From afar some similarities to : Floating Moss, Algae or some Duckweeds.

Uniquely identifiable characteristics : Floats on still fresh waters forming an extensive invasive green or reddish mat and from close-up resembles miniature weather cones.

Water Fern (Azolla Feliculoides) appears similar to Duckweed in that it floats and carpets still freshwater to such an extent that oxygen has no chance to diffuse into the water from the air, and as a result kills fish. It is a controlled weed that is gaining increased prominence in the British Isles from garden pond escapees. It may often grow with Duckweeds of various kinds and or Wollfia.

It grows in spring with remarkable speed, doubling in mass in just a few days. This remarkable ability is bestowed upon it by it being in a mutually beneficial symbiotic relationship with a cyanobacterium (or blue-algae) called Anabaena Azollae which fixes nitrogen for it enabling the phenomenal growth rate. The cyanobacteria are in bright green filamentous strands.

Symbiotic relationships between cyanobacteria (algae) and fungi are well known, the result being called a lichen, but this is the only known symbiotic relationship between an aquatic fern and algae.

Azollas are able to fix nitrogen, unlike a great many other plants (except the Pea Family which can also fix nitrogen). It needs phosphorus fertilizer in order to grow, which is usually supplied in ample needs by run-off from farm-land. It dies back in low temperatures in Winter, but may survive by means of submerged buds. The roots dangle free into the depths of the shallow waters.

Each individual plant of Azolla Feliculoides consists of a short branched stem with minute angular overlapping leaves which are at first bright green. The plants have pendulous roots that dangle in the water whilst the plant floats on water. The plants clump together to form a continuous carpet on the water resembling floating moss. The angular leaves are bright green at first but exposed to strong sunlight or heat-stress develop a pinkish, reddish, orangish or purplish coloration at the edges due to deoxyanthocyanins. The stems of the plant readily pull apart which enables the plant to multiply at astonishing speed. It is nutritious and provides a good source of food for ducks and other pond-life like snails or worms, but is deleterious to fish on account of it preventing the diffusion of oxygen into the water, but it presumably makes some itself during photosynthesis.

Dredged or removed mechanically from shallow waters it can be used as a source of compost rich in available nitrogen and phosphorus. In 2010 on the Bridgewater and Taunton Canal in Somerset British Waterways are trialling the introduction of weevils (short 2mm long beetles) as a weapon against this persistent North American fern.

As a fern, or to be more precise, a Water-Fern, it has no flowers as such. Water-Fern is the only member of the Genus Azolla (at least in the UK).

It floats on water because it contains tiny gas bubbles within cavities in the leaves. On analysis of rapidly growing Water Fern, the gas turns out to be of similar nitrogen to oxygen proportions to that of air.

SYMBIOTIC RELATIONSHIP & NITROGEN FIXATION Azolla species are in a symbiotic relationship with the nitrogen-fixing cyanobacterium Anabaena. Only one species of Azolla is extant in the UK, Water Fern, a recent and mostly un-welcone arrival. Water-Fern is never without its accompanying bacteria, which grows in the mucilaginous material between two envelopes, an internal and an external one. The bacterium uses the enzyme nitrogenase to fix the nitrogen from the air, which are beneficial to the growth of Water Fern. Because Water Fern fixes nitrogen it has the ability to grow extremely fast, with the availability of phosphorus as phosphate being the limiting factor to its growth rate. When phosphorus is freely available to the plant in sufficient concentration (above 0.4mg/litre), a massive bloom results, with Water-Fern taking over the whole surface of the water in a matter of days. The extra nitrogen and phosphorus that the Water Fern has accumulated makes it very useful as a fertilizer when harvested from the water; it has for centuries been used as a green manure in its native countries. Azolla dislikes salt, and any concentration above 10mM NaCl starts to affect the growth; above 40mM and it kills it. However, this is not a method of control! It appears that the nitrogen fixation occurs in the hairs, of which there are three types. More recent data suggests that many different cyanobacteria occur within the leaf cavity - it possesses its own natural mini-ecosystem as a micro-cosm, within which natural selection occurs choosing the appropriate mix of cyanobacteria for the conditions it meets. It is thus very successful at colonising still fresh water. Water-Fern can develop in one of two ways; the most frequent being the free-floating variety on fresh-water, with short roots dangling down into the water. But, on rarer occasions, if the pond or slow-running stream runs dry, it can take to the surface of the damp earth with a root system taking hold in the soil. In these situations it turns reddish very quickly; a result of a stressful situation related to temperature, light intensity and lack of phosphorus. The redness is due to anthocyanin production. The nitrogen, N2, from the air may be stored within the cyanobacteria by granules of Cyanophycin (a co-polymer of Arginine and Aspartic Acid). It is then converted into Ammonia NH3 by the enzyme nitrogenase. Oxygen is poisonous to the non-photosynthetic cyanobacteria; it inhibits nitrogenase from fixing nitrogen gas; so must be prevented from getting into the cyanobacteria. The Water Fern benefits from its host bacterial partner by having a ready supply of available nitrogen (nitrogen that is not N2). The Dye Cyanidin (E-163) can be extracted from this plant.

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