Desmidiales

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.