have chloroplasts with chlorophyll a and b without phycobilins. Land plants are pigmented similarly to green algae and probably developed from them, thus the Chlorophyta is a sister taxon to the plants; sometimes the Chlorophyta, the Charophyta, and land plants are grouped together as the Viridiplantae.
Excavata and Rhizaria Green algae

These groups have green chloroplasts containing chlorophylls a and b.[18] Their chloroplasts are surrounded by four and three membranes, respectively, and were probably retained from ingested green algae.

Chlorarachniophytes, which belong to the phylum Cercozoa, contain a small nucleomorph, which is a relict of the algae's nucleus.

Euglenids, which belong to the phylum Euglenozoa, live primarily in fresh water and have chloroplasts with only three membranes. The endosymbiotic green algae may have been acquired through myzocytosis rather than phagocytosis.[19]

Chromista and Alveolata Red algae

These groups have chloroplasts containing chlorophylls a and c, and phycobilins. The shape varies from plant to plant; they may be of discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon shaped. They have one or more pyrenoids to preserve protein and starch. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with red algae suggest a relationship there.[20]

In the first three of these groups (Chromista), the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and they likely share a common pigmented ancestor, although other evidence casts doubt on whether the heterokonts, Haptophyta, and cryptomonads are in fact more closely related to each other than to other groups.[21][22]

The typical dinoflagellate chloroplast has three membranes, but considerable diversity exists in chloroplasts within the group, and a number of endosymbiotic events apparently occurred.[5] The Apicomplexa, a group of closely related parasites, also have plastids called apicoplasts, which are not photosynthetic, but appear to have a common origin with dinoflagellate chloroplasts.[5]

Gmelin - Historia Fucorum (Titelblatt)
Title page of Gmelin's Historia Fucorum, dated 1768

Linnaeus, in Species Plantarum (1753),[23] the starting point for modern botanical nomenclature, recognized 14 genera of algae, of which only four are currently considered among algae.[24] In Systema Naturae, Linnaeus described the genera Volvox and Corallina, and a species of Acetabularia (as Madrepora), among the animals.

In 1768, Samuel Gottlieb Gmelin (1744–1774) published the Historia Fucorum, the first work dedicated to marine algae and the first book on marine biology to use the then new binomial nomenclature of Linnaeus. It included elaborate illustrations of seaweed and marine algae on folded leaves.[25][26]

W.H.Harvey (1811–1866) and Lamouroux (1813)[27] were the first to divide macroscopic algae into four divisions based on their pigmentation. This is the first use of a biochemical criterion in plant systematics. Harvey's four divisions are: red algae (Rhodospermae), brown algae (Melanospermae), green algae (Chlorospermae), and Diatomaceae.[28][29]

At this time, microscopic algae were discovered and reported by a different group of workers (e.g., O. F. Müller and Ehrenberg) studying the Infusoria (microscopic organisms). Unlike macroalgae, which were clearly viewed as plants, microalgae were frequently considered animals because they are often motile.[27] Even the nonmotile (coccoid) microalgae were sometimes merely seen as stages of the lifecycle of plants, macroalgae, or animals.[30][31]

Although used as a taxonomic category in some pre-Darwinian classifications, e.g., Linnaeus (1753), de Jussieu (1789), Horaninow (1843), Agassiz (1859), Wilson & Cassin (1864), in further classifications, the "algae" are seen as an artificial, polyphyletic group.

Throughout the 20th century, most classifications treated the following groups as divisions or classes of algae: cyanophytes, rhodophytes, chrysophytes, xanthophytes, bacillariophytes, phaeophytes, pyrrhophytes (cryptophytes and dinophytes), euglenophytes, and chlorophytes. Later, many new groups were discovered (e.g., Bolidophyceae), and others were splintered from older groups: charophytes and glaucophytes (from chlorophytes), many heterokontophytes (e.g., synurophytes from chrysophytes, or eustigmatophytes from xanthophytes), haptophytes (from chrysophytes), and chlorarachniophytes (from xanthophytes).

With the abandonment of plant-animal dichotomous classification, most groups of algae (sometimes all) were included in Protista, later also abandoned in favour of Eukaryota. However, as a legacy of the older plant life scheme, some groups that were also treated as protozoans in the past still have duplicated classifications (see ambiregnal protists).

Some parasitic algae (e.g., the green algae Prototheca and Helicosporidium, parasites of metazoans, or Cephaleuros, parasites of plants) were originally classified as fungi, sporozoans, or protistans of incertae sedis,[32] while others (e.g., the green algae Phyllosiphon and Rhodochytrium, parasites of plants, or the red algae Pterocladiophila and Gelidiocolax mammillatus, parasites of other red algae, or the dinoflagellates Oodinium, parasites of fish) had their relationship with algae conjectured early. In other cases, some groups were originally characterized as parasitic algae (e.g., Chlorochytrium), but later were seen as endophytic algae.[33] Some filamentous bacteria (e.g., Beggiatoa) were originally seen as algae. Furthermore, groups like the apicomplexans are also parasites derived from ancestors that possessed plastids, but are not included in any group traditionally seen as algae.

