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 (formerly classified as Cosmarium ralfsii) has no lobes, and the cells are smoothly ellipsoidal in outline. Micrasterias dickiei (formerly classified as Staurodesmus 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.West
Micrasterias adscendens Nordstedt
Micrasterias africana (F.E.Fritsch & F.Rich) Coesel & Van Geest
Micrasterias alata Wallich
Micrasterias ambadiensis (Grönblad & A.M.Scott) Kurt Föster
Micrasterias americana Ehrenberg ex Ralfs
Micrasterias anomala W.B.Turner
Micrasterias apiculata Meneghini ex Ralfs
Micrasterias archeri Coesel & M.Dingley
Micrasterias bangladeshensis A.K.Islam & Haroon
Micrasterias bewsii Fritsch & M.F.Rich
Micrasterias bicoronata A.Kenins
Micrasterias borgei Willi Krieger
Micrasterias bourrellyana J.A.Rino
Micrasterias brachyptera Lundell
Micrasterias ceratofera Joshua
Micrasterias compereana Neustupa, St'astný & Skaloud
Micrasterias conferta Lundell
Micrasterias croasdaleana C.E.M.Bicudo & L.Sormus
Micrasterias crux-africana F.J.Cohn
Micrasterias crux-melitensis Ralfs
Micrasterias cunningtonii G.S.West
Micrasterias decemdentata (Nägeli) W.Archer
Micrasterias denboeri Coesel & Van Geest
Micrasterias denticulata Brébisson ex Ralfs
Micrasterias depauperata Nordstedt
Micrasterias dickiei (Ralfs) Škaloud et al.
Micrasterias divisa Willi Krieger
Micrasterias echinata P.E.Brandham
Micrasterias elegans (W.West & G.S.West) Coesel & Van Geest
Micrasterias elongata (Schmidle) Coesel & Van Geest
Micrasterias euastriellopsis Bharati
Micrasterias excavata (Nordstedt) C.E.M.Bicudo & L.Sormus
Micrasterias fimbriata Ralfs
Micrasterias foersteri Thomasson
Micrasterias foliacea Bailey ex Ralfs
Micrasterias furcata C.Agardh ex Ralfs
Micrasterias groenewaldii Claassen
Micrasterias hardyi G.S.West
Micrasterias hieronymusii Schmidle
Micrasterias horrida C.E.Taft
Micrasterias incisa Ralfs
Micrasterias inflata C.Bernard
Micrasterias integra Nordstedt
Micrasterias jejuensis H.S.Kim
Micrasterias jenneri Ralfs
Micrasterias johnsonii West & G.S.West
Micrasterias khasiae W.B.Turner
Micrasterias koreana H.S.Kim
Micrasterias laticeps Nordstedt
Micrasterias lebrunii Oye
Micrasterias ledouxii A.M.Scott & Croasdale
Micrasterias lundii Bourrelly
Micrasterias lux Joshua
Micrasterias madagascariensis Coesel
Micrasterias mahabuleshwarensis J.Hobson
Micrasterias moebii (O.Borge) West & G.S.West
Micrasterias muricata Bailey ex Ralfs
Micrasterias nordstedtiana Wolle
Micrasterias nordstedtii Thomasson
Micrasterias nylstromica Claassen
Micrasterias oscitans Ralfs
Micrasterias papillifera Brébisson ex Ralfs
Micrasterias pinnatifida Ralfs
Micrasterias piquata R.K.Salisbury
Micrasterias prescottiana C.E.M.Bicudo & L.Sormus
Micrasterias pseudotorreyi Wolle
Micrasterias quadridentata (Nordstedt) Grönblad
Micrasterias quadriverrucosa (Thomasson) C.E.M.Bicudo & L.Sormus
Micrasterias radians W.B.Turner
Micrasterias radiosa Ralfs
Micrasterias ralfsii (Brébisson ex Ralfs) Škaloud et al.
