From afar some similarities to :
Floating Moss, Algae or some
Uniquely identifiable characteristics : It floats on still fresh waters forming an extensive and 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 it kills fish. Water Fern 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 because it is in a mutually beneficial symbiotic relationship with a cyanobacterium (aka a blue-algae) called Anabaena Azollae which fixes essential nitrogen from the air for it, bestowing it with a phenomenal growth rate. The only other plants able to fix atmospheric nitrogen are those from the Pea family, but this is a fern. The cyanobacteria are in bright green filamentous strands within Azolla.
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 algae and an aquatic fern.
It needs phosphorus fertiliser 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.
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 Azolla 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 with great success; the weevil is able to control Water Fern completely. The Weevils are most active around the summer months when Azolla proliferates and they breed quickly, rapidly eating their way through the Water Fern. The weevils are known to feed exclusively on Water Fern so are not a pest on other species of plants.
Water Fern can be successfully eliminated in just a few weeks by biological control methods using the native weevil Stenopelmus rufinasus. The CABI (Centre for Agriculture and Biosciences International) will be able to supply these weevils from stock for the control of Water Fern. There seems to be no other residual effects of the weevils after they have performed their task.
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. Gaseous analysis of rapidly growing Water Fern reveals this gas to have a similar nitrogen to oxygen ratio as that of air.
The Dye Cyanidin (E-163) can be extracted from this plant.
SYMBIOTIC RELATIONSHIP & NITROGEN FIXATION
Azolla species form symbiotic relationships with the filamentous nitrogen-fixing cyanobacteriums Anabaena azollae and Nostoc punctiforme. Cyanobacteria were better known previously as blue-green algae and the name persists in popular culture. Only one species of Azolla is extant in the UK, Water Fern, and it is a recent and mostly un-welcome arrival. Because it is able to capture and fix its own nitrogenous fertiliser with the help of the cyanobacteria, it grows at an astonishing rate, doubling in bio-mass every 2-3 days, the only limit being the availability of another essential mineral, phosphorus. It is thus dubbed a 'super-plant'.
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.
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 fertiliser when harvested from the water; it has for centuries been used as a green manure in its native countries.
Azolla will not grow in sea-water nor in brackish water - it dislikes salt - any concentration above 10mM NaCl starts to adversely affect the growth; above 40mM and it is killed. (The concentration of salt in sea-water is 500mM - far above the toxic levels). However, lacing waters with salt is not a method of control for this pernicious weed!
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 microcosm, 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.
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 clump together to form a continuous carpet on the water resembling floating moss.
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 the 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, see box below on 3-deoxy-anthocyanidins.
The nitrogen, N2, from the air may be stored within the cyanobacteria by granules of Cyanophycin (a co-polymer of two amino acids Arginine and Aspartic Acid and which is found within most cyanobacteria). Cyanophycin is an NPAA (Non-proteinogenic Amino 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 (water-soluble compounds of nitrogen and not the (almost-inert) gas N2).
Another NPAA produced by cyanobacteria within Azolla fern is the neurotoxin BMAA (aka β-MethylAmine-L-Alanine). This non-proteinogenic amino acid is also produced in the leaf petioles of Giant Rhubarb (Gunnera), in certain lichens possessing certain cyanobacterial symbionts, and in Cycad trees. The neurotoxin, BMAA, causes degenerative loco-motor diseases which resembles symptoms expressed by diseases such as Alzheimer's, Parkinson's, Huntingdon's and Amyotrophic lateral sclerosis (ALS) which is displayed by those folk who consume Cycad seeds (the Chamorro population of Guam; a territory of the United States). It has been been proved (at least in monkeys) that BMAA can induce amyloid plaques and tau-tangles within the brain which are associated with Alzheimer's disease. It is also now known that the Amino Acid
Serine can help protect the brain from these tau tangles, and sure enough when Serine was co-administered with BMAA the resulting brain cell death was reduced. With this new found knowledge a human trial is about to begin in order to test the ability of Serine to reduce or stop the build up of amyloid plaques within the brains of Alzheimer's sufferers.
A HEAVY-METAL HYPER-ACCUMULATOR
Water Fern also has the ability to tolerate normally toxic metals in the water, in fact it not only tolerates them but hyperaccumulates them. It is thus a Hyperaccumulator and valuable Metallophyte for the
phytoremediation of water, able to mop up from the water a variety of differing metals including aluminium, arsenic, chromium, copper, iron, lead, manganese, nickel and zinc.
Note that to remove the metals entirely, the Water Fern then has to be harvested from the water and disposed of safely elsewhere. The whole cycle has to be repeated over several seasons in order to bring heavy metal contamination down to acceptable levels, and it may never totally eliminate the problem; it would depend if the metals were continuously leaching from the mud beneath (when a secular equilibrium would be reached).
3-deoxyanthocyanidins and their glycosides are deeply coloured flavonoids, similar to the flavonols with similar names. Thus the 3-deoxy-anthocyanidin Apigenidin has Apigenin as its flavonol counterpart whilst Luteolinidin partners the flavonol Luteolin. The Anthocyanidins normally have an -OH group at position  (shown in red in the structures below), but the 3-deoxy-anthocyanidins lack this, replacing it with just -H. Most 3-deoxy-anthocyanidins are yellow apart from the rarer Tricetinidin which is red. They are found primarily in ferns and mosses, such as the Sphagnum mosses which have a wide colour repertoire. The colours of 3-deoxy-anthocyanidins are said to be insensitive to changes in pH, especially if a carboxylate group is attached at position  (this being the second carbon atom anticlocwise from  and just before ).
Sorghum (Sorghum bicolor) also contains colourful 3-deoxy-anthocyanidins.
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 brightly-coloured
3-deoxyanthocyanidins, which it produces in copious amounts in response to stresses such as bright sunlight or elevated temperatures; a protective mechanism.
The presence of cadmium in the water (a toxic heavy metal pollutant) also induces 3-deoxyanthocyanidin production and accumulation within Water Fern. The 3-deoxyanthocyanidins it produces are the yellow
Luteolinidin and their 5-O-glycosides (attached at the  position) and variously coloured yellow or orange hydrolysis products including acetyl-derivatives of these glycosides. Luteolinidin would be Cyanidin if it possessed the missing -OH group at position .
Water Fern is also said to produce the reddish
Tricetinidin, another 3-deoxyanthocyanidin. Colourful complexes of 3-deoxyanthocyanin glycosides with aluminium have also been experimentally observed.
The 3-deoxyanthocyanidins are Phytoalexins, compounds possessing antimicrobial and antioxidant properties that are synthesized in response to stress or infection by a pathogen.