Dictionary > Cyanobacteria

Cyanobacteria

Cyanobacteria is a phylum comprised of photosynthetic bacteria that live in aquatic habitats and moist soils. Others are considered as an endosymbiont, serving as an endosymbiotic plastids in many eukaryotic cells. Cyanobacteria are found to play a role in producing gaseous oxygen as a byproduct of photosynthesis. They are also believed to be associated with the Great Oxygenation Event. Some of them are nitrogen-fixers. Some live singly or in colonies, forming filaments or spheres.

Etymology

The term cyanobacteria came from the Greek kyanós, meaning “blue” and ‎bacteria. Synonyms: Myxophyceae (Wallroth, 1833); Phycochromaceae (Rabenhorst, 1865); Cyanophyceae (Sachs, 1874); Schizophyceae (Cohn, 1879); Cyanophyta (Steinecke, 1931); Oxyphotobacteria (Gibbons & Murray, 1978).

Classification

In the five-kingdom scheme of classification, Cyanobacteria used to be called Cyanophyta and is one of the phyla of the Kingdom Protista. Other phyla are Euglenophyta, Chrysophyta, Pyrrophyta, Chlorophyta, Phaeophyta, and Rhodophyta.(1) These phyla are groups of plant-like protists due to their photosynthetic capability. They do not have true roots, stems, and leaves as embryophytes do.
Recent studies and findings, though, caused changes in the taxonomic positions and led to newer systems of classification.(2) At present, Cyanophyta (also called blue-green algae) is now referred to as Cyanobacteria, a phylum of bacteria. That is because this clade is comprised of species that are prokaryotic. In phycology, the blue-green algae are the only prokaryotic algae; the rest are eukaryotes. Thus, they are now classified as bacteria belonging to Phylum Cyanobacteria.

Sub-groups

Cyanobacterial sub-groups are classified as follows based on morphology:

  • Chroococcales
  • Pleurocapsales
  • Oscillatoriales
  • Nostocales
  • Stigonematales

General characteristics

A diagram of a typical cyanobacterial cell showing parts.

Cyanobacteria are characterized by the presence of pigments (particularly phycobiliproteins) that account to their blue-greenish colour. The phycobiliproteins are components of the phycobilisomes (light-harvesting antennae for cyanobacterial photosystems). Phycobilisomes are embedded on the intracytoplasmic membranes (thylakoids). These pigments are responsible for the blue-green pigmentation of cyanobacteria and they enable them to synthesize their own sugar through photosynthesis. Some cyanobacteria (e.g. Prochlorothrix, Prochlorococcus, Prochloron) lack phycobilisomes and they have chlorophyll b instead.
Cyanobacteria are prokaryotic. They lack a membrane-bound nucleus. Nevertheless, they have microcompartments. For instance, carboxysome is a compartmentalized cage-like structure surrounded by a protein shell. Cyanobacteria use it for concentrating CO2 and therefore increase the efficiency of RuBisCo (the CO2-fixing enzyme).(3) The thylakoids of cyanobacteria are separate compartments (unlike other photosynthetic bacteria whose thylakoids are continuous with the plasma membrane). Apart from photosynthesis, the thylakoids are also involved in cellular respiration. While the thylakoid machinery for electron transport is used for photosynthesis in the light (during the day) it is then used for respiration in the dark (at night).(4)
They may occur singly (as unicellular organisms) or in colonies (forming filaments, such as Anabaena sp.). Filamentous species may differentiate into vegetative cells (photosynthetic cells), akinetes (spores resistant to harsh environmental conditions), or to heterocysts (cells capable of nitrogen fixation by producing the enzyme nitrogenase). Apart from photosynthesis, cyanobacteria are capable of nitrogen fixation through heterocysts. Some of them are nonmotile whereas others can move by gliding motility. Motile filaments of cyanobacterial cells are called hormogonia. Individual cells may break away from this filament to start a new colony elsewhere. In order to float, they form gas vesicle (a vesicle bounded by a protein sheath and not by a lipid membrane). Cyanobacteria reproduce by binary fission.

Nitrogen fixation

Certain cyanobacteria are nitrogen-fixing organisms. Anabaena is an example. They can fix the atmospheric nitrogen into another form (e.g. ammonia, nitrites, or nitrates) that other organisms (e.g. plants and animals) can readily use and convert into proteins and nucleic acids. Cyanobacteria can fix atmospheric nitrogen through transforming into specialized cells called heterocysts. Heterocyst formation occurs when the environment is anoxic and fixed nitrogen is scarce.

