What are Cyanobacteria and how are they Similar or Different from ‘True’ Plants?

Figure 1: Diagram of a Plant Cell

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Figure 2: Diagram of a Cyanobacteria Cell

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Figure 3: Diagram of a Chloroplast

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‘True’ plants could be seen as organisms which are a member of the kingdom Plantae however there are species in other kingdoms which also show some of their characteristic-for example, those in the Protista, Monera or Fungi. Cyanobacteria, a bacteria, is an example of one such organism.

Cyanobacteria could be described as photosynthetic prokaryotic single-celled organisms which are green/blue in colour. Metcalf and Codd (2004)[1] depicted cyanobacteria to be “common members of the plankton of marine, brackish and freshwaters throughout the world… [with] … a simple structure at subcellular level and lack a nucleus, a characteristic feature defining them, along with bacteria, as prokaryotes.

A similarity between a ‘true’ plant and cyanobacteria is that they both do oxygenic photosynthesis. Thus, this means that they are able to utilise water as an electron donor and uptake carbon dioxide and then convert this, using energy from the sun, into oxygen (a bi-product) and sugars. They both do this by trapping the sun’s light energy in the pigment chlorophyll. However they do so in slightly different ways: plant cells have chloroplasts [Fig 3, Fig1] within them, which is where the chlorophyll is stored however cyanobacteria are able to do this without the aid of chloroplasts, instead they are able to store their photosynthetic pigments in the thylakoid within their cytoplasm [Fig 2]. It is thought, in endosymbiosis theory, that cyanobacteria may have, over time, evolved into the chloroplasts which we know to be in plants today (Gault and Marler (2009)[2]). This also explains the colour difference between them-since plants are usually green while cyanobacteria is usually greeney blue. Since the majority of pigments in plants are chlorophyll, which reflects green light, they mainly appear green. However cyanobacteria are also able to accumulate the pigment phycocyanin which gives a blue colour, along with the green of chlorophyll. In addition they can be other colours for example, red, if they contain the pigment phycoerythrin (Gault and Marler (2009)[3]).

Although cyanobacteria do not contain chloroplasts, the cells do contain many of the same features as plant cells-there are many similarities between Fig. 1 and Fig. 2, even though plants are eukaryotic cells while cyanobacteria are prokaryotic and Staley et al (2007) state that “it was discovered by electron microscopy that their [cyanobacteria] cell structure is truly prokaryotic”[4]. They both contain a cell membrane, which is constructed from a phospholipid bi-layer. They do both contain a cell wall however the composition is different, with cyanobacteria’s being mostly made of peptidoglycan while plant cells usually have a cellulose cell wall. They both also have ribosomes, which are required for the synthesis of DNA. However they do have significantly different RNA sequences and structures and so although the ribosomes have the same function, they do so in a slightly different way. Regardless, these ribosomes are highly important since they both must contain DNA to enable to code for the repair of their own cells and to replicate it and pass their genes on to future generations. But in cyanobacteria it is known as a nucleoid and is circular DNA when in plants it is tightly wound into the nucleus. (Arjun (2011)[5]). A further similarity between the two is that of their reproductive methods. Cyanobacteria do so in an asexual method via fission (where the nucleus of the parent plant divides by mitosis), fragmentation (where a new organism grows from a fragment of the original) or through the formation of hormogia. Organisms within the Plantae kingdom are also able to reproduce asexually e.g. Chlorophytum producing ‘runners’ which are genetically identical to the original. However, plants are also able to reproduce sexually through fertilisation. This means that some plants have reproductive organs, whereas cyanobacteria would never have these. Yet, it is debateable whether ‘true’ plants can be distinguished by the reproduction methods, since there is such a great degree of variation between them.

A difference between ‘true’ plants and cyanobacteria is their varying abilities to fix nitrogen. This means that cyanobacteria can convert inert atmospheric nitrogen into an organic form which other organisms can use for example nitrate or ammonia. They are specialised to do so by having a thick wall and an anaerobic environment. True plants, on the other hand, are not able to do this. They are only able to use the organic form of nitrogen and so usually either rely on man-made fertilisers for it or on symbiotic relationships with diazotrphs e.g. plants in the legume family such as pea plant must do this.

To conclude, I believe that cyanobacteria were given the correct classification and should not be a part of the Plantae kingdom because they break too many of the important rules which govern them. However I do think it is very fair to describe cyanobacteria as ‘plant-like’ because they do both share many characteristics.


Bibliography


[1] Metcalf and Codd (2004), Cyanobacterial toxins in the water environment [Report]

[2] Gault and Marler (2009), Handbook on Cyanobacteria: Biochemistry, Biotechnology and Applications, Nova Science

[3] Gault and Marler (2009), Handbook on Cyanobacteria: Biochemistry, Biotechnology and Applications, Nova Science

[4] Staley et al (2007), Microbial Life, 2nd edition

[5] Arjun (2011), Write a Brief Note on the Structure of Cyanobacteria, http://www.preservearticles.com/201102174059/write-a-brief-note-on-the-structure-of-cyanobacteria.html

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