Azolla is a type of small aquatic fern that lives in symbiosis with microscopic blue-green algae called anabaena azollae. Found in tropical and temperate regions, these ferns float on the top of ponds or streams, clumping together to form green mats across the surface. Azolla is a genus which encompasses seven different species in the Salviria family, although some scientists place it in its own family, Azollaceae. The species filiculoides, mexicana, and caroliniana are found in the United States.
Commonly called mosquito fern or fairy moss, azolla is also sometimes called duckweed fern because of its penchant for growing near duckweed, a small aquatic flowering plant. The fern basically looks like floating moss at first glance. This fern does, however, have tiny overlapping bi-lobed leaves that seem almost scale-like and are attached to stems. Only the upper parts of the leaves contain chlorophyll, so only the tops are green. The roots of the azolla are thin and dangle underneath the foliage, directly in the water.
Azolla can reproduce asexually or sexually, by fragmentation or by spores. Though most ferns produce spores, the azolla is unusual because it produces two kinds of spores, male and female. The spores are first stored in structures on the underside of the leaves called sporocarps. Male sporocarps are larger, containing multiple spores rather than the single spore carried in the female sporocarps. The female spores are actually larger than the males, however, and are termed megaspores.
The algae, or cyanobacteria, anabaena lives in the ovoid cavities of the leaves of the azollas. This algae feeds on nitrogen in a process called nitrogen fixation. This type of symbiotic relationship is more often seen with legumes that have bacteria or algae living in the roots, creating lichen.
Azolla is extremely useful in China and other parts of Asia as nitrogen fertilizer for rice paddies. Called a "green manure," azolla is said to increase rice production by as much as 158 percent. It also allows the continued planting of crops in the same fields year after year, without the need to let a field rest.
In addition to fertilizer, azolla is also used in fish food and garden mulch and is naturally food for various types of water fowl, insects, and crustaceans. It was used to help purify water in the Biosphere II project, which simulated outer space greenhouse conditions. Azolla is also good for control of mosquitoes because its thick mat prevents the larvae from breaking through the water's surface.