Micrasterias rotata Ralfs
Micrasterias sanctipaulensis C.E.M.Bicudo & L.Sormus
Micrasterias schmidleana Coesel & Van Geest
Micrasterias schweickerdtii M.I.Claassen
Micrasterias schweinfurthii Cohn
Micrasterias semiradiata Brébisson ex Kützing
Micrasterias sexpinata (Irénée-Marie & Hilliard) H.Croasdale & G.W.Prescott
Micrasterias simplex Børgesen
Micrasterias spinosa H.S.Kim
Micrasterias stauromorpha W.B.Turner
Micrasterias subaequalis Grönblad
Micrasterias subdenticulata (Nordstedt) Willi Krieger
Micrasterias subincisa Willi Krieger
Micrasterias suboblonga Nordstedt
Micrasterias subtruncata A.I.Lobik
Micrasterias tetraptera West & G.S.West
Micrasterias thomasiana W.Archer
Micrasterias torreyi Bailey
Micrasterias triangularis Wolle
Micrasterias tropica Nordstedt
Micrasterias truncata Brébisson ex Ralfs
Micrasterias verrucosa Bisset
Micrasterias zeylanica F.E.Fritsch
Species names with uncertain taxonomic status
The status of the following species is unresolved:
Micrasterias aculeata M.Rostock
Micrasterias berganii H.V.Hauge
Micrasterias bicaudata (A.K.H.Braun ex Kützing) Kuntze
Micrasterias bioctonaria G.Rabenhorst
Micrasterias boryana (P.J.F.Turpin) Ehrenberg
Micrasterias boryi Kützing
Micrasterias braunii (C.W.Nägeli ex Kützing) Kuntze
Micrasterias comperei R.S.Ganem & P.A.C.Senna
Micrasterias complecta C.G.T.Preuss
Micrasterias convoluta (A.K.J.Corda) Kuntze
Micrasterias cordae A.Braun
Micrasterias coronula Ehrenberg
Micrasterias cruciata Kützing
Micrasterias crucigenia Kützing
Micrasterias decemdentatum (C.W.Nägeli) W.Archer
Micrasterias denticula Istvanfy
Micrasterias duplex (Meyen) Kützing
Micrasterias ecornis Ehrenberg
Micrasterias eichleri Schmidle
Micrasterias enneactis Ehrenberg
Micrasterias extendens W.B.Turner
Micrasterias falcata Corda
Micrasterias floridensis R.K.Salisbury
Micrasterias galeata Borge
Micrasterias ghibellina Meneghini
Micrasterias granulata H.C.Wood
Micrasterias halis Raciborski
Micrasterias hamata (Wolle) F.C.E.Børgesen
Micrasterias heliactis Kützing
Micrasterias heptactis Ehrenberg
Micrasterias hermanniana Reinsch
Micrasterias hexactis Ehrenberg
Micrasterias hexagona G.W.Grant
Micrasterias hexagonalis F.Steinecke
Micrasterias kangofurinensis N.Woodhead & R.D.Tweed
Micrasterias lacerata Kützing
Micrasterias margaritifera (P.J.F.Turpin) L.A.Brébisson & P.Godet
Micrasterias mbugensis E.M.Lind
Micrasterias melitensis G.G.A.Meneghini
Micrasterias mohii (Borge) Authority Unknown
Micrasterias multifida Wolle
Micrasterias napoleonis (P.J.F.Turpin) Kützing
Micrasterias nordstetiana Wolle
Micrasterias oblonga Ehrenberg
Micrasterias ornamentalis O.Borge
Micrasterias paradoxa Kützing
Micrasterias platyptera W.B.Turner
Micrasterias polonica (B.Eichler & R.Gutwinski) West & G.S.West
Micrasterias polycyclia G.Rabenhorst
Micrasterias pseudofurcata Wolle
Micrasterias quadragies-cuspidata (Corda) Ralfs
Micrasterias renicarpa (Turpin) Kützing
Micrasterias ricciaeformis C.Agardh
Micrasterias robusta West & G.S.West
Micrasterias rosula Kützing
Micrasterias selenaea Kützing
Micrasterias senaria Ehrenberg
Micrasterias simplex (Meyen) Kützing
Micrasterias simplex Wolle
Micrasterias simplex Kurt Förster & F.Eckert
Micrasterias sinuata L.A.Brébisson
Micrasterias siolii A.M.Scott & Croasdale
Micrasterias sphaerastrum Kützing
Micrasterias staurastrum Kützing
Micrasterias subfimbriata Wolle
Micrasterias sublagoensis Kurt Förster & F.Eckert
Micrasterias tetracera Kützing
Micrasterias tricera Kützing
Micrasterias tricyclia Ehrenberg
Micrasterias trigemina E.H.P.A.Haeckel
Micrasterias upsaliensis (Cleve) W.Archer
Phylogenetics
Modern molecular phylogenetics suggest the following relationships (not all accepted species are included):
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 (see the Baas Becking hypothesis). 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.
References
External links
Micrasterias Pictures Page
Desmidiaceae
Charophyta genera
Taxa named by Carl Adolph Agardh