Algae
- "Alga" redirects here. For places called Alga, seeAlga (disambiguation). For other uses, see Algae (disambiguation).
| Algae Fossil range: Mesoproterozoic–present[1] | ||
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 A variety of algae growing on the sea bed in shallow waters | ||
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Algae (/ˈældʒiː/ or/ˈælɡiː/; singular alga/ˈælɡə/) is a very large and diverse group ofeukaryotic organisms, ranging from unicellulargenera, such as Chlorellaand the diatoms, tomulticellular forms, such as the giant kelp, a large brown alga that may grow up to 50 meters in length. Most are aquatic andautotrophic and lack many of the distinct cell and tissue types, such as stomata, xylem andphloem, that are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are theCharophyta, a division of algae that includes Spirogyra and the stoneworts.
There is no generally accepted definition of algae. One definition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells".[4] Some authors exclude all prokaryotes[5] and thus do not consider cyanobacteria(blue-green algae) as algae.[6]
Algae constitute a polyphyletic group[5]since they do not include a common ancestor, and although their plastids seem to have a single origin, from cyanobacteria,[2] they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived fromendosymbiotic cyanobacteria. Diatomsare examples of algae with secondary chloroplasts derived from anendosymbiotic red alga.[7]
Algae exhibit a wide range of reproductive strategies, from simpleasexual cell division to complex forms of sexual reproduction.[8]
Algae lack the various structures that characterize land plants, such as the phyllids (leaf-like structures) ofbryophytes, rhizoids in nonvascular plants, and the roots, leaves, and otherorgans that are found in tracheophytes(vascular plants). Most arephototrophic, although some groups[which?] contain members that 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.[9][10] Some other heterotrophic organisms, like theapicomplexans, 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 purpleand green sulfur bacteria. Fossilized filamentous algae from the Vindhyabasin have been dated back to 1.6 to 1.7 billion years ago.[11]
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[hide]Etymology and study[edit]
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]there is no known reason to associate seaweed with temperature. A more likely source is alliga, "binding, entwining."[14]
The Ancient Greek word for seaweed was φῦκος (fūkos or phykos), 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, 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 Fucusappears in a number of taxa.
Classification[edit]
Further information:wikispecies:Algae
Most algae containchloroplasts that are similar in structure tocyanobacteria. Chloroplasts contain circular DNA like that in cyanobacteria and presumably represent reducedendosymbiotic 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.
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| Supergroup affiliation | Members | Endosymbiont | Summary |
|---|---|---|---|
| Primoplantae/ Archaeplastida | Cyanobacteria | These algae have primary chloroplasts, i.e. the chloroplasts are surrounded by two membranesand probably developed through a single endosymbiotic event. The chloroplasts of red algae have chlorophylls a and c (often), andphycobilins, 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, and thus Chlorophyta is a sister taxon to the plants; sometimes Chlorophyta, Charophytaand land plants are grouped together asViridiplantae. | |
| Excavata andRhizaria | Green algae | These groups have green chloroplasts containing chlorophylls a and b.[17] Their chloroplasts are surrounded by four and three membranesrespectively, and were probably retained from ingested green algae. Chlorarachniophytes, which belong to the phylumCercozoa, contain a small nucleomorph, which is a relict of the algae's nucleus. Euglenids, which belong to the phylumEuglenozoa, live primarily in freshwater and have chloroplasts with only three membranes. It has been suggested that the endosymbiotic green algae were acquired through myzocytosis rather than phagocytosis. | |
| Chromista andAlveolata | 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.[18] In the first three of these groups (Chromista), the chloroplast has four membranes, retaining anucleomorph in Cryptomonads, and they likely share a common pigmented ancestor, although other evidence casts doubt on whether theHeterokonts, Haptophyta, and Cryptomonads are in fact more closely related to each other than to other groups.[3][19] The typical dinoflagellate chloroplast has three membranes, but there is considerable diversity in chloroplasts within the group, and it appears that there were a number of endosymbiotic events.[2]The Apicomplexa, a group of closely related parasites, also have plastids called apicoplasts. Apicoplasts are not photosynthetic, but appear to have a common origin with Dinoflagellatechloroplasts.[2] |
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Linnaeus, in Species Plantarum (1753),[20] the starting point for modernbotanical nomenclature, recognized 14 genera of algae, of which only 4 are currently considered among algae.[21] InSystema Naturae, Linnaeus described the genera Volvox and Corallina, 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 onmarine biology to use the then newbinomial nomenclature of Linnaeus. It included elaborate illustrations of seaweed and marine algae on folded leaves.[22][23]
W.H.Harvey (1811—1866) andLamouroux (1813)[24] 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.[25][26]
At this time, microscopic algae were discovered and reported by a different group of workers (e.g., O. F. Müller andEhrenberg) studying the Infusoria(microscopic organisms). Unlikemacroalgae, which were clearly viewed as plants, microalgae were frequently considered animals because they are often motile.[27] Even the non-motile (coccoid) microalgae were sometimes merely seen as stages of the life cycle of plants, macroalgae or animals.[28][29]
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 anddinophytes), euglenophytes andchlorophytes. Later, many new groups were discovered (e.g., Bolidophyceae), and others were splintered from older groups: charophytes and glaucophytes(from chlorophytes), manyheterokontophytes (e.g., synurophytesfrom chrysophytes, oreustigmatophytes from xanthophytes),haptophytes (from chrysophytes) andchlorarachniophytes (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, orCephaleuros, parasites of plants) were originally classified as fungi,sporozoans or protistans of incertae sedis,[30] while others (e.g., the green algae Phyllosiphon and Rhodochytrium, parasites of plants, or the red algaePterocladiophila 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 asendophytic algae.[31] 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[edit]
The first land plants probably evolved from shallow freshwater charophytealgae 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.[32][33]
Morphology[edit]
A range of algalmorphologies are exhibited, and convergence of features in unrelated groups is common. The only groups to exhibit three-dimensional multicellular thalli are the reds andbrowns, and some chlorophytes.[34]Apical growth is constrained to subsets of these groups: theflorideophyte reds, various browns, and the charophytes.[34] The form ofcharophytes 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.[34] Conceptacles are another polyphyletic trait; they appear in the coralline algae and theHildenbrandiales, as well as the browns.[34]
Most of the simpler algae areunicellular 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
- Colonial: small, regular groups of motile cells
- Capsoid: individual non-motile cells embedded in mucilage
- Coccoid: individual non-motile cells with cell walls
- Palmelloid: non-motile cells embedded in mucilage
- Filamentous: a string of non-motile cells connected together, sometimes branching
- Parenchymatous: cells forming athallus with partial differentiation of tissues
In three lines, even higher levels of organization have been reached, with full tissue differentiation. These are thebrown algae,[35]—some of which may reach 50 m in length (kelps)[36]—thered algae,[37] and the green algae.[38]The most complex forms are found among the green algae (see Charalesand 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.
