Anabaena
Anabaena is a genus of filamentous cyanobacteria that exist as plankton. They are known for nitrogen-fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanobacteria that produce neurotoxins, which are harmful to local wildlife, as well as farm animals and pets. Production of these neurotoxins is assumed to be an input into its symbiotic relationships, protecting the plant from grazing pressure.
A DNA sequencing project was undertaken by the United States Department of Energy between 1999 and 2005. This project mapped the complete genome of model organism Anabaena variabilis ATCC 29413, which is 7.2 million base pairs long. A paper detailing the process was published in 2014. The study focused on heterocysts, which convert nitrogen into ammonia. Certain species of Anabaena have been used on rice paddy fields, proving to be an effective natural fertilizer.
Species
List of current, unverified, and preliminary species from AlgaeBase:An. abnormis CAn. aequalis CAn. aerophila CAn. aeruginosa CAn. alatospora CAn. allahabadii CAn. allantospora UAn. anisococca CAn. arenicola UAn. aspera CAn. attenuata CAn. augstumalis CAn. australica CAn. austroafricana CAn. azotica CAn. batophora CAn. beckii CAn. bederi CAn. belmontica CAn. berkeleyana CAn. bernardinensis CAn. bharadwajae UAn. bispora UAn. bolochonceviana CAn. bolochonzewii CAn. bonariensis CAn. borealis UAn. bornetiana CAn. brevis UAn. broomei CAn. calida UAn. carmichaelii CAn. catenula CAn. cavanillesiana CAn. chilensis UAn. codiicola CAn. confervoides UAn. conglobata UAn. contorta CAn. crispa UAn. cupressaphila CAn. cuticularis UAn. cyanea UAn. cylindracea CAn. cylindrica CAn. cylindrospora CAn. debilis UAn. delicatissima CAn. discoidea CAn. distans UAn. duplooyae CAn. echinospora CAn. effusa UAn. elegans CAn. ellipsoidea CAn. elliptica CAn. elongata CAn. endogena UAn. epiphytica CAn. epiphytica CAn. felisii CAn. fragilis UAn. fullebornii CAn. fuscovaginata CAn. galpinii CAn. geitleri CAn. gelatinosa UAn. gerdii CAn. ghosei CAn. gigantea UAn. glauca CAn. gracilis CAn. granularis UAn. gregaria CAn. groenlandica CAn. hatueyi CAn. hederulae UAn. hieronymusii CAn. hispida CAn. hollerbachii CAn. huberi CAn. humicola CAn. hunanensis CAn. hyalina CAn. impalpabilis CAn. imperceptibilis UAn. inaequalis CAn. incerta CAn. incognita CAn. incurva UAn. indica UAn. indica CAn. involuta UAn. irregularis CAn. iyengarii CAn. jaubertiana UAn. jeejiae CAn. jonssonii CAn. karakumica CAn. kemptii UAn. kisselevii CAn. koreana UAn. kwangtungensis CAn. lapponica CAn. laxa CAn. leonardii CAn. levanderi CAn. licheniformis UAn. lohammarii CAn. luteola CAn. manchurica CAn. marina CAn. maritima UAn. marklei CAn. mediocris CAn. mehrai CAn. mesiana CAn. microscopica UAn. minderi CAn. miniata CAn. minima CAn. minuta CAn. minutissima CAn. monilifera CAn. moniliformis UAn. monticulosa UAn. multispora CAn. neapolitana UAn. nodularia UAn. nodularioides CAn. novizelandica CAn. oblonga CAn. obsoleta UAn. orientalis CAn. orthogona CAn. oscillarioides CAn. papillosa CAn. passeriniana UAn. paulseniana CAn. pintoi CAn. pirinica CAn. poulseniana CAn. promecespora CAn. pseudocatenula CAn. pseudoconstricta CAn. pulchra UAn. punctata UAn. ralfsii CAn. ramakaranae CAn. raytonensis CAn. recta CAn. recta UAn. repens CAn. rhodopensis UAn. riparia UAn. robusta CAn. rudis UAn. saaremaaensis CAn. sabulosa UAn. salina CAn. salina UAn. scabra UAn. schauderi CAn. sedowii CAn. shankargarhii CAn. shensiensis CAn. smaragdina UAn. solicola CAn. sphaerica CAn. sphagnicola CAn. spinosa CAn. stillicidiorum UAn. subdelicatula CAn. subperanema UAn. subrigida UAn. subtilissima UAn. subvariabilis CAn. swakopensis CAn. tatarica CAn. tenax UAn. tenuis CAn. tenuis UAn. terrestris UAn. thermophila CAn. thwaitesii UAn. toruloides CAn. torulosa CAn. tschujskaja CAn. tsugarensis CAn. turkestanica CAn. tuzsonii CAn. vaucheriae CAn. verrucosa CAn. vialis UAn. voukii UAn. wallumensis CAn. welshii CAn. westii CAn. willei CAn. wisconsinensis CAn. woodii CAn. yunnanensis nullList of synonimized species from AlgaeBase:An. affinis S →An. akankoensis S →An. ambigua S →An. anisococca S →An. anomala S →An. antarctica S →An. aphanizomenoides S →An. arctica S →An. azollae S → Trichormus azollaeAn. ballygungii S →An. baltica S →An. berezowskii S →An. bergii S →An. bituri S →An. bothae S →An. botulus S →An. bullosa S →An. calcicola S →An. californica S →An. caspica S →An. chalybea