Desmidiales
Symmetrical microscopic algae, bioindicators of freshwater quality.
Desmidiales, commonly called desmids, are an order within the Charophyta, the division of green algae from which land plants evolved. These microscopic, mostly single-celled algae are renowned among amateur and professional microscopists for their striking symmetry, aesthetic appeal, and remarkable diversity of forms. Desmids belong to the class Zygnematophyceae, and while some classifications treat them as a single family, Desmidiaceae, most recognize three to five families within the order. The group encompasses roughly 40 genera and between 5,000 and 6,000 species, found predominantly in freshwater environments, particularly acidic waters with a pH between 4.8 and 7.0. Many species thrive in the fissures between sphagnum moss in marshes, and their sensitivity to environmental changes makes them valuable bioindicators for water and habitat quality.
A distinction exists between saccoderm and placoderm desmids. Saccoderm desmids, placed in the family Mesotaeniaceae within the order Zygnematales, have unconstricted cells lacking median suture lines and mucilage-secreting pores. Placoderm desmids, which correspond to the Desmidiales, possess cells with two symmetrical halves and mucilage-secreting pores in the cell wall. The term "desmid" is used here interchangeably with "placoderm desmid" to refer to the Desmidiales.
Desmids are unicellular and lack flagella. While most species are single-celled, some genera form filaments or non-filamentous colonies where cells are connected by threads or remnants of parent cell walls. The cell is typically divided into two symmetrical compartments separated by a narrow bridge, or isthmus, which houses the spherical nucleus. Each semi-cell contains a large, often folded chloroplast with one or more pyrenoids for carbohydrate storage. The cell wall consists of two halves, or semicells, with an inner cellulose layer and a stronger outer layer often adorned with spines, granules, or warts. This outer layer is impregnated with substances including iron compounds, making it insoluble in an ammoniacal solution of copper oxide. Desmids assume highly symmetrical shapes—elongated, star-shaped, or rotund—and the largest may be visible to the unaided eye. Characteristic crystals of barium sulphate at either end of the cell exhibit continuous Brownian motion, though their function remains unknown. Many desmids secrete translucent muc
- field
- Phycology (algal taxonomy and ecology)
- known_for
- Highly symmetrical, attractive microscopic algae; bioindicators of water quality
- number_of_genera
- Around 40
- number_of_species
- 5,000 to 6,000
- preferred_pH
- 4.8 to 7.0
- class
- Zygnematophyceae
Lore & Background
Desmids are unicellular, though some genera form filaments or non-filamentous colonies. Their cells are often divided into two symmetrical compartments separated by a narrow isthmus containing the nucleus, with each semi-cell housing a large chloroplast. The cell wall has two layers, the inner mainly cellulose, the outer stronger and often ornamented with spines, granules, or warts, and impregnated with iron compounds in some species. Desmids possess characteristic crystals of barium sulfate at each end of the cell, which exhibit continuous Brownian motion, though their function is unknown. Many desmids secrete gelatinous mucilage from pores in the cell wall, which acts as a protective agent; these pores are either uniformly distributed or grouped around ornamentations. Reproduction is most commonly by asexual fission, where the two halves of a cell separate and each develops into a new cell, sometimes resulting in asymmetry. Sexual reproduction occurs through conjugation, as in other Zygnematophyceae, but is rare and has never been observed in many species. Desmids are host to fungal parasites called chytrids and are grazed by microscopic aquatic heterotrophs such as crustaceans, rotifers, and ciliates. Classification recognizes three to five families within the order Desmidiales, though the family Mesotaeniaceae is now placed in the order Zygnematales based on cell wall structure and DNA sequences. Desmids are cosmopolitan but many species are restricted to specific continents or biogeographical realms, likely due to strict ecological requirements and the absence of resting spores, which limits dispersal.
Reader's Guide
Desmidiales, or desmids, are significant as a diverse group of microscopic green algae that serve as sensitive bioindicators of water quality, particularly in acidic, nutrient-poor freshwater habitats. Their strict ecological requirements—preferring low calcium, magnesium, and salinity, and a pH between 4.8 and 7.0—make them valuable for assessing habitat health. With around 40 genera and 5,000 to 6,000 species, desmids exhibit remarkable morphological diversity, from elongated to star-shaped and rotund forms, often highly symmetrical and attractive, which has made them popular subjects for both amateur and professional microscopists. Their cell structure, including barium sulfate crystals of unknown function and mucilage-secreting pores, adds to their biological interest. Ecologically, desmids are host to fungal parasites and grazed by microscopic heterotrophs, yet their interactions with the environment remain relatively unknown. The classification of desmids has been refined by DNA analysis, placing the former family Mesotaeniaceae in the order Zygnematales. Their global distribution, with many species restricted to specific biogeographical realms, underscores their role in understanding freshwater biodiversity and conservation.
Did You Know?
- Desmids possess characteristic crystals of barium sulfate at either end of the cell that exhibit continuous Brownian motion, though their function is completely unknown.
- The largest desmids may be visible to the unaided eye.
- Desmids are sensitive to environmental changes and are useful as bioindicators for water and habitat quality.
- Sexual reproduction in desmids is rare, and many species have never been observed reproducing sexually.
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