Image © Koos Meesters (after Czech material)
Cosmarium obsoletum. Notice semicircular semicells, broad isthmus and
hyaline thickening of the cell wall at the basal angles. Notice also pronounced cell wall pores. In the thickening of the left-hand basal angle of the lower semicell an extra large conical pore is to be observed.
Cell dimensions (L x B): ca 50 x 60 µm
C. obsoletum is a smooth-walled, firm-lined Cosmarium species marked by a broad isthmus and semicells that are depressed semicircular in outline. Most characteristic is the cell wall at the basal angles, i.e., thickened and often perforated by a large conical pore. C. obsoletum has its main point of distribution in tropical regions. From the Netherlands only a few records are known (in oligo-mesotrophic moorland pools) dating from the first half of the last century.
Image © Koos Meesters
Another cell of C. obsoletum (DIC image)
Image © Jan Šťastný (after Czech material)
SEM picture of Cosmarium obsoletum