Relationship to land plants

The first land plants probably evolved from shallow freshwater charophyte algae much like Chara almost 500 million years ago. These probably had an isomorphic alternation of generations and were probably filamentous. Fossils of isolated land plant spores suggest land plants may have been around as long as 475 million years ago.[34][35]

Morphology

Kelp-forest-Monterey
The kelp forest exhibit at the Monterey Bay Aquarium: A three-dimensional, multicellular thallus

A range of algal morphologies is exhibited, and convergence of features in unrelated groups is common. The only groups to exhibit three-dimensional multicellular thalli are the reds and browns, and some chlorophytes.[36] Apical growth is constrained to subsets of these groups: the florideophyte reds, various browns, and the charophytes.[36] The form of charophytes is quite different from those of reds and browns, because they have distinct nodes, separated by internode 'stems'; whorls of branches reminiscent of the horsetails occur at the nodes.[36] Conceptacles are another polyphyletic trait; they appear in the coralline algae and the Hildenbrandiales, as well as the browns.[36]

Most of the simpler algae are unicellular flagellates or amoeboids, but colonial and nonmotile 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 lifecycle of a species, are

  • Colonial: small, regular groups of motile cells
  • Capsoid: individual non-motile cells embedded in mucilage
  • Coccoid: individual non-motile cells with cell walls
  • Palmelloid: nonmotile cells embedded in mucilage
  • Filamentous: a string of nonmotile cells connected together, sometimes branching
  • Parenchymatous: cells forming a thallus with partial differentiation of tissues

In three lines, even higher levels of organization have been reached, with full tissue differentiation. These are the brown algae,[37]—some of which may reach 50 m in length (kelps)[38]—the red algae,[39] and the green algae.[40] The most complex forms are found among the charophyte algae (see Charales and Charophyta), in a lineage that eventually led to the higher land plants. The innovation that defines these nonalgal plants is the presence of female reproductive organs with protective cell layers that protect the zygote and developing embryo. Hence, the land plants are referred to as the Embryophytes.

Physiology

Many algae, particularly members of the Characeae,[41] have served as model experimental organisms to understand the mechanisms of the water permeability of membranes, osmoregulation, turgor regulation, salt tolerance, cytoplasmic streaming, and the generation of action potentials.

Phytohormones are found not only in higher plants, but in algae, too.[42]

Symbiotic algae

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 are:

Lichens

Lichens near Clogher Head (stevefe)
Rock lichens in Ireland

Lichens are defined by the International Association for Lichenology to be "an association of a fungus and a photosynthetic symbiont resulting in a stable vegetative body having a specific structure."[43] The fungi, or mycobionts, are mainly from the Ascomycota with a few from the Basidiomycota. In nature they do not occur separate from lichens. It is unknown when they began to associate.[44] One mycobiont associates with the same phycobiont species, rarely two, from the green algae, except that alternatively, the mycobiont may associate with a species of cyanobacteria (hence "photobiont" is the more accurate term). A photobiont may be associated with many different mycobionts or may live independently; accordingly, lichens are named and classified as fungal species.[45] The association is termed a morphogenesis because the lichen has a form and capabilities not possessed by the symbiont species alone (they can be experimentally isolated). The photobiont possibly triggers otherwise latent genes in the mycobiont.[46]

Trentepohlia is an example of a common green alga genus worldwide that can grow on its own or be lichenised. Lichen thus share some of the habitat and often similar appearance with specialized species of algae (aerophytes) growing on exposed surfaces such as tree trunks and rocks and sometimes discoloring them.

Coral reefs

Coral Reef
Floridian coral reef

Coral reefs are accumulated from the calcareous exoskeletons of marine invertebrates of the order Scleractinia (stony corals). These animals metabolize sugar and oxygen to obtain energy for their cell-building processes, including secretion of the exoskeleton, with water and carbon dioxide as byproducts. Dinoflagellates (algal protists) are often endosymbionts in the cells of the coral-forming marine invertebrates, where they accelerate host-cell metabolism by generating sugar and oxygen immediately available through photosynthesis using incident light and the carbon dioxide produced by the host. Reef-building stony corals (hermatypic corals) require endosymbiotic algae from the genus Symbiodinium to be in a healthy condition.[47] The loss of Symbiodinium from the host is known as coral bleaching, a condition which leads to the deterioration of a reef.

Sea sponges

Endosymbiontic green algae live close to the surface of some sponges, for example, breadcrumb sponges (Halichondria panicea). The alga is thus protected from predators; the sponge is provided with oxygen and sugars which can account for 50 to 80% of sponge growth in some species.[48]

Lifecycle

Rhodophyta, Chlorophyta, and Heterokontophyta, the three main algal divisions, have lifecycles which show considerable variation and complexity. In general, an asexual phase exists 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 permits efficient population increases, but less variation is possible. Commonly, in sexual reproduction of unicellular and colonial algae, two specialized, sexually compatible, haploid gametes make physical contact and fuse to form a zygote. To ensure a successful mating, the development and release of gametes is highly synchronized and regulated; pheromones may play a key role in these processes.[49] Sexual reproduction allows for more variation and provides the benefit of efficient recombinational repair of DNA damages during meiosis, a key stage of the sexual cycle.[50] However, sexual reproduction is more costly than asexual reproduction.[51] Meiosis has been shown to occur in many different species of algae.[52]

Numbers

Taiwan 2009 East Coast ShihTiPing Giant Stone Steps Algae FRD 6581
Algae on coastal rocks at Shihtiping in Taiwan

The Algal Collection of the US National Herbarium (located in the National Museum of Natural History) consists of approximately 320,500 dried specimens, which, although not exhaustive (no exhaustive collection exists), gives an idea of the order of magnitude of the number of algal species (that number remains unknown).[53] Estimates vary widely. For example, according to one standard textbook,[54] in the British Isles the UK Biodiversity Steering Group Report estimated there to be 20,000 algal species in the UK. Another checklist reports only about 5,000 species. Regarding the difference of about 15,000 species, the text concludes: "It will require many detailed field surveys before it is possible to provide a reliable estimate of the total number of species ..."

Regional and group estimates have been made, as well:

  • 5,000–5,500 species of red algae worldwide
  • "some 1,300 in Australian Seas"[55]
  • 400 seaweed species for the western coastline of South Africa,[56] and 212 species from the coast of KwaZulu-Natal.[57] Some of these are duplicates, as the range extends across both coasts, and the total recorded is probably about 500 species. Most of these are listed in List of seaweeds of South Africa. These exclude phytoplankton and crustose corallines.
  • 669 marine species from California (US)[58]
  • 642 in the check-list of Britain and Ireland[59]

and so on, but lacking any scientific basis or reliable sources, these numbers have no more credibility than the British ones mentioned above. Most estimates also omit microscopic algae, such as phytoplankton.

