Azolla (mosquito fern, duckweed fern, fairy moss, water fern) is a genus of seven species of aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, looking nothing like other typical ferns but more resembling duckweed or some mosses. Azolla filiculoides is one of just two fern species for which a reference genome has been published. It is believed that this genus grew so prolifically during the Eocene (and thus absorbed such a large amount of carbon) that it triggered a global cooling event that has lasted to the present.
Azolla is considered an invasive plant in wetlands, freshwater lakes and ditches . It can alter aquatic ecosystems and biodiversity substantially.
Phylogeny
Phylogeny of Azolla
Other species include:
At least six extinct species are known from the fossil record: Azolla intertrappea Sahni & H.S. Rao, 1934 (Eocene, India) Azolla berryi Brown, 1934 (Eocene, Green River Formation, Wyoming) Azolla prisca Chandler & Reid, 1926 (Oligocene, London Clay, Isle of Wight) Azolla tertiaria Berry, 1927 (Pliocene, Esmeralda Formation, Nevada) Azolla primaeva (Penhallow) Arnold, 1955 (Eocene, Allenby Formation, British Columbia) Azolla boliviensis Vajda & McLoughlin, 2005 (Maastrichtian - Paleocene, Eslaboacuten Formation and Flora Formation Bolivia)
Ecology
Azolla is a highly productive plant. It can double its biomass in as little as 1.9 days, depending on growing conditions, and yield can reach 8–10 tonnes fresh matter/ha in Asian rice fields. 37.8 t fresh weight/ha (2.78 t DM/ha dry weight) has been reported for Azolla pinnata in India (Hasan et al., 2009).
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, an extracellular endosymbiont (living outside the host's cells) which fixes atmospheric nitrogen. The typical limiting factor on its growth is phosphorus; thus, 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.
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 resulting from the die off of this Azolla releases nitrogen into the water for the rice plants, providing up to nine tonnes of protein per hectare per year.
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", and may deter the survival of some of the larvae.
Most species can produce large amounts of deoxyanthocyanins in response to various stresses, including bright sunlight and extreme temperatures, 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.
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 also not tolerant of salinity; normal plants cannot survive in greater than 1–1.6‰, and even conditioned organisms die if grown in water with a salinity above 5.5‰.
Azolla filiculoides
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 they are fast growing, and can be abundant and form large mats. The plants are typically red, and have small, water repellent leaves.
Reproduction
Azolla reproduces sexually, and asexually (by splitting).