S →An. circinalis S →An. circularis S →An. citrispora S →An. compacta S →An. compacta S →An. constricta S →An. crassa S →An. curva S →An. cycadearum S →An. cylindrospermoides S →An. cylindrospora S →An. danica S →An. decorticans S →An. delicatula S →An. desikacharyensis S →An. doliolum S →An. ellipsoides S →An. ellipsospora S →An. eucompacta S →An. fallax S →An. farciminiformis S →An. fertilissima S →An. flos-aquae S →An. fragilis S →An. fuellebornii S →An. fusca S →An. gelatinicola S →An. gelatinosa S →An. gelatinosa S →An. globosa S →An. halbfassii S →An. hallensis S →An. hansgirgii S →An. hassallii S →An. helicoidea S →An. heterospora S →An. hoshiarpurensis S →An. humicola S →An. impalpabilis S →An. impalpebralis S →An. impalpebralis S →An. incrassata S →An. indica S →An. infusionum S →An. intricata S →An. isocystoides S →An. issatschenkoi S →An. jacutica S →An. kashiensis S →An. khannae S →An. kisseleviana S →An. knipowitschii S →An. lemmermannii S →An. limicola S →An. limicola S →An. limnetica S →An. longicellularis S →An. lutea S →An. macrospora S →An. major S →An. manguinii S →An. maxima S →An. membranina S →An. mendotae S →An. minispora S →An. minuta S →An. mollis S →An. mucosa S →An. mysorensis S →An. nathii S →An. naviculoides S →An. nygaardii S →An. oryzae S →An. oumiana S →An. parva S →An. perturbata S →An. planctonica S →An. polysperma S →An. polyspora S →An. portoricensis S →An. propinqua S →An. pseudocompacta S →An. pseudoscillatoria S →An. pseudovariabilis S →An. raciborskii S →An. randhawae S →An. reniformis S →An. reverdattoana S →An. rufescens S →An. scheremetieviae S →An. siamensis S →An. sibirica S →An. sigmoidea S →An. skujaelaxum S →An. smithii S →An. solitaria S →An. spiralis S →An. spiroides S →An. stagnalis S →An. steloides S →An. subcylindrica S →An. subtropica S →An. tanganyikae S →An. tenericaulis S →An. tenuis S →An. thermalis S →An. torquesreginae S →An. ucrainica S →An. unispora S →An. utermoehlii S →An. vaginicola S →An. variabilis S → Trichormus variabilisAn. viguieri S →An. volzii S →An. werneri S →An. zinserlingii S →
Recent changes
The genus has significant overlap with Aphanizomenon. Despite An. variabilis having been removed from the genus to form a new Trichormus 1988, it seems to be phylogenetically placed in Anabaena. As a result the position of T. variabilis and the two species of benthic Anabaena may be subject to change. The group around Trichormus variabilis is better-supported by data than grouping it together with main-group Anabaena/''Aphanizomenon, so it's more likely for the two species to be split out. In addition, the position of Trichormus azollae needs change.- In 2009, a 42-species group under the "planktic" main-group, determined by 16S rRNA or gas vesicle morphotype, were transferred to
Nitrogen fixation by ''Anabaena''
Under nitrogen-limiting conditions, vegetative cells differentiate into heterocysts at semiregular intervals along the filaments. Heterocyst cells are terminally specialized for nitrogen fixation. The interior of these cells is micro-oxic as a result of increased respiration, inactivation of O2-producing photosystem II, and formation of a thickened envelope outside of the cell wall. Nitrogenase, sequestered within these cells, transforms dinitrogen into ammonia at the expense of ATP and reductant—both generated by carbohydrate metabolism, a process supplemented, in the light, by the activity of PS I. Carbohydrate, probably in the form of glucose, is synthesized in vegetative cells and moves into heterocysts. In return, nitrogen fixed in heterocysts moves into the vegetative cells, at least in part in the form of amino acids.The fern Azolla forms a symbiotic relationship with the cyanobacterium Anabaena azollae, which fixes atmospheric nitrogen, giving the plant access to this essential nutrient. This has led to the plant being dubbed a "super-plant", as it can readily colonise areas of freshwater, and grow at great speed - doubling its biomass in as little as 1.9 days. The typical limiting factor on its growth is phosphorus, abundance of which, due to chemical runoff, often leads to Azolla blooms. Unlike other known plants, the symbiotic microorganism is transferred directly from one generation to the next. This has made Anabaena azollae completely dependent on its host, as several of its genes are either lost or have been transferred to the nucleus in Azolla's cells.