Cosmarium porteanum Archer, 1860
Most likely ID: n.a.
Synonym: Cosmarium portianum
Sampling location: Sima Moor (Austria)
Phylogenetic tree: Cosmarium porteanum
Diagnosis:
- semi-cell transversely elliptica
- cell ellipsoid in apical view
- length 25–42 µm, width 20–30
- cell wall covered with vertical rows of granules
- one pyrenoid per semi-cell
- one furcoid chloroplast per semi-cell
- sinus deep and wide
- isthmus elongated, neck-shaped
I have only ever found Cosmarium porteanum in the Sima Moor in Austria. The alga was only found sporadically in the samples. The most striking feature of Cosmarium porteanum is the neck-like elongated isthmus. This makes the semi-cells appear clearly separated from each other. The cell wall is evenly covered with granules.
The name Cosmarium portianum is often found in the literature and on the Internet. However, the species was named after George Porte, which is why Cosmarium porteanum is the correct name.
The similar species Cosmarium reniforme var. apertum also has a slightly elongated isthmus and the cell wall is also granulated. However, there are two pyrenoids in each of the semi-cells of Cosmarium reniforme var. apertum and the species is considerably larger than Cosmarium porteanum with a length of 44–46 µm.
Fig. 1 a-b: Cosmarium porteanum. L = 35 µm. Two focal planes of a slightly squashed specimen. Obj. 100 X.
Fig. 2 a-c: Cosmarium porteanum. L = 35 µm. Three focal planes of the same specimen as shown in fig. 1 a-b in brightfield illumination. Obj. 100 X.