Cyanobacterial circadian rhythm

Circadian rhythm was once thought to be an exclusive feature of eukaryotes. Later on, scientists found that certain cyanobacteria also display circadian rhythm.

Evolution

According to endosymbiotic theory, eukaryotes that have acquired the ability to photosynthesize are those that have evolved from the primitive eukaryotes that ingested primitive photosynthetic prokaryotes, such as cyanobacteria. A primary endosymbiotic event led to the evolution of the three primary endosymbiotic eukaryotes: green plants, red algae, and glaucophytes. These three groups make up the monophyletic group, Archaeplastida. The primitive cyanobacterial cell inside the eukaryote is theorized to have eventually become the plastid (chloroplast) that is known today. The chloroplast and the cyanobacterial cell seem to share common features, i.e. morphologically, phylogenetically, genetically, and biochemically. Following the primary endosymbiosis, secondary and tertiary endosymbiotic events ensued, and these are believed to have led to later lineages of photosynthetic eukaryotes.

Ecology

Cyanobacterial blooms

Cyanobacteria are found in aquatic habitats and moist soil. They can also cause algal blooms in aquatic environments, especially those that are stagnant, calm or slowly flowing. The algal bloom by cyanobacteria appears like a scum that is blue-green in colour. It can contain cyanotoxins (toxins produced by cyanobacteria) that can cause serious illness or kill when consumed in certain concentrations. It can cause shellfish poisoning and fish kill. Cyanobacterial blooms are intensified by anthropogenic eutrophication of aquatic habitats. Rising temperatures, vertical stratification, increased CO2 in the atmosphere, and high concentration of phosphorus can cause cyanobacterial population to grow exponentially. Cyanotoxins consist of neurotoxins, hepatotoxins, cytotoxins, and endotoxins that can cause respiratory failure to animals that ingest them through contaminated water.

Soil stabilization

In terrestrial habitats, such as a damp soil, cyanobacteria help stabilize soil. Their growth prevents erosion. They help retain water. For instance, Microcoleus vaginatus is a cyanobacterium that produces a polysaccharide sheath that binds soil particles and helps retain water.(5)

Oxygen cycle contributor

Cyanobacteria are key players in the oxygen cycle. Prochlorococcus sp. alone is credited for contributing much oxygen (about half) by photosynthesis in the open ocean.(6)

Heterotrophy

Some cyanobacteria can become heterotrophs. Some heterorophic parasitic cyanobacteria can cause disease to their invertebrate host, such as black band disease.(7)

See also

References

  1. Pascher, A. (1914). “Über Flagellaten und Algen “. Berichte der deutsche botanischen Gesellschaft 32: 136–160.
  2. The NCBI taxonomy database. Retrieved from http://www.ncbi.nlm.nih.gov/taxonomy.
  3. Long, B. M., Badger, M. R., Whitney, S. M., & Price, G. D. (October 2007). “Analysis of carboxysomes from Synechococcus PCC7942 reveals multiple Rubisco complexes with carboxysomal proteins CcmM and CcaA”. The Journal of Biological Chemistry. 282 (40): 29323–35. doi:10.1074/jbc.M703896200.
  4. Armstronf, J. E. (2015). How the Earth Turned Green: A Brief 3.8-Billion-Year History of Plants. The University of Chicago Press. Retrieved from https://www.press.uchicago.edu/ucp/books/book/chicago/H/bo16465693.html
  5. Belnap, J. & Gardner, J. S. (1993). “Soil Microstructure in Soils of the Colorado Plateau: The Role of the Cyanobacterium Microcoleus Vaginatus”. The Great Basin Naturalist. 53 (1): 40–47.
  6. Nadis S (December 2003). “The cells that rule the seas” (PDF). Scientific American. 289 (6): 52–3. Bibcode:2003SciAm.289f..52N. doi:10.1038/scientificamerican1203-52.
  7. Kristiansen, A. (1964). “Sarcinastrum urosporae, a Colourless Parasitic Blue-green Alga” (PDF). Phycologia. 4 (1): 19–22. doi:10.2216/i0031-8884-4-1-19.1.

© Biology Online. Content provided and moderated by Biology Online Editors