Azolla

• Azolla cariniana
• Azolla imbricata
• Azolla nilotica Decne. ex Mett.
• Azolla pinnata R.Br.

Drawing of Azolla filiculoides, about 5 mm. Upper green leaves perform photo synthesis, lower lack chlorophyll.

• Azolla cristata Kaulf. (this name takes priority over Azolla caroliniana Willd.)
• Azolla filiculoides Lam.
• Azolla rubra R.Br.

Sources:[4][5][6][7]

At least six extinct species are known from the fossil record:

• Azolla intertrappea Sahni & H.S. Rao, 1934 (Eocene, India)[8]
• Azolla berryi Brown, 1934 (Eocene, Green River Formation, Wyoming)[8]
• Azolla prisca Chandler & Reid, 1926 (Oligocene, London Clay, Isle of Wight)[8]
• Azolla tertiaria Berry, 1927 (Pliocene, Esmeralda Formation, Nevada)[8]
• Azolla primaeva (Penhallow) Arnold, 1955 (Eocene, Allenby Formation, British Columbia)[8]
• Azolla boliviensis Vajda & McLoughlin, 2005 (Maastrichtian - Paleocene, Eslaboacuten Formation and Flora Formation Bolivia)[9]

Azolla filiculoides (red azolla) is the only member of this genus and of the family Azollaceae in Tasmania. It is a common native aquatic plant in Tasmania. It is common behind farm dams and other still waterbodies. The plants are small (usually only a few cm across) and float, but can be abundant and form large mats. The plants are typically red, and have small, water repellent leaves.

Azolla floats 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 fixes atmospheric nitrogen. The plant can readily colonise areas of freshwater, and grow at great speed - doubling its biomass every two to three days. The typical limiting factor on its growth is phosphorus. An abundance of phosphorus, due for example to eutrophication or chemical runoff, often leads to Azolla blooms. Unlike all other known plants, its symbiotic microorganism transfers directly from one generation to the next. A, azollae is completely dependent on its host, as several of its genes have either been lost or transferred to the nucleus in Azolla's cells.[12]

SEM image of Azolla surface

The nitrogen-fixing capability of Azolla has led to widespread use as a biofertiliser, especially in parts of southeast Asia. The plant has been used to bolster agricultural productivity in China for over a thousand years. When rice paddies are flooded in the spring, they can be planted with Azolla, which then quickly multiplies to cover the water, suppressing weeds. The rotting plant material releases nitrogen into the water for the rice plants, providing up to nine tonnes of protein per hectare per year.[13]

Azolla are weeds in many parts of the world, entirely covering some bodies of water. The myth that no mosquito can penetrate the coating of fern to lay its eggs in the water gives the plant its common name "mosquito fern",[14] and may deter the survival of some of the larvae.

Most species can produce large amounts of deoxyanthocyanins in response to various stresses,[15] including bright sunlight and extreme temperatures,[16][17] causing the water surface to appear to be covered with an intensely red carpet. Herbivore feeding induces accumulation of deoxyanthocyanins and leads to a reduction in the proportion of polyunsaturated fatty acids in the fronds, thus lowering their palatability and nutritive value.[18]

Azolla cannot survive winters with prolonged freezing, so is often grown as an ornamental plant at high latitudes where it cannot establish itself firmly enough to become a weed. It is not tolerant of salinity; normal plants cannot survive in greater than 1–1.6‰, and even conditioned organisms die given salinity above 5.5‰.[19]

In addition to its traditional cultivation as a bio-fertilizer for wetland paddy, Azolla is finding increasing use for sustainable production of livestock feed.[22] Azolla is rich in protein, essential amino acids, vitamins and minerals. Studies describe feeding Azolla to dairy cattle, pigs, ducks, and chickens, with reported increases in milk production, weight of broiler chickens and egg production of layers, as compared to conventional feed. One FAO study describes how Azolla integrates into a tropical biomass agricultural system, reducing the need for inputs.[23] Concerns about biomagnification exist because the plant may contain the neurotoxin BMAA that remains present in the bodies of animals consuming it and BMAA has been documented as passing along the food chain.[24] Azolla may contain this substance that is a possible cause of neurodegenerative diseases.[25][26][27] Azolla has been suggested as a foodstuff for human consumption, however, no long-term studies of the safety of eating Azolla have been made on humans.[28] Previous studies attributed neurotoxin production to Anabaena flos-aquae species, which is also a type of nitrogen-fixing cyanobacteria.[29] Further research may be needed to ascertain if A. azollae produces neurotoxins.

Azolla has been used for at least one thousand years in rice paddies as a companion plant, to fix nitrogen and to block out light to prevent competition from other plants. Rice is planted when tall enough to poke through the Azolla layer. Mats of mature Azolla can also be used as a weed-suppressing mulch.

Rice farmers used Azolla as a rice biofertilizer 1500 years ago. The earliest known written record of this practice is in a book written by Jia Ssu Hsieh (Jia Si Xue) in 540 A.D on The Art of Feeding the People (Chih Min Tao Shu). By the end of the Ming dynasty in the early 17th century, Azolla’s use as a green compost was documented in local records.[30]

The myth that no mosquito can penetrate the coating of fern to lay its eggs in the water gives the plant its common name "mosquito fern".[14] Azolla have been used to control mosquito larvae in rice fields. The plant grows in a thick mat on the surface of the water, making it more difficult for the larvae to reach the surface to breathe, effectively choking the larvae.[31]