Algae

Algae(UK:/ˈælɡiː/AL-ghee,US:/ˈældʒiː/AL-jee;sg.:alga/ˈælɡə/AL-gə) is an informal term for anyorganismsof a large and diverse group ofphotosyntheticeukaryotes, which includespeciesfrom multiple distinctclades. Such organisms range fromunicellularmicroalgaesuch asChlorella,Protothecaand thediatoms, tomulticellularmacroalgaesuch as thegiant kelp, a largebrown algawhich may grow up to 50 metres (160 ft) in length. Most algae are aquatic organisms and lack many of the distinct cell and tissue types, such asstomata,xylemandphloemthat are found inland plants. The largest and most complex marine algae are calledseaweeds. In contrast, the most complex freshwater forms are theCharophyta, adivisionofgreen algaewhich includes, for example,Spirogyraandstoneworts. Algae that are carried passively by water areplankton, specificallyphytoplankton.
Algae constitute apolyphyleticgroupsince they do not include acommon ancestor, and although theirchlorophyll-bearingplastidsseem to have a single origin (fromsymbiogenesiswithcyanobacteria),they were acquired in different ways. Green algae are a prominent example of algae that have primarychloroplastsderived fromendosymbiontcyanobacteria.Diatomsand brown algae are examples of algae with secondary chloroplasts derived from endosymbioticred algae, which they acquired viaphagocytosis.Algae exhibit a wide range of reproductive strategies, from simpleasexualcell division to complex forms ofsexual reproductionviaspores.
Algae lack the various structures that characterizeplants(which evolved from freshwater green algae), such as the phyllids (leaf-like structures) andrhizoidsofbryophytes(non-vascular plants), and theroots,leavesand otherxylemic/phloemicorgansfound intracheophytes(vascular plants). Most algae areautotrophic, although some aremixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either byosmotrophy,myzotrophyorphagotrophy. Some unicellular species of green algae, manygolden algae,euglenids,dinoflagellates, and other algae have becomeheterotrophs(also called colorless or apochlorotic algae), sometimesparasitic, relying entirely on external energy sources and have limited or no photosynthetic apparatus.Some other heterotrophic organisms, such as theapicomplexans, are also derived from cells whose ancestors possessed  chlorophyllic plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from cyanobacteria that produceoxygenas abyproductof splittingwater molecules, unlike other organisms that conductanoxygenic photosynthesissuch aspurpleandgreen sulfur bacteria. Fossilized filamentous algae from theVindhyabasin have been dated to 1.6 to 1.7 billion years ago.
Because of the wide range of algae types, they have increasingly different industrial and traditional applications in human society. Traditionalseaweed farmingpractices have existed for thousands of years and have strong traditions inEast Asianfood cultures. More modernalgacultureapplications extend thefood traditionsfor other applications, including cattle feed, using algae forbioremediationor pollution control, transforming sunlight intoalgae fuelsor other chemicals used in industrial processes, and in medical and scientific applications. A 2020 review found that these applications of algae could play an important role incarbon sequestrationtomitigate climate changewhile providing lucrative value-added products for global economies.
The singularalgais the Latin word for 'seaweed' and retains that meaning in English.Theetymologyis obscure. Although some speculate that it is related to Latinalgēre, 'be cold',no reason is known to associate seaweed with temperature. A more likely source isalliga, 'binding, entwining'.
TheAncient Greekword for 'seaweed' wasφῦκος(phŷkos), 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 theBiblicalפוך(pūk), 'paint' (if not that word itself), acosmetic eye-shadowused by theancient Egyptiansand other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, or blue.
The study of algae is most commonly calledphycology(fromGreekphykos'seaweed'); the termalgologyis falling out of use.
One definition of algae is that they "havechlorophyllas their primaryphotosynthetic pigmentand lack a sterile covering of cells around theirreproductive cells".On the other hand, the colorlessProtothecaunderChlorophytaare all devoid of any chlorophyll. Althoughcyanobacteriaare often referred to as "blue-green algae", most authorities exclude allprokaryotes, including cyanobacteria, from the definition of algae.
The algae containchloroplaststhat are similar in structure to cyanobacteria. Chloroplasts contain circularDNAlike that in cyanobacteria and are interpreted as representing reduced endosymbioticcyanobacteria. 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.
