Azolla

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Mosquito fern

Azolla caroliniana

Scientific classification

Kingdom:	Plantae
Division:	Pteridophyta
Class:	Pteridopsida
Order:	Salviniales
Family:	Azollaceae Wettst.
Genus:	Azolla Lam.

Species

Azolla caroliniana Willd.
Azolla filiculoides Lam.
Azolla japonica Franch. & Sav.
Azolla mexicana Presl
Azolla microphylla Kaulf.
Azolla nilotica Decne. ex Mett.
Azolla pinnata R.Br.

Azolla (mosquito fern, duckweed fern, fairy moss, water fern) is a genus of seven species of aquatic ferns, the only genus in the family Azollaceae. They are extremely reduced in form and specialized, looking nothing like conventional ferns but more resembling duckweed or some mosses.

They float on the surface of water by means of numerous, small, closely-overlapping scale-like leaves, with their roots hanging in the water. They form a symbiotic relationship with the cyanobacterium Anabaena azollae, which allows the plants to fix nitrogen from the air.

SEM image of Azolla surface

Because of their nitrogen-fixing capability, they have enabled an increase in agricultural productivity in parts of southeast Asia. When rice paddies are flooded in the spring, they can be inoculated with Azolla, which then quickly multiplies to cover the water, suppressing weeds. As the plants die, they contribute nitrogen to the rice plants, and as the rice paddy dries out, the Azolla all eventually die, making an exceptional green manure, providing up to nine tonnes of protein per hectare per year FAO figures. As such, Azolla may be used in place of agrochemicals.

Azolla are also serious weeds in many parts of the world, covering bodies of water so thickly that no water is exposed. This is where they derive the name 'mosquito fern', from the belief that no mosquito can penetrate the coating of fern to lay its eggs in the water^{[citation needed]}. Azolla is reputed to be able to grow so quickly that it can double its mass in three days under good conditions.

Most of the species can produce large amounts of anthocyanins in bright sunlight, creating an intense red color and causing the water surface to appear to be covered with a red carpet.

Mosquito ferns are safe to grow in cool temperate areas with prolonged freezing in winter, as they cannot overwinter in these conditions. They are often grown as an ornamental plant by water-garden hobbyists.

[edit] Reproduction

Azolla on the Canning River, Western Australia

Azolla can reproduce asexualy by breaking off. Each branch that breaks off forms a new plant.

Azolla can also reproduce sexually. Like all ferns, Azolla produce spores. Unlike most ferns, Azolla produces two kinds of spores. During the summer months, numerous spherical structures called sporocarps form on the undersides of the branches. The male sporocarp is greenish or reddish and looks like the egg mass of an insect or spider. It is two millimeters in diameter, and inside are numerous male sporangia. Male spores (microspores) are extremely small and are produced inside each microsporangium. One very curious thing about microspores is that they tend to stick together in little clumps or masses called massulae.

Female sporocarps are much smaller, and only contain one sporangium and one functional spore. Since an individual female spore is considerably larger than a male spore, it is termed a megaspore.

Azolla has microscopic male and female gametophytes that develop inside the male and female spores. The female gametophyte protrudes from the megaspore and bears one to several archegonia, each containing a single egg. The microspore forms a male gametophyte with a single antheridium which produces eight swimming sperm (Scagel et al. 1966). The barbed glochidia on the male spore clusters presumably cause them to cling to the female megaspores, thus facilitating fertilization.

[edit] Climatic paleontology

Azolla covering the Canning River

A study of arctic climatology reported that azolla may have had a significant role in reversing a greenhouse effect that occurred 55 million years ago that caused the region around the north pole to turn into a hot tropical environment. This research conducted by the Institute of Environmental Biology at Utrecht University claims that large dense patches of azolla growing around freshwater lakes formed by the climate change eventually consumed enough carbon dioxide for the greenhouse effect to reverse.