The most recent estimate suggests 72,500 algal species worldwide.[60]

Distribution

The distribution of algal species has been fairly well studied since the founding of phytogeography in the mid-19th century.[61] Algae spread mainly by the dispersal of spores analogously to the dispersal of Plantae by seeds and spores. This dispersal can be accomplished by air, water, or other organisms. Due to this, spores can be found in a variety of environments: fresh and marine waters, air, soil, and in or on other organisms.[61] Whether a spore is to grow into an organism depends on the combination of the species and the environmental conditions where the spore lands.

The spores of freshwater algae are dispersed mainly by running water and wind, as well as by living carriers.[61] However, not all bodies of water can carry all species of algae, as the chemical composition of certain water bodies limits the algae that can survive within them.[61] Marine spores are often spread by ocean currents. Ocean water presents many vastly different habitats based on temperature and nutrient availability, resulting in phytogeographic zones, regions, and provinces.[62]

To some degree, the distribution of algae is subject to floristic discontinuities caused by geographical features, such as Antarctica, long distances of ocean or general land masses. It is, therefore, possible to identify species occurring by locality, such as "Pacific algae" or "North Sea algae". When they occur out of their localities, hypothesizing a transport mechanism is usually possible, such as the hulls of ships. For example, Ulva reticulata and U. fasciata travelled from the mainland to Hawaii in this manner.

Mapping is possible for select species only: "there are many valid examples of confined distribution patterns."[63] For example, Clathromorphum is an arctic genus and is not mapped far south of there.[64] However, scientists regard the overall data as insufficient due to the "difficulties of undertaking such studies."[65]

Ecology

Phytoplankton Lake Chuzenji
Phytoplankton, Lake Chūzenji

Algae are prominent in bodies of water, common in terrestrial environments, and are found in unusual environments, such as on snow and ice. Seaweeds grow mostly in shallow marine waters, under 100 m (330 ft) deep; however, some such as Navicula pennata have been recorded to a depth of 360 m (1,180 ft).[66]

The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column (phytoplankton) provide the food base for most marine food chains. In very high densities (algal blooms), these algae may discolor the water and outcompete, poison, or asphyxiate other life forms.

Algae can be used as indicator organisms to monitor pollution in various aquatic systems.[67] In many cases, algal metabolism is sensitive to various pollutants. Due to this, the species composition of algal populations may shift in the presence of chemical pollutants.[67] To detect these changes, algae can be sampled from the environment and maintained in laboratories with relative ease.[67]

On the basis of their habitat, algae can be categorized as: aquatic (planktonic, benthic, marine, freshwater, lentic, lotic),[68] terrestrial, aerial (subareial),[69] lithophytic, halophytic (or euryhaline), psammon, thermophilic, cryophilic, epibiont (epiphytic, epizoic), endosymbiont (endophytic, endozoic), parasitic, calcifilic or lichenic (phycobiont).[70]

Cultural associations

In classical Chinese, the word is used both for "algae" and (in the modest tradition of the imperial scholars) for "literary talent". The third island in Kunming Lake beside the Summer Palace in Beijing is known as the Zaojian Tang Dao, which thus simultaneously means "Island of the Algae-Viewing Hall" and "Island of the Hall for Reflecting on Literary Talent".

Uses

Algae Harvester
Harvesting algae

Agar

Agar, a gelatinous substance derived from red algae, has a number of commercial uses.[71] It is a good medium on which to grow bacteria and fungi, as most microorganisms cannot digest agar.

Alginates

Alginic acid, or alginate, is extracted from brown algae. Its uses range from gelling agents in food, to medical dressings. Alginic acid also has been used in the field of biotechnology as a biocompatible medium for cell encapsulation and cell immobilization. Molecular cuisine is also a user of the substance for its gelling properties, by which it becomes a delivery vehicle for flavours.

Between 100,000 and 170,000 wet tons of Macrocystis are harvested annually in New Mexico for alginate extraction and abalone feed.[72][73]

Energy source

To be competitive and independent from fluctuating support from (local) policy on the long run, biofuels should equal or beat the cost level of fossil fuels. Here, algae-based fuels hold great promise,[74][75] directly related to the potential to produce more biomass per unit area in a year than any other form of biomass. The break-even point for algae-based biofuels is estimated to occur by 2025.[76]

Fertilizer

Inisheer landscape
Seaweed-fertilized gardens on Inisheer

For centuries, seaweed has been used as a fertilizer; George Owen of Henllys writing in the 16th century referring to drift weed in South Wales:[77]

This kind of ore they often gather and lay on great heapes, where it heteth and rotteth, and will have a strong and loathsome smell; when being so rotten they cast on the land, as they do their muck, and thereof springeth good corn, especially barley ... After spring-tydes or great rigs of the sea, they fetch it in sacks on horse backes, and carie the same three, four, or five miles, and cast it on the lande, which doth very much better the ground for corn and grass.

Today, algae are used by humans in many ways; for example, as fertilizers, soil conditioners, and livestock feed.[78] Aquatic and microscopic species are cultured in clear tanks or ponds and are 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.

Nutrition

Dulse
Dulse, a type of edible seaweed

Naturally growing seaweeds are an important source of food, especially in Asia. They provide many vitamins including: A, B1, B2, B6, niacin, and C, and are rich in iodine, potassium, iron, magnesium, and calcium.[79] In addition, commercially cultivated microalgae, including both algae and cyanobacteria, are marketed as nutritional supplements, such as spirulina,[80] Chlorella and the vitamin-C supplement from Dunaliella, high in beta-carotene.