Phylogenybased onplastidnot nucleocytoplasmic genealogy:
Linnaeus, inSpecies Plantarum(1753),the starting point for modernbotanical nomenclature, recognized 14 genera of algae, of which only four are currently considered among algae.InSystema Naturae, Linnaeus described the generaVolvoxandCorallina, and a species ofAcetabularia(asMadrepora), among the animals.
In 1768,Samuel Gottlieb Gmelin(1744–1774) published theHistoria Fucorum, the first work dedicated to marine algae and the first book onmarine biologyto use the then new binomial nomenclature of Linnaeus. It included elaborate illustrations of seaweed and marine algae on folded leaves.
W. H. Harvey(1811–1866) andLamouroux(1813)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.
At this time, microscopic algae were discovered and reported by a different group of workers (e.g.,O. F. MüllerandEhrenberg) studying theInfusoria(microscopic organisms). Unlikemacroalgae, which were clearly viewed as plants,microalgaewere frequently considered animals because they are often motile.Even the nonmotile (coccoid) microalgae were sometimes merely seen as stages of the lifecycle of plants, macroalgae, or animals.
Although used as a taxonomic category in some pre-Darwinian classifications, e.g., Linnaeus (1753),de Jussieu (1789),Lamouroux (1813), Harvey (1836), 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(cryptophytesanddinophytes),euglenophytes, andchlorophytes. Later, many new groups were discovered (e.g.,Bolidophyceae), and others were splintered from older groups:charophytesandglaucophytes(from chlorophytes), manyheterokontophytes(e.g.,synurophytesfrom chrysophytes, oreustigmatophytesfrom xanthophytes),haptophytes(from chrysophytes), andchlorarachniophytes(from xanthophytes).
With the abandonment of plant-animal dichotomous classification, most groups of algae (sometimes all) were included inProtista, later also abandoned in favour ofEukaryota. However, as a legacy of the older plant life scheme, some groups that were also treated asprotozoansin the past still have duplicated classifications (seeambiregnal protists).
Some parasitic algae (e.g., the green algaeProtothecaandHelicosporidium, parasites of metazoans, orCephaleuros, parasites of plants) were originally classified asfungi,sporozoans, orprotistansofincertae sedis,while others (e.g., the green algaePhyllosiphonandRhodochytrium, parasites of plants, or the red algaePterocladiophilaandGelidiocolax mammillatus, parasites of other red algae, or the dinoflagellatesOodinium, 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 asendophyticalgae.Some filamentous bacteria (e.g.,Beggiatoa) were originally seen as algae. Furthermore, groups like theapicomplexansare also parasites derived from ancestors that possessed plastids, but are not included in any group traditionally seen as algae.
Algae arepolyphyleticthus their origin cannot be traced back to single hypotheticalcommon ancestor. It is thought that they came into existence when photosyntheticcoccoidcyanobacteriagotphagocytizedby aunicellularheterotrophiceukaryote (aprotist),giving rise to double-membranous primaryplastids. Suchsymbiogenicevents (primary symbiogenesis) are believed to have occurred more than 1.5 billion years ago during theCalymmianperiod, early inBoring Billion, but it is difficult to track the key events because of so much time gap.Primary symbiogenesis gave rise to threedivisionsofarchaeplastids, namely theViridiplantae(green algaeand laterplants),Rhodophyta(red algae) andGlaucophyta("grey algae"), whose plastids further spread into other protist lineages through eukaryote-eukaryotepredation, engulfments and subsequent endosymbioses (secondary and tertiary symbiogenesis).This process of serial cell "capture" and "enslavement" explains the diversity of photosynthetic eukaryotes.
Recentgenomicandphylogenomicapproaches have significantly clarified plastidgenome evolution, thehorizontal movementofendosymbiontgenesto the "host"nucleargenome, and plastid spread throughout the eukaryotictree of life.
Fossils of isolatedsporessuggestland plantsmay have been around as long as 475million years ago(mya) during theLate Cambrian/Early Ordovicianperiod,fromsessileshallowfreshwatercharophytealgae much likeChara,which likely got stranded ashore whenriverine/lacustrinewater levelsdropped duringdry seasons.These charophyte algae probably already developed filamentousthalliandholdfaststhat superficially resembledplant stemsandroots, and probably had an isomorphicalternation of generations. They perhaps evolved some 850 myaand might even be as early as 1Gyaduring the late phase of theBoring Billion.
