Desmids (Gr.desmos, bond or chain) are an order (Desmidiales) in the Charophyta, a division of green algae that forms a sister group to the land plants (Embryophyta). The desmids are often treated as members of the Zygnematales, as family Desmidiaceae. The Desmidiales comprise around 40 genera and 5,000 to 6,000species, found mostly but not exclusively in fresh water. Many species may be found in the fissures between patches of sphagnum moss in marshes. With a pH level of approximately 4.0, sphagnum peat provides the ideal environment for this flora. The structure of these algae is unicellular, while the cell is sometimes divided into two symmetricalcompartments separated by a narrow bridge or isthmus, wherein the spherical nucleus is located. Each semi-cell houses a large, often folded chloroplast for photosynthesizing. One or more pyrenoids can be found. These form carbohydrates for energy storage. Desmids assume a variety of highly symmetrical and generally attractive shapes, among those elongated, star-shaped and rotund configurations, which provide the basis for their classification. The largest among them may be visible to the unaided eye. Desmidiales lack in flagellae. Desmids commonly reproduce by asexual fission, however, in adverse conditions, Desmidiales may reproduce sexually through a process of conjugation, which are also found among the Zygnematales. These two groups are closely related, and comprise the class Zygnematophyceae. They resemble the algae genus Spirogyra as some of them are filamentous, such as the species Desmidium swartzii. However, these filaments are arranged in a helix pattern.
As photosynthetic algae they benefit by releasing oxygen into the air and manufacturing glucose as food, in turn being predated upon by amoebae and other invertebrates which are then consumed by higher-level predators, thus benefiting the food chain. They are known to absorb large amounts of strontium and barium from the water, thus recycling these metals.
Survey of Clare Island 1990 - 2005, noting the Desmidiales recorded. Ed. Guiry, M.D., John, D.M., Rindi, F. and McCarthy, T.K. 2007. New Survey of Clare Island. Volume 6: The Freshwater and Terrestrial Algae. Royal Irish Academy. ISBN 978-1-904890-31-7