desmid

Any green freshwater algae in the family Desmidiaceae. Desmids are most frequent in acidic or soft waters such as Sphagnum bogs, or shallow-water habitats where there is a concentration of organic acids.

Most desmids are single celled and bear a superficial resemblance to diatoms but differ in being green and having a cell wall composed of cellulose and pectic materials instead of silica. The wall is further characterised by having variously shaped pores through which mucilage is secreted, often giving the appearance of spines. Their growth is inhibited in water containing high concentrations of calcium.