The Zygnematales comprise 18 genera, with more than more than 600 species. Spirogyra and Zygnema are the best-known members of the order, and because of the beauty of these organisms when seen under a microscope, they are commonly used in biology classes as examples of the "green algae". However, the Zygnematales are not representative of the green algae in general, and have many unusual features which suggest a closer relationship to the plants than to the other green algae.
Almost all species of Zygnematales live in freshwater environments, where they are important as primary producers. They are commonly called "pondscum", and make up the filamentous periphyton in ponds, growing on and around the larger aquatic plants.
Mature zygnemataleans are filamentous and unbranched, growing as long chains of cells connected end-to-end. The filament grows when cells in the chain divide, developing into two new full-sized cells. Some species will produce rhizoids at one end of the filament to help anchor them to objects in the water. Each cell is encased in a cell wall composed of three layers, two cellulosic inner layers and and outer layer of mucus, which probably helps to keep the alga moist.
Like all organisms called "algae", zygnemataleans have plastids which allow them to manufacture their own food. There is a great deal of variation in chloroplast morphology in the group. They are stellate in Zygnema, while in Spirogyra the chloroplasts are ribbonlike and are wound around the inside of the cell like a spiral, giving the genus its name. The photo at left shows this in Spirogyra submaxima. The "bumps" visisble along the chloroplast are pyrenoids used to store starch reserves.
Members of the order may reproduce in one of two ways, asexually by fragmentation, or sexually by conjugation. Conjugation is a very distinctive mode of reproduction which only occurs among algae in the Zygnematales and Desmidiales. The process begins with a parallel pairing of two filaments, aligning neighboring cells. Then, a bridge-like conjugation tube is formed to connect neighboring cells, and the amoeboid cells from one filament move over into the neighboring filament. Alternatively in some species, cells from both filaments will move into the cojugation tube. In either case, the cells meet and fuse, producing a diploid zygote which forms a spore. The zygote will undergo meiosis directly to produce the next generation. Mature algae are therefore haploid, though polyploidy is known in some taxa, resulting in size variation. Like plants, zygnemataleans have uniparental inheritance of plastids; the plastids of only one gamete are passed on to the next generation.
Fossils of the group are mostly Cenozoic, though some Permian and Carboniferous fossils are also known. Zygospores are the most common fossils found; filaments appear to be much more fragile, and are only known from the Eocene. Fossil zygospores have been used to infer paleoecology of the regions in which they occur, suggesting that shallow freshwater pools or lakes were present in the past. Perhaps the best-known in the fossil record are the keeled spores of the genus Debarya (called Peltacystia when it occurs as fossil spores), though it is a rare alga today.
Photo of Spirogyra submaximacourtesy Richard M. McCourt.