Algae are national foods of many nations: China consumes more than 70 species, including fat choy, a cyanobacterium considered a vegetable; Japan, over 20 species such as nori and aonori;[81] Ireland, dulse; Chile, cochayuyo.[82] Laver is used to make "laver bread" in Wales, where it is known as bara lawr; in Korea, gim. It is also used along the west coast of North America from California to British Columbia, in Hawaii and by the Māori of New Zealand. Sea lettuce and badderlocks are salad ingredients in Scotland, Ireland, Greenland, and Iceland. Algae is being considered a potential solution for world hunger problem.[83][84][85]

The oils from some algae have high levels of unsaturated fatty acids. For example, Parietochloris incisa is very high in arachidonic acid, where it reaches up to 47% of the triglyceride pool.[86] Some varieties of algae favored by vegetarianism and veganism contain the long-chain, essential omega-3 fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Fish oil contains the omega-3 fatty acids, but the original source is algae (microalgae in particular), which are eaten by marine life such as copepods and are passed up the food chain.[87] Algae have emerged in recent years as a popular source of omega-3 fatty acids for vegetarians who cannot get long-chain EPA and DHA from other vegetarian sources such as flaxseed oil, which only contains the short-chain alpha-linolenic acid (ALA).

Pollution control

  • Sewage can be treated with algae,[88] reducing the use of large amounts of toxic chemicals that would otherwise be needed.
  • Algae can be used to capture fertilizers in runoff from farms. When subsequently harvested, the enriched algae can be used as fertilizer.
  • Aquaria and ponds can be filtered using algae, which absorb nutrients from the water in a device called an algae scrubber, also known as an algae turf scrubber.[89][90][91][92]

Agricultural Research Service scientists found that 60–90% of nitrogen runoff and 70–100% of phosphorus runoff can be captured from manure effluents using a horizontal algae scrubber, also called an algal turf scrubber (ATS). Scientists developed the ATS, which consists of shallow, 100-foot raceways of nylon netting where algae colonies can form, and studied its efficacy for three years. They found that algae can readily be used to reduce the nutrient runoff from agricultural fields and increase the quality of water flowing into rivers, streams, and oceans. Researchers collected and dried the nutrient-rich algae from the ATS and studied its potential as an organic fertilizer. They found that cucumber and corn seedlings grew just as well using ATS organic fertilizer as they did with commercial fertilizers.[93] Algae scrubbers, using bubbling upflow or vertical waterfall versions, are now also being used to filter aquaria and ponds.

Polymers

Various polymers can be created from algae, which can be especially useful in the creation of bioplastics. These include hybrid plastics, cellulose based plastics, poly-lactic acid, and bio-polyethylene.[94] Several companies have begun to produce algae polymers commercially, including for use in flip-flops[95] and in surf boards.[96]

Bioremediation

The alga Stichococcus bacillaris has been seen to colonize silicone resins used at archaeological sites; biodegrading the synthetic substance.[97]

Pigments

The natural pigments (carotenoids and chlorophylls) produced by algae can be used as alternatives to chemical dyes and coloring agents.[98] The presence of some individual algal pigments, together with specific pigment concentration ratios, are taxon-specific: analysis of their concentrations with various analytical methods, particularly high-performance liquid chromatography, can therefore offer deep insight into the taxonomic composition and relative abundance of natural algae populations in sea water samples.[99][100]

Stabilizing substances

Carrageenan, from the red alga Chondrus crispus, is used as a stabilizer in milk products.

Additional images

Algae bladder 4290

Algae bladder

See also

References

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Bibliography

General

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  • Fritsch, F.E. (1935/1945). The Structure and Reproduction of the Algae. I. and II. Cambridge, England: Cambridge University Press
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  • Smith, G.M. (1938). Cryptogamic Botany, vol. 1. McGraw-Hill, New York.

Regional

Britain and Ireland
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  • Morton, Osborne (1 December 2003). "The Marine Macroalgae of County Donegal, Ireland". Bulletin of the Irish Biogeographical Society. 27: 3–164.
Australia
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New Zealand
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Europe
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  • Gayral, Paulette (1966). Les Algues de côtes françaises (manche et atlantique), notions fondamentales sur l'écologie, la biologie et la systématique des algues marines (in French). Paris: Doin, Deren et Cie.
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  • Míguez Rodríguez, Luís (1998). Algas mariñas de Galicia: bioloxía, gastronomía, industria (in Galician). Vigo: Edicións Xerais de Galicia. ISBN 978-84-8302-263-4.
  • Otero, J. (2002). Guía das macroalgas de Galicia (in Galician). A Coruña: Baía Edicións. ISBN 978-84-89803-22-0.
  • Bárbara, I.; Cremades, J. (1993). Guía de las algas del litoral gallego (in Spanish). A Coruña: Concello da Coruña – Casa das Ciencias.
Arctic
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Greenland
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Faroe Islands
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Canary Islands
  • Børgesen, Frederik (1936) [1925, 1926, 1927, 1929, 1930]. Marine Algae from the Canary Islands. København: Bianco Lunos.
Morocco
  • Gayral, Paulette (1958). Algues de la côte atlantique marocaine (in French). Casablanca: Rabat [Société des sciences naturelles et physiques du Maroc].
South Africa
  • Stegenga, H.; Bolton, J.J.; Anderson, R.J. (1997). Seaweeds of the South African West Coast. Bolus Herbarium, University of Cape Town. ISBN 978-0-7992-1793-3.
North America

External links

Agar

Agar () or agar-agar, is a jelly-like substance, obtained from red algae.Agar is a mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin. It forms the supporting structure in the cell walls of certain species of algae, and is released on boiling. These algae are known as agarophytes, and belong to the Rhodophyta (red algae) phylum.Agar has been used as an ingredient in desserts throughout Asia, and also as a solid substrate to contain culture media for microbiological work. Agar can be used as a laxative, an appetite suppressant, a vegetarian substitute for gelatin, a thickener for soups, in fruit preserves, ice cream, and other desserts, as a clarifying agent in brewing, and for sizing paper and fabrics.The gelling agent in agar is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from tengusa (Gelidiaceae) and ogonori (Gracilaria). For commercial purposes, it is derived primarily from ogonori. In chemical terms, agar is a polymer made up of subunits of the sugar galactose.