A range of algalmorphologiesis exhibited, andconvergenceof features in unrelated groups is common. The only groups to exhibit three-dimensional multicellularthalliare theredsandbrowns, and somechlorophytes.Apical growth is constrained to subsets of these groups: theflorideophytereds, various browns, and the charophytes.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 thehorsetailsoccur at the nodes.Conceptaclesare anotherpolyphyletictrait; they appear in thecoralline algaeand theHildenbrandiales, as well as the browns.
Most of the simpler algae are unicellularflagellatesoramoeboids, 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 thelifecycleof 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 charophyte algae (seeCharalesandCharophyta), 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 theEmbryophytes.
The term algal turf is commonly used but poorly defined. Algal turfs are thick, carpet-like beds of seaweed that retainsedimentand compete with foundation species likecoralsandkelps, and they are usually less than 15 cm tall. Such a turf may consist of one or more species, and will generally cover an area in the order of a square metre or more. Some common characteristics are listed:
Many algae, particularly species of theCharaceae,have served as model experimental organisms to understand the mechanisms of the water permeability of membranes,osmoregulation,salt tolerance,cytoplasmic streaming, and the generation ofaction potentials.Plant hormonesare found not only in higher plants, but in algae, too.
Some species of algae formsymbiotic relationshipswith 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:
Lichensare defined by theInternational Association for Lichenologyto be "an association of a fungus and a photosyntheticsymbiontresulting in a stable vegetative body having a specific structure".The fungi, or mycobionts, are mainly from theAscomycotawith a few from theBasidiomycota. In nature, they do not occur separate from lichens. It is unknown when they began to associate.One or moremycobiont associates with the same phycobiont species, 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.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.
Trentepohliais 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 reefsare accumulated from thecalcareousexoskeletons ofmarine invertebratesof the orderScleractinia(stonycorals). These animalsmetabolizesugar and oxygen to obtain energy for their cell-building processes, includingsecretionof the exoskeleton, with water andcarbon dioxideas byproducts. Dinoflagellates (algal protists) are oftenendosymbiontsin 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 genusSymbiodiniumto be in a healthy condition.The loss ofSymbiodiniumfrom the host is known ascoral bleaching, a condition which leads to the deterioration of a reef.
Endosymbionticgreen 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.
Rhodophyta,Chlorophyta, andHeterokontophyta, the three main algaldivisions, have life cycles which show considerable variation and complexity. In general, an asexual phase exists where the seaweed's cells arediploid, a sexual phase where the cells arehaploid, followed by fusion of the male and femalegametes. 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.Sexual reproduction allows for more variation and provides the benefit of efficient recombinational repair of DNA damages duringmeiosis, a key stage of the sexual cycle.However, sexual reproduction is more costly than asexual reproduction.Meiosis has been shown to occur in many different species of algae.
TheAlgal Collection of the US National Herbarium(located in theNational 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).Estimates vary widely. For example, according to one standard textbook,in theBritish Isles, theUK Biodiversity Steering Group Reportestimated 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:
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.
The distribution of algal species has been fairly well studied since the founding ofphytogeographyin the mid-19th century.Algae spread mainly by the dispersal ofsporesanalogously to the dispersal ofcryptogamicplantsbyspores. Spores can be found in a variety of environments: fresh and marine waters, air, soil, and in or on other organisms.Whether a spore is to grow into an adult organism depends on 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.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.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.
To some degree, the distribution of algae is subject to floristic discontinuities caused by geographical features, such asAntarctica, long distances of ocean or general land masses. It is, therefore, possible to identify species occurring by locality, such as "Pacificalgae" or "North Seaalgae". When they occur out of their localities, hypothesizing a transport mechanism is usually possible, such as the hulls of ships. For example,Ulva reticulataandU. fasciatatravelled from the mainland toHawaiiin this manner.
Mapping is possible for select species only: "there are many valid examples of confined distribution patterns."For example,Clathromorphumis an arctic genus and is not mapped far south of there.However, scientists regard the overall data as insufficient due to the "difficulties of undertaking such studies."