Physiology[edit]
Many algae, particularly members of the Characeae,[39] 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.[40]
Symbiotic algae[edit]
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 as follows.
Lichens[edit]
Main article: Lichens
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Lichens are defined by the International Association for Lichenology to be "an association of a fungus and a photosynthetic symbiontresulting in a stable vegetative body having a specific structure."[41] The fungi, or mycobionts, are mainly from the Ascomycota with a few from theBasidiomycota. They are not found alone in nature; but when they began to associate is not known.[42] One mycobiont associates with the same phycobiont species, rarely two, from the green algae, except that alternatively the mycobiont may associate with a species ofcyanobacteria (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.[43] 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). It is possible that the photobiont triggers otherwise latent genes in the mycobiont.[44]
Coral reefs[edit]
Coral reefs are accumulated from thecalcareous exoskeletonsof marine invertebrates of the orderScleractinia (stony corals). Theseanimals metabolize sugar and oxygento obtain energy for their cell-building processes, including secretion of the exoskeleton, with water and carbon dioxide as byproducts. Dinoflagellates(algal protists) are oftenendosymbionts in the cells of the coral-forming marine invertebrates, where they accelerate host-cell metabolism by generating immediately available sugar and oxygen throughphotosynthesis using incident light and the carbon dioxide produced by the host. Reef-building stony corals (hermatypic corals) requireendosymbiotic algae from the genusSymbiodinium to be in a healthy condition.[45] The loss of Symbiodiniumfrom the host is known as coral bleaching, a condition which leads to the deterioration of a reef.
Sea sponges[edit]
Main article: Sea sponge
Green algae live close to the surface of some sponges, for example, breadcrumb sponge (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.[46]
Life-cycle[edit]
Rhodophyta, Chlorophyta andHeterokontophyta, the three main algalphyla, have life-cycles which show considerable variation and complexity. In general, there is an asexual phase where the seaweed's cells are diploid, a sexual phase where the cells arehaploid 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 azygote. 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.[47] 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.[48] However, sexual reproduction is more costly than asexual reproduction.[49] Meiosis has been shown to occur in many different species of algae.[50]
For more details on this topic, seeConceptacle.
Numbers[edit]
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).[51] Estimates vary widely. For example, according to one standard textbook,[52] in the British Isles the UK Biodiversity Steering Group Report estimated there to be 20000 algal species in the UK. Another checklist reports only about 5000 species. Regarding the difference of about 15000 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:
- 5000–5500 species of red algae worldwide
- "some 1300 in Australian Seas"[53]
- 400 seaweed species for the western coastline of South Africa,[54] and 212 species from the coast of KwaZulu-Natal.[55] 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 inList of seaweeds of South Africa. These exclude phytoplankton and crustose corallines.
- 669 marine species from California (US)[56]
- 642 in the check-list of Britain and Ireland[57]
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.[58]
Distribution[edit]
The distribution of algal species has been fairly well studied since the founding of phytogeography in the mid-19th century AD.[59] Algae spread mainly by the dispersal of sporesanalogously to the dispersal of Plantae by seeds and spores. Spores are everywhere in all parts of the Earth: the waters fresh and marine, the atmosphere, free-floating and in precipitation or mixed with dust, thehumus and in other organisms, such as humans. Whether a spore is to grow into an organism depends on the combination of the species and the environmental conditions of where the spore lands.
The spores of fresh-water algae are dispersed mainly by running water and wind, as well as by living carriers.[60]The bodies of water into which they are transported are chemically selective.[clarification needed] Marine spores are spread by currents. Ocean water is temperature selective, resulting in phytogeographic zones, regions and provinces.[61]
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, it is usually possible to hypothesize a transport mechanism, such as the hulls of ships. For example, Ulva reticulata and Ulva 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."[62] For example, Clathromorphum is an arctic genus and is not mapped far south of there.[63] On the other hand, scientists regard the overall data as insufficient due to the "difficulties of undertaking such studies."[64]
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