AlgaeBase

AlgaeBase is a global species database of information on all groups of algae, as well as one group of flowering plants, the sea-grasses.AlgaeBase evolved from Michael Guiry's seaweed website, and has grown into a database of algae from throughout the world, and in freshwater, terrestrial, and brackish as well as marine habitats. By 2005, the database contained about 65,000 names, and by September 2006, 122,240 species and infraspecific names were in the database, with 5,826 images, 38,290 bibliographic items, 138,706 distributional records. Currently, the data for the marine algae, particularly seaweeds, are the most complete. About 30,000 species of algae are included, of which the Rhodophyta (6000 species), marine Chlorophyta (1500 species), and Phaeophyceae (1755 species) are the most complete. The diatoms and the smaller freshwater green algae are currently (August 2010) the most incomplete groups.

Programming is carried out by VisualID (http://www.visualid.com) (Pier Kuipers and Caoilte Guiry) and the compilation of the data was funded by the Irish government Department of Education and Science's PRTLI 3 and 4 programmes (http://www.hea.ie/en/prtli).

Algae fuel

Algae fuel, algal biofuel, or algal oil is an alternative to liquid fossil fuels that uses algae as its source of energy-rich oils. Also, algae fuels are an alternative to commonly known biofuel sources, such as corn and sugarcane. When made from seaweed (macroalgae) it can be known as seaweed fuel or seaweed oil.

Several companies and government agencies are funding efforts to reduce capital and operating costs and make algae fuel production commercially viable. Like fossil fuel, algae fuel releases CO2 when burnt, but unlike fossil fuel, algae fuel and other biofuels only release CO2 recently removed from the atmosphere via photosynthesis as the algae or plant grew. The energy crisis and the world food crisis have ignited interest in algaculture (farming algae) for making biodiesel and other biofuels using land unsuitable for agriculture. Among algal fuels' attractive characteristics are that they can be grown with minimal impact on fresh water resources, can be produced using saline and wastewater, have a high flash point, and are biodegradable and relatively harmless to the environment if spilled. Algae cost more per unit mass than other second-generation biofuel crops due to high capital and operating costs, but are claimed to yield between 10 and 100 times more fuel per unit area. The United States Department of Energy estimates that if algae fuel replaced all the petroleum fuel in the United States, it would require 15,000 square miles (39,000 km2), which is only 0.42% of the U.S. map, or about half of the land area of Maine. This is less than ​1⁄7 the area of corn harvested in the United States in 2000.The head of the Algal Biomass Organization stated in 2010 that algae fuel could reach price parity with oil in 2018 if granted production tax credits. However, in 2013, Exxon Mobil Chairman and CEO Rex Tillerson said that after committing to spend up to $600 million over 10 years on development in a joint venture with J. Craig Venter's Synthetic Genomics in 2009, Exxon pulled back after four years (and $100 million) when it realized that algae fuel is "probably further" than 25 years away from commercial viability. In 2017, Synthetic Genomics and ExxonMobil reported a breaktrough in the joint research into advanced biofuels. The breakthrough was that they managed to double lipid content (from 20 % in its natural form to 40-55 percent) in a genetically engineered strain of Nannochloropsis gaditana. On the other hand, Solazyme, Sapphire Energy, and Algenol, among others have begun commercial sale of algal biofuel in 2012 and 2013, and 2015, respectively. By 2017, most efforts had been abandoned or changed to other applications, with only a few remaining.

Algal bloom

An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems, and is often recognized by the discoloration in the water from their pigments. The term algae encompasses many types of aquatic photosynthetic organisms, both macroscopic, multicellular organisms like seaweed and microscopic, unicellular organisms like cyanobacteria. Algal bloom commonly refers to rapid growth of microscopic, unicellular algae, not macroscopic algae. An example of a macroscopic algal bloom is a kelp forest. Algal blooms are the result of a nutrient, like nitrogen or phosphorus from fertilizer runoff, entering the aquatic system and causing excessive growth of algae. An algal bloom affects the whole ecosystem; it can have benign results like simply feeding higher tropic levels to more harmful effects like blocking the sunlight from reaching other organisms, causing a depletion of oxygen levels in the water, and, depending on the organism, secreting toxins into the water. The process of the oversupply of nutrients leading to algae growth and oxygen depletion is called eutrophication. Blooms that can injure animals or the ecology are called "harmful algal blooms" (HAB), and can lead to fish die-offs, cities cutting off water to residents, or states having to close fisheries.

Archaeplastida

The Archaeplastida (or kingdom Plantae sensu lato) are a major group of autotrophic eukaryotes, comprising the red algae (Rhodophyta), the green algae, and the land plants, together with a small group of freshwater unicellular algae called glaucophytes. Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events. All of these organisms have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly from endosymbiotic cyanobacteria. In all other groups besides the amoeboid Paulinella chromatophora, the chloroplasts are surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae.

The cells of the Archaeplastida typically lack centrioles and have mitochondria with flat cristae. They usually have a cell wall including cellulose, and food is stored in the form of starch. However, these characteristics are also shared with other eukaryotes. The main evidence that the Archaeplastida form a monophyletic group comes from genetic studies, which indicate their plastids probably had a single origin. This evidence is disputed. Based on the evidence to date, it is not possible to confirm or refute alternative evolutionary scenarios to a single primary endosymbiosis. Photosynthetic organisms with plastids of different origin (such as brown algae) do not belong to the Archaeplastida.

