| Actinastrum | |
|---|---|
 | |
| Actinastrum hantzschii | |
Scientific classification | |
| Clade: | Viridiplantae |
| Division: | Chlorophyta |
| Class: | Trebouxiophyceae |
| Order: | Chlorellales |
| Family: | Chlorellaceae |
| Genus: | Actinastrum Lagerheim, 1882 |
| Type species | |
| Actinastrum hantzschii Lagerheim[1] | |
| Species | |
See below | |
Actinastrum is agenus of freshwatergreen algae.[1] It was first described byGustaf Lagerheim in 1882.[2] Members of the genus are commonly found ineutrophic freshwater ponds and lakes, and have acosmopolitan distribution.[1]
Traditionally,Actinastrum was placed within the familyCoelastraceae based on morphology (spherical, three-dimensional colonies and reproduction by autospores). However,molecular phylogenetic studies have shown that the type speciesActinastrum hantzschii is closely related toChlorella, another autospore-forming genus in the familyChlorellaceae.[3][4]
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 acoenobium. The colonies are often aggregated to form compound coenobia. Cells are 7–40 μm long and 1–8 μ wide, much longer than broad, and are cylindrical, cigar-shaped, or pointed. Thecell wall is smooth, thin, and not thickened. Cells are uninucleate (with a singlenucleus) and a contain a singlechloroplast, which is elongated, parietal and contains a singlepyrenoid.[5][1][6]
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.[6] Spores are released by the dissolution of the parentalcell wall. Sexual reproduction has not been observed in this genus.[1]
As of 2025[update],AlgaeBase accepts ten species, including one fossil species:[1]
Species inActinastrum 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.[6]