Algae are prominent in bodies of water, common in terrestrial environments, and are found in unusual environments, such as onsnowandice. Seaweeds grow mostly in shallow marine waters, under 100 m (330 ft) deep; however, some such asNaviculapennatahave been recorded to a depth of 360 m (1,180 ft).A type of algae,Ancylonema nordenskioeldii, was found inGreenlandin areas known as the 'Dark Zone', which caused an increase in the rate of melting ice sheet.The same algae was found in theItalian Alps, after pink ice appeared on parts of the Presena glacier.
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 marinefood chains. In very high densities (algal blooms), these algae may discolor the water and outcompete, poison, orasphyxiateother life forms.
Algae can be used asindicator organismsto monitor pollution in various aquatic systems.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.To detect these changes, algae can be sampled from the environment and maintained in laboratories with relative ease.
On the basis of their habitat, algae can be categorized as:aquatic(planktonic,benthic,marine,freshwater,lentic,lotic),terrestrial,aerial(subaerial),lithophytic,halophytic(oreuryhaline),psammon,thermophilic,cryophilic,epibiont(epiphytic,epizoic),endosymbiont(endophytic, endozoic),parasitic,calcifilicorlichenic(phycobiont).
Inclassical Chinese, the word藻is used both for "algae" and (in the modest tradition of theimperial scholars) for "literary talent". The third island inKunming Lakebeside theSummer Palacein Beijing is known as the Zaojian Tang Dao (藻鑒堂島), which thus simultaneously means "Island of the Algae-Viewing Hall" and "Island of the Hall for Reflecting on Literary Talent".
Agar, agelatinoussubstance derived from red algae, has a number of commercial uses.It is a good medium on which to grow bacteria and fungi, as most microorganisms cannot digest agar.
Alginic acid, or alginate, is extracted frombrown algae. Its uses range from gelling agents in food, to medical dressings. Alginic acid also has been used in the field ofbiotechnologyas abiocompatible mediumfor cell encapsulation and cell immobilization.Molecular cuisineis 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 ofMacrocystisare harvested annually inNew Mexicoforalginateextraction andabalonefeed.
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,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.
For centuries, seaweed has been used as a fertilizer;George Owen of Henllyswriting in the 16th century referring to drift weed inSouth Wales:
Today, algae are used by humans in many ways; for example, asfertilizers,soil conditioners, and livestock feed.Aquatic and microscopic species are cultured in clear tanks or ponds and are either harvested or used to treat effluents pumped through the ponds.Algacultureon a large scale is an important type ofaquaculturein some places.Maerlis commonly used as a soil conditioner.
Algae are used as foods in many countries: China consumes more than 70 species, includingfat choy, a cyanobacterium considered a vegetable; Japan, over 20 species such asnoriandaonori;Ireland,dulse;Chile,cochayuyo.Laveris used to makelaverbreadinWales, where it is known asbara lawr. InKorea,green laveris used to makegim.
Three forms of algae used as food:
The oils from some algae have high levels ofunsaturated fatty acids. Some varieties of algae favored byvegetarianismandveganismcontain the long-chain, essentialomega-3 fatty acids,docosahexaenoic acid(DHA) andeicosapentaenoic 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 ascopepodsand are passed up the food chain.
Agricultural Research Servicescientists found that 60–90% of nitrogen runoff and 70–100% of phosphorus runoff can be captured frommanure effluentsusing a horizontal algae scrubber, also called analgal 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.Algae scrubbers, using bubbling upflow or vertical waterfall versions, are now also being used to filter aquaria and ponds.
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.Several companies have begun to produce algae polymers commercially, including for use in flip-flopsand in surf boards.
The algaStichococcus bacillarishas been seen to colonize silicone resins used at archaeological sites;biodegradingthe synthetic substance.
The naturalpigments(carotenoidsandchlorophylls) produced by algae can be used as alternatives to chemicaldyesand coloring agents.The presence of some individual algal pigments, together with specific pigment concentration ratios, are taxon-specific: analysis of their concentrations with various analytical methods, particularlyhigh-performance liquid chromatography, can therefore offer deep insight into the taxonomic composition and relative abundance of natural algae populations in sea water samples.
Carrageenan, from the red algaChondrus crispus, is used as a stabilizer in milk products.