A Microbial Biorealm page on the genus Anabaena azollae
Higher Order Taxa:
Bacteria; Cyanobacteria, Nostocales, Nostocaceae, Anabaena,
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 it’s 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 waterfern whose small leaves accumulate on the surface of bodies of water creating mats. A. 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. A. Azollae is a model organism commonly used in labs for the study of gene differentiation in the formation of heterocysts.
- The complete genome of Anabaena is 7.2 million base pairs long. The sequenced strain, known as Anabaena sp. strain PCC 7120, was sequence in a collaborative effort between Japanese researchers led by Takakazu Kaneko, of the Kazusa DNA Research Institute in Chiba, Japan, and C. Peter Wolk, of the Michigan State University in East Lansing. Anabaena has a single circular chromosome of 6.4 million base pairs and six smaller circular plasmids. Researchers have identified 5,368 genes in the chromosome, 45 % of which are functional. More than 60 genes involved in various processes of heterocyst formation and nitrogen fixation were assigned to the chromosome. The genomes of many other strains of Anabaena have been sequenced (NCBI Taxonomy Browser), however only several strains of Anabaena Azollae have been sequenced (strains 0708 and PCC 7120). Sequencing of the Anabaena genome has the potential to aid researchers in the study of the genetics and physiology of cellular differentiation and nitrogen fixation (as exhibited in the heterocycsts of Anabaena). Information on the genome of Anabaena Azollae 0708 can be found on KEGG GENOME.
- Genes that cause cellular differentiation has been the focus of much research to date. Out of the 5,368 possible genes, only two genes that control the initiation of heterocyst formation and the frequency of cell differentiation have been identified. The hetR gene codes for a protein required for heterocyst differentiation and is only transcribed in differentiating cells. The patS gene has been found to regulate the frequency at which cells develop into heterocysts.
Cell structure, metabolism & life cycle
- The vegetative cell of Anabaena Azollae is a small phototrophic cyanobacteria. It has a filamentous structure composed of smaller circular cells, which are typically blue-green in color. Like all phototrophic bacteria, A. Azollae uses solar energy along with bacteriochlorophyll and CO2 to carry out photosynthesis in the cytoplasm of its cells. Photosynthetic electron transport in Anabaena proceeds from oxidation of H2O in Photosystem II in which ATP and NADPH are formed through the movement of excited electrons down a series of proteins to Photosystem I. In Photosystem I, electrons are separated from an organic electron donor and are ultimately transferred to form NADH or NADPH. This electron transport system is represented in Z-scheme photosynthesis.
- A. Azollae typically undergoes a type of asexual reproduction called fragmentation; a process in which, a section of the cyanobacteria chain will split off from the parent chain and float away. These new sections called hormogonia are small motile segments of the parent unit. Over time hormogonia will begin to elongate via the separation of adjacent cell walls and form their own chains. Fragmentation can also occur when a live portion of a chain separates from a section of dead cells.
- How A. Azollae is passed on to new generations of Azolla is not completely understood. It is believed that hormogonia may become entrapped by the embryo of the Azolla plant during differentiation of its shoot apex and dorsal lobe primordia of the first leaves.
- Heterocysts function as the sites for nitrogen fixation under aerobic conditions and are formed in response to a lack of fixed nitrogen. Terminal and intercalary heterocysts are created from Anabaena vegetative cells, which have undergone morphological differentiation as well as by biochemical changes. Although they vary in size, heterocysts are usually larger, paler, and more homogeneous than the cells from which they are formed. Some of the major changes that occur include the production of three additional cell walls (including one composed of glycolopid which forms a hydrophobic barrier to oxygen), production of nitrogenase and other proteins for nitrogenfixation, production of enzymes that eliminate oxygen from the cell, rearrangement of the thylakoid, and the up-regulation of glycolytic enzymes.
- The mature heterocysts contain no functional photosystem II and cannot produce oxygen. Instead, they only contain photosystem I, which enables them to carry out cyclic photophosphorylation and ATP regeneration. Loss of photosystem II means that heterocysts are no longer capable of producing necessary reductants required for nitrogen fixation. These need to be supplied by neighboring vegetative cells largely in the form of sucrose, produced during photosynthesis. These changes provide appropriate conditions for the functioning of oxygen-sensitive nitrogenase.
- Under environmental stress, such as light limitation, cold temperatures, or phosphate starvation, filamentous cyanobacteria such as Anabaena form specialized spores called akinetes. An akinete forms a long, oval cell adjacent to a heterocyst, where it stores nitrogen and developes a thickened envelope. Like other types of spores, akinetes are able to resist desiccation and remain viable for long periods of time. Akinetes can lie dormant for decades until favorable conditions allow them to germinate and grow in to new vegetative filaments.
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.
- 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.
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