By: Ridaul Hayatin
Anabaena is a genus of filamentous cyanobacteria that exists as plankton. It is known for its nitrogen fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanobacteria that produce neurotoxins, which are harmful to local wildlife, as well as farm animals and pets. Production of these neurotoxins is assumed to be an input into its symbiotic relationships, protecting the plant from grazing pressure.
A DNA sequencing project was undertaken in 1999, which mapped the complete genome of Anabaena, which is 7.2 million base pairs long. The study focused on heterocysts, which convert nitrogen into ammonia. Certain species of Anabaena have been used on rice paddy fields, proving to be an effective natural fertilizer.
A. Description and significance
Anabaena azollae is a small filamentous phototrophic cyanobacteria generally seen as a multicellular organism with two distinct, interdependent cell types. The first is a small, circular, photoautotrophic “vegetative” cell that performs oxygenic photosynthesis and is typically blue green in color. The second is a heterocyst; a larger, paler, more homogenous cell produced by Anabaena to fix atmospheric nitrogen. A. Azollae, although it can exist on its own, is usually found within ovoid cavities inside the leaves of the water fern Azolla. Azolla (also known as mosquito fern, duckweed fern, or fairy moss) is a genus of common water fern whose small leaves accumulate on the surface of bodies of water creating mats. Anabaena azollae and Azolla have formed a symbiotic relationship in which the cyanobacterium receives carbon and nitrogen sources from the plant in exchange for fixed nitrogen. This relationship has proven to be useful to humans in the production of food; specifically in the fertilization of rice paddies. Azolla is used as an organic biofertilizer to increase the nitrogen levels in the rice paddies. Anabaena azollae is a model organism commonly used in labs for the study of gene differentiation in the formation of heterocysts.
B. Ecology (Including Pathogenesis)
Anabaena Azollae maintains a mutually beneficial symbiotic relationship with the water fern Azolla, which provides the cyanobacteria with a safe environment in exchange for nitrogen. Azolla is grown in tropical and temperate climates in calm bodies of water. A. Azollae are usually found within ovoid cavities, located within the plant’s dorsal leaves, and are connected to the external environment by pores. Epidermal cells inside the leaf cavity of Azolla project inwards forming numerous multicellular hairs with wart like outgrowths. These hairs are thought to serve as a pathway for the free exchange of nutrients between A. Azollae and Azolla. When free living, A. Azollae only develops between 5-10% of its cells into heterocysts. However, when living in conjunction with Azolla, A. Azollae will increase its heterocyst production up to 25-30%.
Although Anabaena azollae is not classified as a pathogen, Azolla is known to produce “toxic blooms” in their environment. A. Azollae is able to fix nitrogen so efficiently that populations of Azolla have been known to double their biomass every two to three days. The products are dangerous or deadly to animals and humans due to various cyanotoxins that are released.
C. Interesting feature
For centuries the symbiotic relationship between A. Azollae and Azolla has benefited farmers across the world. Because of the “in house” nitrogen source provided by A. Azollae, Azolla has been used as “green manure” in China and other countries to fertilize rice paddies and increase rice production. Azolla is either incorporated as green manure at the beginning of the cropping season or grown as a dual crop along with rice, in the standing water of flooded fields. The nitrogen fixed by the cyanobacteria is either released upon decay Azolla or leached into the water from the growing Azolla and is available for uptake by the rice crop. Additionally, Azolla can promote aerobic transformations such as methane oxidation through enhanced aeration of the flood water in rice fields. This fern is responsible with providing between 50-75% of the nitrogen required by the rice crop. According to some reports, Azolla has helped increase rice yields as much as 158% per year.
In addition to fertilizer, Azolla is also used in fish food and garden mulch and is a natural food for various types of insects. Azolla can be used and as a water purifier and for the control of weeds, mosquitoes, and the reduction of ammonia volatilization that accompanies the application of chemical nitrogen fertilizers.