Micrasterias
Micrasterias is a unicellular green alga of the order Desmidiales. Its species vary in size reaching up to hundreds of microns.
Micrasterias displays a bilateral symmetry, with two mirror image semi-cells joined by a narrow isthmus containing the nucleus of the organism. This dual semi-cell structure is unique to the group of green algae to which Micrasterias belongs. Each semi-cell contains a single large chloroplast, the site of photosynthesis for Micrasterias. Chloroplasts within Micrasterias contain chlorophyll a and chlorophyll b and the enzymes required for photosynthesis. The sugar created is used to provide energy for the organism or, if not used, taken up by many small round pyrenoids which are embedded in the chloroplast. They convert the sugar to a starch for storage.
Micrasterias can produce both asexually and sexually. Asexual reproduction occurs via mitosis. When this occurs the genetic material of Micrasterias is duplicated and two small semi-cells grow between the original semi-cells, gradually increasing in size. Sexual reproduction occurs through a process called conjugation whereby two organisms come together and fuse their haploid cells to form a diploid zygote. This zygote typically forms a thick protective wall which can allow the organism to remain dormant for many months to survive cold winters and long droughts. When adequate conditions resume, the zygospore will germinate, undergo meiosis, and produce new haploid algal cells.
Description
Micrasterias species are symmetrical and generally consist of two flattened, identical portions called semicells that are almost entirely filled with chloroplasts, with a nucleus that lies at the center where the two semicells are joined together. The gaps between the two semicells are joined by an isthmus. Each semicell is further divided into a polar lobe and two lateral lobes. These lobes can be further subdivided up to the fourth order. Some species, such as Micrasterias laticeps, have a very different morphology, with unbranched lobes. Except for a single filament-forming species, Micrasterias foliacea, it is found as single cells. As is common in the green algae, the chloroplasts of Micrasterias contain pyrenoids.Two species of Micrasterias have different morphologies to species traditionally placed in this genus, but molecular phylogenetic analyses demonstrate that they are embedded within Micrasterias. Micrasterias ralfsii has no lobes, and the cells are smoothly ellipsoidal in outline. Micrasterias dickiei is triradiate in polar view instead of flattened, and has three spines on each semicell.
Similar genera
Micrasterias is generally easy to identify due to its shape and typically large size.The genus Pseudomicrasterias has been split off from the genus Micrasterias, and as of 2023, contains 2 species, formerly known as Micrasterias arcuata. They have a similar morphology to the simpler species of Micrasterias, but molecular phylogenetic analyses show that they are not related.
The genus Prescottiella contains a single species, Prescottiella sudanensis, formerly known as Micrasterias sudanensis. It is distinguished from Micrasterias in that its semicells are not identical, making the cell asymmetrical along one axis; the spines of one semicell curve towards the isthmus, while the spines of the other semicell curve away.
Species
Accepted species
Micrasterias comprises the following species:Micrasterias abrupta West & G.S.WestMicrasterias adscendens NordstedtMicrasterias africana Coesel & Van GeestMicrasterias alata WallichMicrasterias ambadiensis Kurt FösterMicrasterias americana Ehrenberg ex RalfsMicrasterias anomala W.B.TurnerMicrasterias apiculata Meneghini ex RalfsMicrasterias archeri Coesel & M.DingleyMicrasterias bangladeshensis A.K.Islam & HaroonMicrasterias bewsii Fritsch & M.F.RichMicrasterias bicoronata A.KeninsMicrasterias borgei Willi KriegerMicrasterias bourrellyana J.A.RinoMicrasterias brachyptera LundellMicrasterias ceratofera JoshuaMicrasterias compereana Neustupa, St'astný & SkaloudMicrasterias conferta LundellMicrasterias croasdaleana C.E.M.Bicudo & L.SormusMicrasterias crux-africana F.J.CohnMicrasterias crux-melitensis RalfsMicrasterias cunningtonii G.S.WestMicrasterias decemdentata W.ArcherMicrasterias denboeri Coesel & Van GeestMicrasterias denticulata Brébisson ex RalfsMicrasterias depauperata NordstedtMicrasterias dickiei Škaloud et al.Micrasterias divisa Willi KriegerMicrasterias echinata P.E.BrandhamMicrasterias elegans Coesel & Van GeestMicrasterias elongata Coesel & Van GeestMicrasterias euastriellopsis BharatiMicrasterias excavata C.E.M.Bicudo & L.SormusMicrasterias fimbriata RalfsMicrasterias foersteri ThomassonMicrasterias foliacea Bailey ex RalfsMicrasterias furcata C.Agardh ex RalfsMicrasterias groenewaldii ClaassenMicrasterias hardyi G.S.WestMicrasterias hieronymusii SchmidleMicrasterias horrida C.E.TaftMicrasterias incisa RalfsMicrasterias inflata C.BernardMicrasterias integra NordstedtMicrasterias jejuensis H.S.KimMicrasterias jenneri RalfsMicrasterias johnsonii West & G.S.WestMicrasterias khasiae