Azolla

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. Azolla nilotica Decne. ex Mett.Azolla pinnata R.Br. Azolla cristata Kaulf. (this name takes priority over Azolla caroliniana Willd.)Azolla filiculoides Lam. At least six extinct species are known from the fossil record: Azolla is a highly productive plant. It doubles its biomass in 3–10 days, depending on 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 filiculoides (red azolla) is the only member of this genus and of the family Azollaceae in Tasmania. It is a very common native aquatic plant in Tasmania. It is particularly common on farm dams and other still water bodies. The plants are small (usually only a few cm across) and float, but can be very abundant and form large mats. The plants are typically red, and have very 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, giving the plant access to the essential nutrient. This has led to the plant being dubbed a 'super-plant', as it 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, another essential mineral. An abundance of phosphorus, due for example to eutrophication or chemical runoff, often leads to Azolla blooms. Unlike all other known plants, the symbiotic microorganism is transferred directly from one generation to the next. This has made Anabaena azollae completely dependent on its host, as several of its genes are either lost or has been transferred to the nucleus in Azolla’s cells. The nitrogen-fixing capability of Azolla has led to Azolla being widely used as a biofertiliser, especially in parts of southeast Asia. Indeed, 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 inoculated with Azolla, which then quickly multiplies to cover the water, suppressing weeds. The rotting plant material releases nitrogen to 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'.