Cosmarium circulare

(Hassall ex Ralfs) Kützing, 1849

Most likely ID: n.a.


Synonym: n.a.


Sampling location: Simmelried


Phylogenetic tree: Cosmarium circulare



  • semi-cells almost hemi-spherical, apex sometimes slightly flattened
  • length 34–120 µm, width 35–105 µm
  • sinus deep, narrow, dilated internally
  • isthmus 14–25 µm wide
  • cell wall covered with fine pores
  • two pyrenoids per semi-cell
Cosmarium circulare

I only find Cosmarium circulare in the Simmelried. However, I have only been able to detect it there since 2023. I have no earlier records from the Simmelried.


I recognize Cosmarium circulare mainly by the cell shape (circular or broadly oval), the smooth, only finely dotted cell wall and the two pyrenoids per half cell. The similar species Cosmarium ralfsii has more than 2 pyrenoids per half cell.


Fig. 1: Cosmarium circulare. L = 49–52 µm. An accumulation of 4 specimens. Obj. 40 X.


Fig. 2 a-b: Cosmarium circulare. L = 55 µm. Two focal planes of a slightly squashed specimen. Note the fine pores covering the cell wall (b). Obj. 100 X.


Fig. 3 a-b: Cosmarium circulare. L = 53 µm. Two focal planes of a second specimen. Obj. 100 X.


Fig. 4 a-b: Cosmarium circulare. L = 51 µm. Two focal planes of a third specimen in brightfield illumination. Obj. 100 X.