Azolla is a genus of seven species of aquatic ferns in the family Salviniaceae (order Salviniales), within the class Polypodiopsida. Despite being true ferns, Azolla species are so reduced in form and so highly specialized that they superficially resemble mosses or duckweeds rather than conventional ferns. They float freely on the surface of still or slow-moving water, supported by numerous tiny, closely overlapping scale-like leaves, with their roots trailing in the water below.
What distinguishes Azolla from virtually all other plants is its intimate symbiosis with the nitrogen-fixing cyanobacterium Anabaena azollae (now often placed in Trichormus). This microorganism lives in cavities within the fern's leaves and fixes atmospheric nitrogen, enabling Azolla to thrive in nutrient-poor waters. Uniquely among known plants, the symbiont is transmitted directly from one plant generation to the next via spores — and A. azollae has become so dependent on its host that several of its genes have been lost or transferred to Azolla's own cell nucleus.
Azolla is extraordinarily productive: under favourable conditions it can double its biomass in under two days, and yields in Asian paddy fields can reach 8–10 tonnes of fresh matter per hectare. This productivity, combined with its nitrogen-fixing ability, has made it a valued biofertiliser across Southeast Asia for more than a thousand years, particularly in China and India, where it is planted in flooded rice paddies to suppress weeds and enrich the soil.
The genus is also notable in deep time: it is hypothesised that Azolla grew so prolifically during the Eocene epoch that its mass uptake of carbon dioxide contributed to a significant global cooling event — the so-called "Azolla event" — whose effects may persist to the present day. Azolla filiculoides is one of only two fern species for which a full reference genome has been sequenced and published.
Outside its native range, Azolla can become highly invasive, forming dense mats up to 30 cm thick that block light, deplete oxygen, and displace native aquatic vegetation and wildlife. A. filiculoides is considered especially problematic and has naturalised in the United Kingdom and many other regions.
Etymology
The name Azolla is derived from the Greek azo (to dry) and ollyo (to kill), referring to the plant's inability to survive desiccation — it dies quickly when removed from water.
Distribution
Azolla species occur naturally across tropical, subtropical and warm-temperate regions worldwide. Several species have been widely introduced beyond their native ranges and have become invasive, most notably A. filiculoides in the United Kingdom, Western Europe, South Africa and parts of Australasia, where it can blanket entire water bodies and disrupt native aquatic ecosystems.
Ecology
Azolla floats on the water surface and forms a unique obligate symbiosis with the cyanobacterium Anabaena azollae, which fixes atmospheric nitrogen directly inside the fern's leaf cavities. Growth is typically limited by phosphorus availability, and nutrient enrichment from eutrophication or agricultural runoff can trigger dense blooms. Most species produce red deoxyanthocyanin pigments under stress (intense light, temperature extremes or herbivory), turning entire water surfaces a vivid red. Azolla is intolerant of salinity above about 1–1.6‰ and cannot survive prolonged freezing, except for A. filiculoides which endures temperatures as low as −22 °C.
Cultural Uses
Azolla has been cultivated as a biofertiliser in Chinese rice paddies for over a thousand years. When flooded paddies are seeded with Azolla in spring, it rapidly multiplies to cover the water surface, suppressing weed growth. As it dies back it releases fixed nitrogen into the water, supplying rice plants with the equivalent of up to nine tonnes of protein per hectare per year. Its use as a green manure is widespread across Southeast and East Asia, and it has attracted modern interest as a sustainable, low-input nitrogen source for organic farming.
History
During the Eocene epoch (roughly 56–34 million years ago), Azolla is hypothesised to have grown prolifically across the Arctic Ocean when it was a warm, stratified freshwater sea. The resulting massive drawdown of atmospheric CO₂ — known as the Azolla event — is proposed as a contributing factor to the long-term global cooling that followed and continues today. More recently, the full genome of A. filiculoides was sequenced and published, making it one of the first ferns to have its reference genome completed and providing insights into the evolution of its nitrogen-fixing symbiosis.
