Cosmarium is a single-celled placoderm desmid. The
cells are deeply divided in the middle by a short isthmus
that contains the nucleus. The two semicells are rounded
in front view and flattened, oval, or elliptic in side view.
The older half of the cell wall secretes mucilage, while the
younger half is lifted off of the substrate at a slight angle.
The mucilage swells as it absorbs water and propels the cell
forward. The cell wall may be smooth with pores or ornamented
with granules, pits, or warts. The vegetative cells
usually do not have spines. Each semicell has at least one
central chloroplast (usually 2 or 4) with pyrenoids. Cosmarium
sometimes produces thick-walled resting cells (Vashishta
et al., 2005). We selected Cosmarium sp. because
of its availability in Iran and also it could be cultured in
growth media easily.