The archaeplastidans fall into two main evolutionary lines. The red algae are pigmented with chlorophyll a and phycobiliproteins, like most cyanobacteria, and accumulate starch outside the chloroplasts. The green algae and land plants – together known as Viridiplantae (Latin for "green plants") or Chloroplastida – are pigmented with chlorophylls a and b, but lack phycobiliproteins, and starch is accumulated inside the chloroplasts. The glaucophytes have typical cyanobacterial pigments, and are unusual in retaining a cell wall within their plastids (called cyanelles).Archaeplastida should not be confused with the older and obsolete name Archiplastideae, which refers to cyanobacteria and other groups of bacteria.

Botanical name

A botanical name is a formal scientific name conforming to the International Code of Nomenclature for algae, fungi, and plants (ICN) and, if it concerns a plant cultigen, the additional cultivar or Group epithets must conform to the International Code of Nomenclature for Cultivated Plants (ICNCP). The code of nomenclature covers "all organisms traditionally treated as algae, fungi, or plants, whether fossil or non-fossil, including blue-green algae (Cyanobacteria), chytrids, oomycetes, slime moulds and photosynthetic protists with their taxonomically related non-photosynthetic groups (but excluding Microsporidia)."The purpose of a formal name is to have a single name that is accepted and used worldwide for a particular plant or plant group. For example, the botanical name Bellis perennis denotes a plant species which is native to most of the countries of Europe and the Middle East, where it has accumulated various names in many languages. Later, the plant was introduced worldwide, bringing it into contact with more languages. English names for this plant species include: daisy, English daisy, and lawn daisy. The cultivar Bellis perennis 'Aucubifolia' is a golden-variegated horticultural selection of this species.

Brown algae

The brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Most brown algae live in marine environments, where they play an important role both as food and as habitat. For instance, Macrocystis, a kelp of the order Laminariales, may reach 60 m (200 ft) in length and forms prominent underwater kelp forests. Kelp forests like these contain a high level of biodiversity. Another example is Sargassum, which creates unique floating mats of seaweed in the tropical waters of the Sargasso Sea that serve as the habitats for many species. Many brown algae, such as members of the order Fucales, commonly grow along rocky seashores. Some members of the class, such as kelps, are used by humans as food.

Between 1,500 and 2,000 species of brown algae are known worldwide. Some species, such as Ascophyllum nodosum, are important in commercial use because they have become subjects of extensive research in their own right. They have environmental significance as well, through carbon fixation.Brown algae belong to the group Heterokontophyta, a large group of eukaryotic organisms distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among heterokonts in developing into multicellular forms with differentiated tissues, but they reproduce by means of flagellated spores and gametes that closely resemble cells of other heterokonts. Genetic studies show their closest relatives to be the yellow-green algae.

Cell wall

A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mechanism. Cell walls are present in most prokaryotes (except mollicute bacteria), in algae, fungi and eukaryotes including plants but are absent in animals. A major function is to act as pressure vessels, preventing over-expansion of the cell when water enters.

The composition of cell walls varies between species and may depend on cell type and developmental stage. The primary cell wall of land plants is composed of the polysaccharides cellulose, hemicelluloses and pectin. Often, other polymers such as lignin, suberin or cutin are anchored to or embedded in plant cell walls. Algae possess cell walls made of glycoproteins and polysaccharides such as carrageenan and agar that are absent from land plants. In bacteria, the cell wall is composed of peptidoglycan. The cell walls of archaea have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls made of the N-acetylglucosamine polymer chitin. Unusually, diatoms have a cell wall composed of biogenic silica.

Chlorophyta

Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a highly paraphyletic group of all the green algae within the green plants (Viridiplantae) and thus includes about 7,000 species of mostly aquatic photosynthetic eukaryotic organisms. In newer classifications, it refers to the sister of the streptophytes/charophytes. The clade Streptophyta consists of the Charophyta in which the Embryophyta emerged. In this sense the Chlorophyta includes only about 4,300 species. About 90% of all known species live in freshwater.

Like the land plants (bryophytes and tracheophytes), green algae contain chlorophyll a and chlorophyll b and store food as starch in their plastids.

With the exception of Palmophyllophyceae, Trebouxiophyceae, Ulvophyceae and Chlorophyceae, which show various degrees of multicellularity, all the Chlorophyta lineages are unicellular. Some members of the group form symbiotic relationships with protozoa, sponges, and cnidarians. Others form symbiotic relationships with fungi to form lichens, but the majority of species are free-living. Some conduct sexual reproduction, which is oogamous or isogamous. All members of the clade have motile flagellated swimming cells. While most species live in freshwater habitats and a large number in marine habitats, other species are adapted to a wide range of land environments. For example, Chlamydomonas nivalis, which causes Watermelon snow, lives on summer alpine snowfields. Others, such as Trentepohlia species, live attached to rocks or woody parts of trees. Monostroma kuroshiense, an edible green alga cultivated worldwide and most expensive among green algae, belongs to this group.

Cyanobacteria

Cyanobacteria , also known as Cyanophyta, are a phylum consisting of free-living bacteria as well as the endosymbiotic plastids which appear sister to Gloeomargarita. They commonly obtain their energy through photosynthesis. They are the only photosynthetic prokaryotes able to produce oxygen. The name cyanobacteria comes from the color of the bacteria (Greek: κυανός, romanized: kyanós, lit. 'blue'). Cyanobacteria, which are prokaryotes, are also called "blue-green algae", though some modern botanists restrict the term algae to eukaryotes. Cyanobacteria appear to have originated in freshwater or a terrestrial environment.Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed.Phototrophic eukaryotes such as green plants perform photosynthesis in plastids that are thought to have their ancestry in cyanobacteria, acquired long ago via a process called endosymbiosis. These endosymbiotic cyanobacteria in eukaryotes then evolved or differentiated into specialized organelles such as chloroplasts, etioplasts and leucoplasts.

