Characteristic cyanobacteria, morphology, toxicity

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Basil Manning

The cyanobacteria, Formerly known as blue green algae, they are a phylum of bacteria formed by the only prokaryotes capable of using sunlight for energy and water as a source of electrons in photosynthesis (oxygenic photosynthesis)..

Like higher plants, they contain pigments that allow them to carry out oxygenated photosynthesis. This phylum includes around 2000 species in 150 genera, with a wide range of shapes and sizes..

Oscillatoria sp. By Wiedehopf20 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

Cyanobacteria are very ancient organisms. Microfossils with great similarity to modern cyanobacteria have been found in deposits dating back 2.1 billion years. Characteristic biomarker molecules of cyanobacteria have also been found in 2.7 and 2.5 billion year old marine deposits..

Due to the ability of cyanobacteria to produce and release oxygen as a by-product of photosynthesis, it is believed that its appearance on earth allowed the modification of the atmosphere, causing a large oxygenation event..

The increase in oxygen may have caused a decrease in atmospheric methane concentration approximately 2.4 to 2.1 billion years ago, causing the extinction of many species of anaerobic bacteria..

Some strains of cyanobacteria species can produce potent toxins in aquatic environments. These toxins are secondary metabolites that are released into the environment when environmental conditions are extreme, in eutrophic environments, with high concentrations of mineral nutrients such as phosphorus and particular conditions of pH and temperature..

Article index

  • 1 General characteristics
  • 2 Morphology
  • 3 Systematic
  • 4 Toxicity
  • 5 References

Characteristics general

Cyanobacteria are gram-negative staining bacteria, which can be unicellular or form colonies in the shape of filaments, sheets, or hollow spheres..

Within this diversity, different types of cells can be observed:

  • Vegetative cells are those that are formed under favorable environmental conditions, in which photosynthesis occurs.
  • Akinetes, endospores produced under harsh environmental conditions.
  • Heterocytes, thick-walled cells, contain the enzyme nitrogenase, which is involved in nitrogen fixation in anaerobic environments.

Cyanobacteria are the simplest organisms that exhibit circadian cycles, oscillations of biological variables at regular intervals of time associated with periodic environmental changes during the day. The circadian clock in cyanobacteria works from the KaiC phosphorylation cycle.

Cyanobacteria are distributed in a great diversity of terrestrial and aquatic environments: bare rocks, temporarily wet rocks in deserts, fresh water, oceans, moist soil, and even in Antarctic rocks.

They can form part of plankton in bodies of water, form phototrophic biofilms on exposed surfaces, or establish a symbiotic relationship with plants or lichen-forming fungi..

Some cyanobacteria play an important role in ecosystems. Microcoleus vaginatus Y M. vaginatus stabilizes the soil using a polysaccharide sheath that binds to sand particles and absorbs water.

Bacteria of the genus Prochlorococcus produce more than half of the photosynthesis of the open ocean, making an important contribution to the global oxygen cycle.

Various species of cyanobacteria, such as Aphanizomenon flos-aquae Y Arthrospira platensis (Spirulina), are harvested or cultivated as food sources, animal feed, fertilizers and health products.

Morphology

Cyanobacterial cells have a highly differentiated, gram-negative cell wall with a plasma membrane and an outer membrane separated by a periplasmic space..

In addition, they have an internal system of thylakoid membranes where the electron transfer chains that are involved in photosynthesis and respiration reside. These different membrane systems give these bacteria a unique complexity.

They do not have flagella. Some species have moving filaments called hormogonia, which allow them to glide on surfaces..

The multicellular filamentous forms, such as the genus Oscillatoria, are capable of generating an undulating movement through oscillation of the filament.

Other species that inhabit columns of water form gas vesicles, formed by a protein sheath, which give them buoyancy..

Hormogonia are made up of thin cells with sharp cells at the ends. These cells are released and mobilized, sprouting in places far from the main colony, where new colonies start..

Systematic

The classification of cyanobacteria at the highest taxonomic levels has been hotly debated. These bacteria were initially classified as blue-green algae (Cyanophyta), according to botanical codes. These initial studies were based on morphological and physiological characteristics..

Later, in the 1960s, when the prokaryotic characteristics of these microorganisms were established, cyanobacteria were reclassified under the bacteriological code.

In 1979 5 sections were proposed that correspond to 5 orders: section I = Chroococcales, section II = Pleurocapsales, section III = Oscillatoriales, section IV = Nostocales and section V = Stigonematales.

The taxonomic system of cyanobacteria was radically changed with the introduction of electron microscopy and molecular and genetic methods..

The taxonomy of cyanobacteria has been reviewed almost continuously in the last 50 years, in which radically different proposals have been generated. The debate on the classification of cyanobacteria still continues.

The latest proposals for phylogenetic trees for this phylum propose the use of the orders: Gloeobacterales, Synechococcales, Oscillatoriales, Chroococcales, Pleurocapsales, Spirulinales, Rubidibacter / Halothece, Chroococcidiopsidales y Nostocales. These orders are composed of monophyletic genera, composed of many species.

Toxicity

It is estimated that there are 150 genera of cyanobacteria containing approximately 2000 species, of which about 46 have some toxin-producing strain.

In aquatic ecosystems, the abundance of cyanobacteria can reach very high levels when environmental conditions are appropriate for their growth, which favors the accumulation of secondary metabolites in the cytoplasm..

When environmental conditions become unfavorable, with increases in the concentrations of mineral nutrients such as phosphorus, cyanobacteria die, producing cell lysis and the release of toxins into the environment..

Two main types of toxins have been identified: hepatotoxins and neurotoxins. Neurotoxins are produced mainly by species and strains of the genera: Anabaena, Aphanizomenon, Oscillatoria, Trichodesmium Y Cylindrospermopsis.

Neurotoxins act rapidly, causing death from respiratory arrest within minutes of ingesting high concentrations of toxin. Saxitoxin is a paralyzing neurotoxin, listed in Annex 1 of the Chemical Weapons Convention.

Hepatotoxins are produced by the genera Microcystis, Anabaena, Nodularia, Oscillatoria, Nostoc Y Cylindrospermopsis. They cause the most common type of poisoning related to cyanobacteria. They work more slowly and can lead to death a few hours or days after poisoning.

References

  1. Dmitry A. Los. (2017). Cyanobacteria: Omics and Manipulation | Book. Caister Academic Press. Moscow, Russia. 256 pp.
  2. Komárek, J., Kaštovský, J., Mareš, J. Y & JOhansen, J.R. (2014). Taxonomic classification of cyanoprokaryotes (cyanobacterial genera) 2014, using a polyphasic approach. Preslia 86: 295-335.
  3. Gupta, R.C. Handbook of Toxicology of Chemical Warfare Agents. (2009). Academic Press. Pp 1168.
  4. Howard-Azzeh, M., L. Shamseer, H. E. Schellhorn, and R. S. Gupta. (2014). Phylogenetic analysis and molecular signatures defining a monophyletic clade of heterocystous cyanobacteria and identifying its closest relatives. Photosynthesis Research, 122 (2): 171-185.
  5. Roset J, Aguayo S, Muñoz MJ. (2001). Detection of cyanobacteria and their toxins. Journal of Toxicology, 18: 65-71.
  6. Wikipedia contributors. (2018, October 2). Cyanobacteria. In Wikipedia, The Free Encyclopedia. Retrieved 10:40, October 12, 2018, from en.wikipedia.org

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