Spirogyra is a free-floating filamentous alga, of the order Zygnematales. It grows in fresh eutrophic water, water rich in nutrients. In high summer, it’s often seen tangled together in large slimy bright green masses that may be interspersed with bubbles of oxygen. More than 400 species of Spirogyra exist.
Spirogyra is pond scum. This eukaryotic organism is a protist that can photosynthesize, that is, an alga. Under magnification, its cylindrical cells can be seen joined end to end in long filaments. As an alga, spirogyra has chloroplasts, green organelles that make sugar from carbon dioxide and water using the sun’s energy.
In spirogyra, the chloroplasts are arranged in spiral ribbons that seem to curl up the filaments of the colony like the stripes of a green and transparent candy cane. Along the ribbons of the chloroplasts are small round nodules. These are the pyrenoids, centers for making starch. The striped filaments may be 10 to 100 microns wide and sometimes many centimeters long.
Filaments of spirogyra grow by cell division. The filaments fragment, and their component cells split almost as an amoeba would, producing two new cells. This is mitosis. However, spirogyra does also have sex.
In spirogyra, sexual reproduction is conjugation. It comes in two forms. In one, adjacent cells of a filament unite to reproduce. Two cells build papillae, conjugation tubes, out from their sides. Where the papillae meet, the wall between them dissolves, and the male cytoplasm migrates through the canal to join with the genetic material of the other cell. The female cell is the only partner to contribute genetic material to the chloroplasts, as in higher plants. This is called lateral conjugation.
In scalariform conjugation, two separate filaments of spirogyra align side by side. They then grow out papillae, conjugation tubes, from their sides, and the process proceeds as in lateral reproduction.
Generally speaking, reproduction by fragmentation takes place in lush times. Spirogyra filaments grow until they break, and then the fragments grow. Small sub-surface numbers of spirogyra quickly grow into large floating mats.
Conjugation prepares for tougher times, as ephemeral ponds dry up, or as winter approaches.
The result of conjugation is a zygote that is tough and hardy. Inside a thick cell wall, it can survive a cold winter on the bottom of a pond or watercourse, to split and begin a new filament in the spring. In some species, the spirogyra zygote survives when seasonal ponds dry up, to revive when rains renew the pond.
Spirogyra is an alga found in fresh water throughout the world. It is remarkable for the spiral stripes of its filaments, and for its varied forms of reproduction.