Filamentous pond algae
Kingdom Protista
Phylum Chlorophyta
Not to be confused with the jazz-fusion band Spyro Gyra (definitely not pond scum), genus Spirogyra is a free-floating multicellular alga. It is found mostly in freshwater ditches, ponds and puddles (although some species do thrive in streams). It is filamentous, forming unbranching strings which may gather together at the water's surface to form dense mats (also known as Pond Silk, Mermaid's Tresses and pond scum). Each cell has a spiral of ribbonlike chloroplasts that extend along its length, making the plant a beautiful thing to behold (although you really need a microscope for best results).

The genus comprises about 400 species spread over all continents except Antarctica, all of which reproduce both asexually (by cell fission) and sexually (by exchanging genetic material). Each cell is covered in sticky mucilage which protects it to some extent against dessication in dry weather. The filaments vary in thickness between species (10-100 µm) - some are barely visible to the naked eye, others are large enough that the chloroplasts may be seen with a magnifying lens.

Sexual reproduction takes place during the warmer weather; two nearby filaments begin to grow lateral extensions known as conjugation tubes, and the adjoining cells fuse temporarily, to allow one cell to transfer its genetic contents into the other. A zygospore forms in the receiving cell, formed from both cells' genetic matter. This spore is capable of surviving quite harsh conditions, (it is covered with sporopollenin, a highly resistant biopolymer) such as severe drought or freezing temperatures. Once favourable conditions are restored, the spore undergoes meiosis to form new spirogyra chains.

During the sexual reproductive phase, the algal mat is more likely to be found at the surface of the water, and the mat may have a brown tinge due to the presence of the zygospores. Where nutrient levels are high, these mats may become so dense as to block sunlight from lower reaches of a pond, which can create quite a nuisance to owners of fishing or ornamental waters. They can also form in freshwater aquaria, and whilst chemical control is possible, prevention is better than cure, but lowering the levels of artificial fertilizers.

As a further point of historical interest, this was the first alga to be examined using Leeuwenhock's microscope, in 1670.

Encyclopædia Britannica