Micrasterias Fimbriata

''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, 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

{{Taxonbar, from=Q149783

Desmidiaceae
Charophyta genera
Taxa named by Carl Adolph Agardh