By producing and releasing oxygen (as a byproduct of photosynthesis), cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing one, causing the Great Oxygenation Event and the "rusting of the Earth", which dramatically changed the composition of the Earth's life forms and led to the near-extinction of anaerobic organisms.

Dinoflagellate

The dinoflagellates (Greek δῖνος dinos "whirling" and Latin flagellum "whip, scourge") are a classification subgroup of protista.They are a large group of flagellate eukaryotes that constitute the phylum Dinoflagellata. Most are marine plankton, but they also are common in freshwater habitats. Their populations are distributed depending on sea surface temperature, salinity, or depth. Many dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy). In terms of number of species, dinoflagellates are one of the largest groups of marine eukaryotes, although this group is substantially smaller than diatoms. Some species are endosymbionts of marine animals and play an important part in the biology of coral reefs. Other dinoflagellates are unpigmented predators on other protozoa, and a few forms are parasitic (for example, Oodinium and Pfiesteria). Some dinoflagellates produce resting stages, called dinoflagellate cysts or dinocysts, as part of their lifecycles.

Dinoflagellates are considered to be protists, with their own division, Dinoflagellata.About 1,555 species of free-living marine dinoflagellates are currently described. Another estimate suggests about 2,000 living species, of which more than 1,700 are marine (free-living, as well as benthic) and about 220 are from fresh water. The latest estimates suggest a total of 2,294 living dinoflagellate species, which includes marine, freshwater, and parasitic dinoflagellates.A bloom of certain dinoflagellates can result in a visible coloration of the water, colloquially known as red tide, which can cause shellfish poisoning if humans eat contaminated shellfish. Some dinoflagellates also exhibit bioluminescence—primarily emitting blue-green light.

Glaucophyte

The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater unicellular algae, less common today than they were during the Proterozoic. Only 15 species have been described, but more species are likely to exist. Together with the red algae (Rhodophyta) and the green algae plus land plants (Viridiplantae or Chloroplastida), they form the Archaeplastida. However, the relationships among the red algae, green algae and glaucophytes are unclear, in large part due to limited study of the glaucophytes.The glaucophytes are of interest to biologists studying the development of chloroplasts because some studies suggest they may be similar to the original algal type that led to green plants and red algae in that they may be basal Archaeplastida.Unlike red and green algae, glaucophytes only have asexual reproduction.

Green algae

The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate divisions, as well as the potentially more basal Mesostigmatophyceae, Chlorokybophyceae and Spirotaenia.The land plants, or embryophytes, are thought to have emerged from the charophytes. Therefore, cladistically, embryophytes belong to green algae as well. However, because the embryophytes are traditionally classified as neither algae nor green algae, green algae are a paraphyletic group. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them. The clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid and filamentous forms, and macroscopic, multicellular seaweeds. There are about 8,000 species of green algae. Many species live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.

A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in euglenids and chlorarachniophytes were acquired from ingested green algae, and in the latter retain a nucleomorph (vestigial nucleus). Green algae are also found symbiotically in the ciliate Paramecium, and in Hydra viridissima and in flatworms. Some species of green algae, particularly of genera Trebouxia of the class Trebouxiophyceae and Trentepohlia (class Ulvophyceae), can be found in symbiotic associations with fungi to form lichens. In general the fungal species that partner in lichens cannot live on their own, while the algal species is often found living in nature without the fungus. Trentepohlia is a filamentous green alga that can live independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family Graphidaceae. Also the macroalga Prasiola calophylla (Trebouxiophyceae) is terrestrial, and

Prasiola crispa, which live in the supralittoral zone, is terrestrial and can in the Antarctic form large carpets on humid soil, especially near bird colonies.

International Code of Nomenclature for algae, fungi, and plants

The International Code of Nomenclature for algae, fungi, and plants (ICN) is the set of rules and recommendations dealing with the formal botanical names that are given to plants, fungi and a few other groups of organisms, all those "traditionally treated as algae, fungi, or plants". It was formerly called the International Code of Botanical Nomenclature (ICBN); the name was changed at the International Botanical Congress in Melbourne in July 2011 as part of the Melbourne Code which replaced the Vienna Code of 2005.

The current version of the code is the Shenzhen Code adopted by the International Botanical Congress held in Shenzhen, China, in July 2017. As with previous codes, it took effect as soon as it was ratified by the congress (on 29 July 2017), but the documentation of the code in its final form was not published until 26 June 2018.

The name of the Code is partly capitalized and partly not. The lower-case for "algae, fungi, and plants" indicates that these terms are not formal names of clades, but indicate groups of organisms that were historically known by these names and traditionally studied by phycologists, mycologists, and botanists. This includes blue-green algae (Cyanobacteria); fungi, including chytrids, oomycetes, and slime moulds; photosynthetic protists and taxonomically related non-photosynthetic groups. There are special provisions in the ICN for some of these groups, as there are for fossils.

The ICN can only be changed by an International Botanical Congress (IBC), with the International Association for Plant Taxonomy providing the supporting infrastructure. Each new edition supersedes the earlier editions and is retroactive back to 1753, except where different starting dates are specified.For the naming of cultivated plants there is a separate code, the International Code of Nomenclature for Cultivated Plants, which gives rules and recommendations that supplement the ICN.

Photosynthesis

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις, synthesis, "putting together". In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.

In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle; some bacteria use different mechanisms, such as the reverse Krebs cycle, to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about eight times the current power consumption of human civilization.

Photosynthetic organisms also convert around 100–115 billion tons (91-104 petagrams) of carbon into biomass per year.

