TAXONOMY
The Old METHOD
Cyanobacteria are decended from the domain Eubacteria. There are five groups of Cyanobacteria:
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Unicellular
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Pleurocapsalean
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Ocillatorian
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Nostocalean
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Branching
Image taken from: https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Phylogenetic_tree.svg/450px-Phylogenetic_tree.svg.png
Image taken from: https://c1.staticflickr.com/9/8003/7629005292_9be8b307ed_b.jpg
Pleurocapsalean
Pleurocapsalean reproduce by multiple fission and form cells called baeocytes, which is like an endospore. Multiple fission is where a cell divides on multiple planes to form smaller and smaller cells. These cells are surrounded by a firm sheath.
Image taken from: http://www.landcareresearch.co.nz/__data/assets/image/0016/62242/oscillatoria2.jpg
Nostocalean
Filamentous cyanobacteria that produce cells called heterocysts for nitrogen fixation and other cells called akinites that break away from their filaments. Nostoc is an important model organism
Unicellular
These reproduce by binary fission, along one plane. These cells can also be found in large aggregates of single cells that but work independently. An example is Synechocystis an important model organism.
Image taken from: http://www.denniskunkel.com/DK/Bacteria/11299C.html
Oscillatorian
This group is home to the largest known cyanobacteria, Oscillatoria princeps, at 60um in diameter. The group itself are made up of multicellular filamentous cyanobacteria which divide by binary fission across a single plane.
Branching
These genera are multicellular and when the cells divide from a filamentous strand of cells which can divide down a different plane in order to branch away from the strand.
However, this is just not accurate enough for modern day needs of classification
Modern Approaches to Grouping Cyanobacteria
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Over time it has been found grouping phenotypically isn't a fair way to group cyanobacteria. This is as a result of some bacteria falling into multiple categories.
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Its better to now compare by biochemical and protein analysis.
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An example is 16S rRNA analysis.
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16S rRNA has a very highly conserved protein and DNA sequence allowing examination of the change of sequence overtime. Comparing these sequences allows creation of a phylogenetic tree or cladogram of cyanobacteria.
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From this it is possible to recategorise the cyanobacteria by their biochemical and genotypic similarities into more closely related groups of cyanobacteria.
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However after a large scientific discussion it was found that the most suitable way to group cyanobacteria was under Phylum.
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The bacteria are grouped as genera and classes under a polyphasic approach that was much needed to renew the grouping into a more accurate form.
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Each genera contains cyanobateria with different characteristics as well as very similar 16S RNA strands.
The 16S RNA sequencing method
Image taken from:
http://scielo.isciii.es/img/revistas/im/v8n3/06-Gonzalez-Fig1.gif
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From the phylogenetic tree on the right it is clear that there are 5 main groups, 9 in total, compared to the old method that has 5.
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Within these new groups some remain very similar to the old groups such as Nostocales and Oscillatorales, however they include more species.
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The first group is made of one genus - Gloeobacterales:
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the only group to have no thylakoids.
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Image taken from:
Image taken from: http://protist.i.hosei.ac.jp/pdb/images/Prokaryotes/Chroococcaceae/Synechococcus/sp_05.jpg
A new adapted phylogenetic tree of cyanobacteria based on 31 conserved protein sequences. All suitable complete and draft data on genomes available in April 2014 were utilised. The tree was calculated using a maximum likelihood algorithm with the bootstrap values given at the nodes. The cyanobacterial orders included the current system are highlighted on the tree.
Image taken from: Komarek et al., 2014
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The second is Synechococcales:
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They contain filamentous and unicellular organisms that may be in form of single cells as well as colonies.
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There are over seventy genera are part of this order, some of which may not be in the same ancestral group.
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More study is required to identify whether they do belong to this group
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The third order is Spirulinales:
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Their main characteristic is having coiled trichromes without sheaths.
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They are also recognised by distinctive ecology and cytology.
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Image taken from:
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The fourth order is Chroococcales:
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Coccoid cyanobacteria with irregular thylakoid arrangement
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They reproduce by equal binary fission or by budding.
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Image taken from:
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The Chroococcidiopsidales:
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The group contains small number of genera
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These also p[roduce baeocytes but divide in three or more planes.
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Image taken from:
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The next order – the Pleurocapsales:
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Characterised by the formation of baeocytes and division by binary fission.
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Image taken from:
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The Oscillatoriales:
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Includes bacteria that are filamentous
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These cells undergo binary fission only in a single plane at right angles to the long axis.
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The last order is the Nostocales:
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It clusters filamentous cyanobacteria with special cells such as heterocytes or akinetes.
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Image taken from:
As it can be seen from the biochemical method of taxonomy, it's extremely important to make sure the classification is accurate.
The genus Gloeobacter and the Synechococcus. They both are in the group containing unicellular bacteria that divide by binary fission. However the research of their genomes showed that they belong to the two different orders, which means that they are not as closely related as previously thought.