Azolla
A tiny aquatic fern that once cooled the planet.
Azolla—known as mosquito fern, water fern, or fairy moss—is a genus of seven aquatic fern species in the family Salviniaceae. These plants look nothing like typical ferns; they are extremely reduced and specialized, more closely resembling mosses or duckweeds. One species, *Azolla filiculoides*, is one of only two ferns with a published reference genome. During the Eocene epoch, this genus is thought to have grown so abundantly that it pulled vast amounts of carbon from the atmosphere, triggering a global cooling event that persists today.
Where it is not native, Azolla can become invasive. It alters aquatic ecosystems by covering the water’s surface, blocking sunlight from underwater plants, and depleting oxygen, which reduces biodiversity.
**Phylogeny** The genus includes species such as *A. mexicana*. At least six extinct species are known from fossils: *A. intertrappea* (Eocene, India), *A. berryi* (Eocene, Wyoming), *A. prisca* (Oligocene, Isle of Wight), *A. tertiaria* (Pliocene, Nevada), *A. primaeva* (Eocene, British Columbia), and *A. boliviensis* (Maastrichtian–Paleocene, Bolivia).
**Ecology** Azolla is highly productive, doubling its biomass in as little as 1.9 days under good conditions. In Asian rice paddies, it can yield 8–10 tonnes of fresh matter per hectare; in India, *A. pinnata* has been reported at 37.8 tonnes fresh weight per hectare (2.78 tonnes dry weight). The plant floats on water using small, overlapping scale-like leaves, with roots dangling below. It forms a symbiotic relationship with the cyanobacterium *Anabaena azollae*, which lives outside the plant’s cells and fixes nitrogen. Growth is typically limited by phosphorus, so runoff or eutrophication can trigger blooms. Uniquely, this microbe is passed directly from one generation of Azolla to the next, and it is completely dependent on the fern—several of its genes have been lost or transferred to the Azolla nucleus.
Because of its nitrogen-fixing ability, Azolla is widely used as a biofertilizer, especially in Southeast Asia. In China, it has boosted agricultural productivity for over a thousand years. When rice paddies are flooded in spring, Azolla is planted; it rapidly covers the water, suppressing weeds. When it dies and rots, it releases nitrogen into the water, providing up to nine tonnes of protein per hectare per year.
Quick Facts
- Taxon
- Azolla
- Type Species
- Azolla filiculoides
- Type Species Authority
- Lam.
Facts from the source article.
Lore & Background
Azolla floats on the surface of water by means of numerous small, closely overlapping scale-like leaves, with its roots hanging in the water. It forms a symbiotic relationship with the cyanobacterium Anabaena azollae, which lives outside the cells of its host and fixes atmospheric nitrogen. Unlike all other known plants, this symbiotic microorganism transfers directly from one generation to the next. 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. 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‰.
Reader's Guide
Azolla is notable for its nitrogen-fixing capability, which 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. Azolla is also finding increasing use for sustainable production of livestock feed, being 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. However, concerns exist because the plant may contain the neurotoxin β-methylamino-L-alanine (BMAA), which has been documented as passing along the food chain and is a possible cause of neurodegenerative diseases. Studies published in 2024 have found that the Azolla–Nostoc azollae superorganism does not contain BMAA or their isomers, but further research may be needed to ascertain whether A. azollae is a healthy foodstuff for humans. Azolla may also establish as an invasive plant in areas where it is not native, altering aquatic ecosystems and biodiversity by exhausting oxygen and covering the water surface.
Did You Know?
- The genus is believed to have triggered a global cooling event during the Eocene by absorbing massive amounts of carbon.
- Azolla filiculoides is one of only two fern species for which a reference genome has been published.
- The plant's symbiotic cyanobacterium, Anabaena azollae, transfers directly from one generation to the next, unlike any other known plant.
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