{{Short description|Genus of algae}} {{Automatic taxobox | image = Actinastrumhantzii EPA.jpg | image_caption = ''Actinastrum hantzschii'' | taxon = Actinastrum | authority = Lagerheim, 1882 | type_species = ''Actinastrum hantzschii'' | type_species_authority = Lagerheim<ref name=AlgaeBase/> | subdivision_ranks = Species | subdivision = See below }}
'''''Actinastrum''''' is a genus of freshwater green algae.<ref name=AlgaeBase>{{AlgaeBase genus|id=43466|title=''Actinastrum'' Lagerheim, 1882 |access-date=2025-01-09}}</ref> It was first described by Gustaf Lagerheim in 1882.<ref>{{cite journal | author = Gustaf Lagerheim | title = Bidrag till kännedomen om Stockholmstraktens Pediastréer, Protococcacéer och Palmellacéer | journal = Öfversigt af Kongl. Vetenskaps-Akademiens Förhandlingar | volume = 39 | issue = 2 | pages = 47–81 | date = February 1882}}</ref> Members of the genus are commonly found in eutrophic freshwater ponds and lakes, and have a cosmopolitan distribution.<ref name=AlgaeBase/>
Traditionally, ''Actinastrum'' was placed within the family Coelastraceae based on morphology (spherical, three-dimensional colonies and reproduction by autospores). However, molecular phylogenetic studies have shown that the type species ''Actinastrum hantzschii'' is closely related to ''Chlorella'', another autospore-forming genus in the family Chlorellaceae.<ref>{{cite journal |vauthors= Wolf M, Kreinitz L, Hepperle D |title= Phylogenetic position of ''Actinastrum hantzschii'' LAGERHEIM 1882 (Chlorophyta, Trebouxiophyceae) |journal= Algological Studies |volume= 104 |pages= 59–67 |date= April 2002 |doi= 10.1127/algol_stud/104/2002/59 }}</ref><ref>{{cite journal |vauthors= Luo W, Pröschold T, Bock C, Krienitz L |title= Generic concept in Chlorella-related coccoid green algae (Chlorophyta, Trebouxiophyceae) |journal= Plant Biology |volume= 12 |issue= 3 |pages= 545–553 |date= April 2010 |doi= 10.1111/j.1438-8677.2009.00221.x |pmid= 20522192 |bibcode= 2010PlBio..12..545L }}</ref>
==Description== ''Actinastrum'' consists of eight (sometimes two, four or 16) cells, which are attached together at a common point to form a star-shaped colony, termed a coenobium. The colonies are often aggregated to form compound coenobia. Cells are 7–40 μm long and 1–8 μm wide, much longer than broad, and are cylindrical, cigar-shaped, or pointed. The cell wall is smooth, thin, and not thickened. Cells are uninucleate (with a single nucleus) and a contain a single chloroplast, which is elongated, parietal and contains a single pyrenoid.<ref name=Shubert2014>{{cite book |editor-first1=John D.|editor-last1=Wehr|editor-first2=Robert G.|editor-last2=Sheath|editor-first3=J. Patrick|editor-last3=Kociolek |date= 2014 |edition=2 |title= Freshwater Algae of North America: Ecology and Classification |last1= Shubert|first1=Elliot| last2=Gärtner |first2=Georg |chapter= Chapter 7. Nonmotile Coccoid and Colonial Green Algae |url= |location= |publisher= Elsevier Inc. |page= |isbn=978-0-12-385876-4 }}</ref><ref name=AlgaeBase/><ref name=Komarek_Fott1983>{{cite book | title= Chlorophyceae (Grünalgen), Ordnung Chlorococcales | last1=Komárek | first1=J. | last2=Fott | first2=B. | date=1983 | publisher= E. Schweizerbart'sche Verlagsbuchhandlung | series=Das Phytoplankton des Süßwassers | language=German | pages=1044 }}</ref>
''Actinastrum'' reproduces asexually, by forming 4-16 autospores usually arranged into a coenobium. The first protoplast division occurs transversely, the second perpendicular to it, and the third occurs along the long axis of the cell.<ref name=Komarek_Fott1983/> Spores are released by the dissolution of the parental cell wall. Sexual reproduction has not been observed in this genus.<ref name=AlgaeBase/>
==Species== {{As of|2025}}, AlgaeBase accepts ten species, including one fossil species:<ref name=AlgaeBase/> * ''Actinastrum aciculare'' *†''Actinastrum bansaense'' * ''Actinastrum cerastioides'' * ''Actinastrum fluviatile'' * ''Actinastrum gracillimum'' * ''Actinastrum hantzschii'' * ''Actinastrum mixtum'' * ''Actinastrum schroeteri'' * ''Actinastrum subcornutum'' * ''Actinastrum tetaniforme''
Species in ''Actinastrum'' are distinguished based on the overall shape of the cells. However, the genus is very morphologically, and the cell shape changes considerably during its life cycle and in response to external conditions.<ref name=Komarek_Fott1983/>
==References== {{Reflist|1}} {{Taxonbar|from=Q4676838}}
Category:Trebouxiophyceae genera Category:Chlorellaceae Category:Freshwater algae