W.B.TurnerMicrasterias koreana H.S.KimMicrasterias laticeps NordstedtMicrasterias lebrunii OyeMicrasterias ledouxii A.M.Scott & CroasdaleMicrasterias lundii BourrellyMicrasterias lux JoshuaMicrasterias madagascariensis CoeselMicrasterias mahabuleshwarensis J.HobsonMicrasterias moebii West & G.S.WestMicrasterias muricata Bailey ex RalfsMicrasterias nordstedtiana WolleMicrasterias nordstedtii ThomassonMicrasterias nylstromica ClaassenMicrasterias oscitans RalfsMicrasterias papillifera Brébisson ex RalfsMicrasterias pinnatifida RalfsMicrasterias piquata R.K.SalisburyMicrasterias prescottiana C.E.M.Bicudo & L.SormusMicrasterias pseudotorreyi WolleMicrasterias quadridentata GrönbladMicrasterias quadriverrucosa C.E.M.Bicudo & L.SormusMicrasterias radians W.B.TurnerMicrasterias radiosa RalfsMicrasterias ralfsii Škaloud et al.Micrasterias rotata RalfsMicrasterias sanctipaulensis C.E.M.Bicudo & L.SormusMicrasterias schmidleana Coesel & Van GeestMicrasterias schweickerdtii M.I.ClaassenMicrasterias schweinfurthii CohnMicrasterias semiradiata Brébisson ex KützingMicrasterias sexpinata H.Croasdale & G.W.PrescottMicrasterias simplex BørgesenMicrasterias spinosa H.S.KimMicrasterias stauromorpha W.B.TurnerMicrasterias subaequalis GrönbladMicrasterias subdenticulata Willi KriegerMicrasterias subincisa Willi KriegerMicrasterias suboblonga NordstedtMicrasterias subtruncata A.I.LobikMicrasterias tetraptera West & G.S.WestMicrasterias thomasiana W.ArcherMicrasterias torreyi BaileyMicrasterias triangularis WolleMicrasterias tropica NordstedtMicrasterias truncata Brébisson ex RalfsMicrasterias verrucosa BissetMicrasterias zeylanica F.E.FritschSpecies names with uncertain taxonomic status
The status of the following species is unresolved:Micrasterias aculeata M.RostockMicrasterias berganii H.V.HaugeMicrasterias bicaudata KuntzeMicrasterias bioctonaria G.RabenhorstMicrasterias boryana EhrenbergMicrasterias boryi KützingMicrasterias braunii KuntzeMicrasterias comperei R.S.Ganem & P.A.C.SennaMicrasterias complecta C.G.T.PreussMicrasterias convoluta KuntzeMicrasterias cordae A.BraunMicrasterias coronula EhrenbergMicrasterias cruciata KützingMicrasterias crucigenia KützingMicrasterias decemdentatum W.ArcherMicrasterias denticula IstvanfyMicrasterias duplex KützingMicrasterias ecornis EhrenbergMicrasterias eichleri SchmidleMicrasterias enneactis EhrenbergMicrasterias extendens W.B.TurnerMicrasterias falcata CordaMicrasterias floridensis R.K.SalisburyMicrasterias galeata BorgeMicrasterias ghibellina MeneghiniMicrasterias granulata H.C.WoodMicrasterias halis RaciborskiMicrasterias hamata F.C.E.BørgesenMicrasterias heliactis KützingMicrasterias heptactis EhrenbergMicrasterias hermanniana ReinschMicrasterias hexactis EhrenbergMicrasterias hexagona G.W.GrantMicrasterias hexagonalis F.SteineckeMicrasterias kangofurinensis N.Woodhead & R.D.TweedMicrasterias lacerata KützingMicrasterias margaritifera L.A.Brébisson & P.GodetMicrasterias mbugensis E.M.LindMicrasterias melitensis G.G.A.MeneghiniMicrasterias mohii Authority UnknownMicrasterias multifida WolleMicrasterias napoleonis KützingMicrasterias nordstetiana WolleMicrasterias oblonga EhrenbergMicrasterias ornamentalis O.BorgeMicrasterias paradoxa KützingMicrasterias platyptera W.B.TurnerMicrasterias polonica West & G.S.WestMicrasterias polycyclia G.RabenhorstMicrasterias pseudofurcata WolleMicrasterias quadragies-cuspidata RalfsMicrasterias renicarpa KützingMicrasterias ricciaeformis C.AgardhMicrasterias robusta West & G.S.WestMicrasterias rosula KützingMicrasterias selenaea KützingMicrasterias senaria EhrenbergMicrasterias simplex KützingMicrasterias simplex WolleMicrasterias simplex Kurt Förster & F.EckertMicrasterias sinuata L.A.BrébissonMicrasterias siolii A.M.Scott & CroasdaleMicrasterias sphaerastrum KützingMicrasterias staurastrum KützingMicrasterias subfimbriata WolleMicrasterias sublagoensis Kurt Förster & F.EckertMicrasterias tetracera KützingMicrasterias tricera KützingMicrasterias tricyclia EhrenbergMicrasterias trigemina E.H.P.A.HaeckelMicrasterias upsaliensis W.ArcherPhylogenetics
Modern molecular phylogenetics suggest the following relationships :Even though Triploceras was recovered as embedded within Micrasterias, there was low statistical support for this placement, so it remains a separate genus.
Habitat and distribution
As with other desmids, Micrasterias grows in freshwater habitats. It prefers oligotrophic to mesotrophic lakes and bogs, often associated with aquatic plants.Freshwater microalgae, along with other microscopic organisms, are often presumed to be cosmopolitan in distribution. However, many desmids are an exception to this, likely because of their high morphological complexity allowing for easier identification, and the fact that they mostly do not form resting spores that would allow for wider dispersal. In particular, a number of Micrasterias species are restricted to certain biogeographical realms or continents. For example, Micrasterias muricata appears to be endemic to North America, while Micrasterias ceratofera is restricted to Southeast Asia and northern Australia.