Invasive species (X): Azolla, the amazing water fern

Azolla is a rare-looking genus of ferns . These plants, unlike common ferns, are tiny and grow in floating colonies on bodies of water.

The genus has several described species, but so far only three have been accepted, due to the difficulty of taxonomically classifying these plants. One of these species is Azolla filiculoides, commonly called a water fern or mosquito fern.

The plant is native to temperate regions of America. Each individual is 2.5-10cm in size, with triangular leaves that allow it to float on water. Its overall appearance is fluffy and it is, frankly speaking, a pretty plant. For this reason its use in aquariums is popular and it is because of this that its accidental introduction occurs outside its native area.

Azolla, the water fern as an invasive species

The problem associated with Azolla filiculoides is that it completely covers the masses of water where it is introduced, forming a floating carpet. This is disastrous for the environment, since it prevents the passage of light into the water column, which alters the composition of the phytoplankton (microscopic algae that carry out photosynthesis, and therefore require light) and consequently the entire ecological structure of rivers and lakes .

In addition, the growth of populations is extremely fast. Cases have been described of populations that double their biomass in just three days if the light and nutrient conditions allow it. Growth stops once the plant has completely covered the water surface.

The reason the plant is called a mosquito fern is that, it is said, the density of the carpet it forms is so great that it does not allow mosquitoes to lay their eggs in the water.

Eliminating such a plant is by no means an easy task, since it reproduces asexually from fragments or sexually thanks to spores. This means that even if the entire surface layer of a river invaded by Azolla was removed , the population could survive thanks to small fragments and regenerate over time. On the other hand, the spores adhere to aquatic birds or fish, which allows them to be transported long distances.

Due to its rapid growth and the impacts it causes on the environment, it is classified as an aggressive invasive species in many countries. This is the case in Spain, where the introduction of the species into the natural environment, as well as its possession, trade or transport, is prohibited. This is because, as mentioned before, many of the introductions into the medium occur accidentally.

 The Azolla event

Bioclimatic studies have revealed a global cooling that occurred 49 million years ago, during the Eocene, thanks to the increase in Azolla populations in what is now the Arctic Ocean.

At that time that portion of the sea was a body of fresh water, compatible with the development of aquatic plants. The plants covered an area of ​​about 4 million square kilometers of sea and the population survived for almost a million years.

During this period the ferns fixed a large amount of carbon dioxide , which is a greenhouse gas and causes global warming. Later the ocean became salty and the ferns died en masse, releasing part of the accumulated carbon and warming the planet again.

However, the global temperature after the event was lower than the temperature before, so Azolla contributed to lasting climate change . Although it was not the only factor that intervened, it is accepted that the fern population was the most important factor.

It was at this time, in addition, that the ice masses at the poles that exist today were formed. Thus, a tiny fern that today is an ecological problem in some countries helped to establish the current climate.

 The benefits of the water fern

One use that has been proposed for this species is bioremediation . That is, using living beings to deal with the problem of pollution produced by humans. The populations of Azolla are capable of removing water from the metals as chromium, nickel, copper, zinc or lead . Once the contaminants are removed, the plants are harvested, dried and treated as solid waste, much more manageable than liquid waste.

On the other hand, Azolla makes symbiotic associations with the cyanobacterium Anabaena azollae . Cyanobacteria are bacteria with nitrogen fixation capacity. Nitrogen is an element of great importance for living beings, as it is the molecular basis of proteins.

Plants do not have the ability to directly take advantage of the nitrogen present in the air (which makes up 78% of the air, no less), nor do fungi or animals. Thus, this fundamental stage of biochemical processes falls on the bacteria.

The importance of nitrogen

The bacteria fix nitrogen in compounds which are retained on the floor, where it is used by plants and subsequently by animals. But intensive crops require more nitrogen from the soil than the bacteria can provide, so the nitrogen contribution is made artificially.

Guano was traditionally used , excrement produced by birds and collected on islands to be sold to farmers. Later, in the early 20th century, the German chemist Fritz Haber discovered a method to extract nitrogen from the air and transform it into ammonia. This allowed the production of fertilizers on a large scale and a major improvement for agriculture.

More than 100 million tons of fertilizer are produced annually thanks to this process, which consumes around 10% of the energy produced worldwide and allows billions of people to be fed.

In other words; Fritz Haber’s discovery is not only a global business of utmost importance, but it allowed the world’s population to increase substantially. His Nobel Prize was well deserved, no doubt.

Returning to cyanobacteria, there are plants that perform symbiosis with them and can fix nitrogen directly from the air . This is the case of legumes, which were used in crop rotations to enrich the soil with nitrogen and allow higher crop yields.

This use of legumes was discovered before cyanobacteria, proteins, and nitrogen, but no one was awarded the Nobel for it. Hence the importance of putting the findings in writing.

Legumes intended for food, such as soybeans, do not require an external supply of nitrogen. But other crops, like rice, do require the use of nitrogen fertilizers. Rice is the staple of the diet of a large part of the population, so its cultivation is widespread and causes a great impact on the environment. This is where Azolla makes its appearance.

 Azolla as organic fertilizer

Rice cultivation occurs in flooded land. In several Asian countries it has become common to plant Azolla in rice fields months before planting the rice itself. Azolla covers the water and prevents weed growth. Subsequently the ferns die, having contributed nitrogen to the cultivation fields.

This avoids having to use industrial fertilizers, which are expensive and pollute aquifers due to excess nitrogen filtered through the soil. The growth of Azolla allows reaching 9 tons of this green manure per hectare per year.

It would therefore be a plant of interest for agriculture in countries where rice is grown. But it is not the case of the rivers of Spain, where Azolla is a headache for ecologists .

In summary, Azolla is a problematic plant if it is introduced into a natural environment where it can alter the biotic community, but it can also be used in agriculture so that it is carried out in a sustainable and environmentally friendly way.

It is an interesting contrast worthy of reflection, because like any other living being, it can be beneficial or harmful depending on the environment in which it grows. As for its role in the Earth’s climate development, it is staggering.

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