Spirogyra

Spirogyra is a genus of filamentous green algae in the family Zygnemataceae, within the Zygnematales. It is widely distributed in freshwater habitats, including ponds, ditches, and slow-moving streams, where it often forms bright green, ribbon-like filaments near the surface or on submerged vegetation.

Morphology and structure: Spirogyra filaments are uniseriate, composed of cylindrical cells arranged end to end. Each cell contains a single, large chloroplast that forms a distinctive spiral (helical) arrangement, giving the filaments a twisted appearance. The chloroplasts may include a series of lobes and a central nucleus. The cells secrete a mucilaginous envelope that facilitates gliding movement along surfaces. Unlike many algae, Spirogyra lacks flagellated cells at all life stages.

Reproduction and life cycle: Spirogyra can reproduce asexually by fragmentation, producing new filaments from broken pieces. Sexual reproduction occurs by conjugation between filaments of opposite mating types, typically in a scalariform (ladder-like) arrangement. Conjugation tubes form between adjacent cells, cytoplasm migrates from one cell to the other, and plasmogamy results in the formation of a thick-walled zygospore. The zygospore undergoes meiosis to produce haploid products, which germinate to form new filaments. The species generally exhibits a haplontic life cycle, with the diploid stage confined to the resting zygospore.

Ecology and significance: Spirogyra contributes to freshwater ecosystems as a primary producer and as a food source for small invertebrates. Its conspicuous green mats or films are common in nutrient-rich, sunlit waters. Spirogyra is frequently used in teaching laboratories to illustrate algal morphology and the sexual reproduction of zygnematalean algae. Occurrences can increase in eutrophic conditions, sometimes forming visible algal displays.