Plant

Plants are mainly multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes (the archaea and bacteria). By one definition, plants form the clade Viridiplantae (Latin name for "green plants"), a group that includes the flowering plants, conifers and other gymnosperms, ferns and their allies, hornworts, liverworts, mosses and the green algae, but excludes the red and brown algae.

Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria. Their chloroplasts contain chlorophylls a and b, which gives them their green color. Some plants are parasitic or mycotrophic and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although asexual reproduction is also common.

There are about 320,000 species of plants, of which the great majority, some 260–290 thousand, produce seeds. Green plants provide a substantial proportion of the world's molecular oxygen, and are the basis of most of Earth's ecosystems. Plants that produce grain, fruit and vegetables also form basic human foods and have been domesticated for millennia. Plants have many cultural and other uses, as ornaments, building materials, writing material and, in great variety, they have been the source of medicines and psychoactive drugs. The scientific study of plants is known as botany, a branch of biology.

Red algae

Red algae, or Rhodophyta ( roh-DOF-it-ə, ROH-də-FY-tə; from Ancient Greek ῥόδον (rhodon), meaning 'rose', and φυτόν (phyton), meaning 'plant'), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority of species (6,793) are found in the Florideophyceae (class), and mostly consist of multicellular, marine algae, including many notable seaweeds. Approximately 5% of the red algae occur in freshwater environments with greater concentrations found in warmer areas. Except for two coastal cave dwelling species in the asexual class Cyanidiophyceae, that diverged from other red algae about 1.3 billion years ago, there are no terrestrial species, which may be due to an evolutionary bottleneck where the last common ancestor lost about 25% of its core genes and much of its evolutionary plasticity.The red algae form a distinct group characterized by having eukaryotic cells without flagella and centrioles, chloroplasts that lack external endoplasmic reticulum and contain unstacked (stroma) thylakoids, and use phycobiliproteins as accessory pigments, which give them their red color. Red algae store sugars as floridean starch, which is a type of starch that consists of highly branched amylopectin without amylose, as food reserves outside their plastids. Most red algae are also multicellular, macroscopic, marine, and reproduce sexually. The red algal life history is typically an alternation of generations that may have three generations rather than two.Chloroplasts evolved following an endosymbiotic event between an ancestral, photosynthetic cyanobacterium and an early eukarytoic phagotroph. This event (termed primary endosymbiosis) resulted in the origin of the red and green algae, and the glaucophytes, which make up the oldest evolutionary lineages of photosynthetic eukaryotes. A secondary endosymbiosis event involving an ancestral red alga and a heterotrophic eukaryote resulted in the evolution and diversification of several other photosynthetic lineages such as Cryptophyta, Haptophyta, Stramenopiles (or Heterokontophyta), and Alveolata. In addition to multicellular brown algae, it is estimated that more than half of all known species of microbial eukaryotes harbor red-algal-derived plastids.The coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here. Red algae such as dulse (Palmaria palmata) and laver (nori/gim) are a traditional part of European and Asian cuisines and are used to make other products such as agar, carrageenans and other food additives.Red algae are divided into the Cyanidiophyceae, a class of unicellular and thermoacidophilic extremophiles found in sulphuric hot springs and other acidic environments, an adaptation partly made possible by horizontal gene transfers from prokaryotes,, with about 1% of their genome having this origin, and two sister clades called SCRP (Stylonematophyceae, Compsopogonophyceae, Rhodellophyceae and Porphyridiophyceae) and BF (Bangiophyceae and Florideophyceae), which are found in both marine and freshwater environments. The SCRP clade are microalgae, consisting of both unicellular forms and multicellular microscopic filaments and blades. The BF are macroalgae, seaweed that usually do not grow to more than about 50 cm in length, but a few species can reach lengths of 2 m. Most rhodophytes are marine with a worldwide distribution, and are often found at greater depths compared to other seaweeds. While this was formerly attributed to the presence of pigments (such as phycoerythrin) that would permit red algae to inhabit greater depths than other macroalgae by chromatic adaption, recent evidence calls this into question (e.g. the discovery of green algae at great depth in the Bahamas). Some marine species are found on sandy shores, while most others can be found attached to rocky substrata. Freshwater species account for 5% of red algal diversity, but they also have a worldwide distribution in various habitats; they generally prefer clean, high-flow streams with clear waters and rocky bottoms, but with some exceptions. A few freshwater species are found in black waters with sandy bottoms and even fewer are found in more lentic waters. Both marine and freshwater taxa are represented by free-living macroalgal forms and smaller endo/epiphytic/zoic forms, meaning they live in or on other algae, plants, and animals. In addition, some marine species have adopted a parasitic lifestyle and may be found on closely or more distantly related red algal hosts.

Reef

A reef is a bar of rock, sand, coral or similar material, lying beneath the surface of water.

Many reefs result from natural, abiotic processes—deposition of sand, wave erosion planing down rock outcrops, etc.—but the best known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and coralline algae.

Artificial reefs (e.g. shipwrecks) sometimes have a role in enhancing the physical complexity of featureless sand bottoms, in order to attract a diverse assemblage of organisms, especially algae and fish.

Earth's largest reef system is the Great Barrier Reef in Australia, at a length of over 2,300 kilometres (1,400 miles).

Seaweed

Seaweed, or macroalgae, refers to several species of macroscopic, multicellular, marine algae. The term includes some types of Rhodophyta (red), Phaeophyta (brown) and Chlorophyta (green) macroalgae. Seaweed species such as kelps provide essential nursery habitat for fisheries and other marine species and thus protect food sources; other species, such as planktonic algae, play a vital role in capturing carbon, producing up to 90% of Earth's oxygen. Understanding these roles offers principles for conservation and sustainable use. Mechanical dredging of kelp, for instance, destroys the resource and dependent fisheries.

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