Should Bacteria and Archaea belong to the
The main purpose of this essay is to find out if Archaea and Bacteria should be classified as two different Kingdoms or as a single one. As organisms, bacteria and archaea both are microscopic and prokaryotic (not possessing a true nucleus). These prokaryotes are very abundant on Earth and inhabit a wide spread of areas, including extreme ones. Both are an example of the most ancient living cells, which have appeared over 3.5 billion years ago. Correct classification of these organisms is important in order to trace the evolutionary history from the very beginning and make a clearer picture of the common ancestor, if it existed. This work will explain the basic taxonomy principles and compare some of the main similarities and differences between archaea and bacteria.
Taxonomy is a section of science which puts organisms into biological groups and names them, based on their characteristic features. The history of this science started when ancient Greek and Roman scientists classified certain animal and plant species known to them (http://www.nhm.ac.uk). Since that time, the way of classifying organisms has changed several times, as the ways of gathering information about the species improved. Scientists of different times proposed their own ways of classification schemes which included the four-Kingdom version with Monera, as a collective name for prokaryotic organisms (Scamardella, 1999). This was followed by five-Kingdom classification by Whittaker (1969), again introducing Kingdom Monera, which became very popular that time. However, some scientists opposed the initial dichotomic division of organisms into prokaryotes and eukariotes. Woese (1977) proposed the three-domain or six-Kingdom division, where Kingdom Eubacteria and Kingdom Archaea were classified separately (see Figure 2). Overall, there was no united opinion about Kingdoms in the 20th century. Terms, such as "urkingdoms", "superkingdoms" or "primary kingdoms" could be met in numerous articles, while scientists tried to find the right way of organism classification (Cary, 2009). Each of them used a different approach towards the organism features and structure, causing a range of different subdivisions.
Figure 2: widely used three-domain taxonomic classification of organisms, acquired from: www.nature.com
Simply looking at an archaeon and bacterium under a microscope does not always give an idea of distinguishing one from another. Both of these organisms come in a range of different shapes, are similar in size and organelle content (Brooker, 2014). They are also widely spread and can be found in any environment, however, archaea are more adapted to living in extreme habitats, such as ocean depths or very hot, cold and acidic areas of the world. Depending on their habitat, prokaryotes express different features, such as ability to fix nitrogen from atmosphere, perform photosynthesis or anaerobic respiration (Alberts, 2014). Most of the differences start to appear on biochemical, rather than cellular level.
Talking about cellular organisation, both archaea and bacteria possess a cell wall that contributes to their shape and protection. Bacterial cell walls can be thick and rich in peptidoglycan – Gram-positive, or have a thin peptidoglycan (murein) layer sandwiched between inner and outer membranes - Gram-negative (Gupta, 1998). Archaeal cell wall lacks peptidoglycan, which is an unusual negative trait. Cavicchioli (2007), who described this feature, compared it to a landscape having no lakes. Instead of murein, most of the archaea are covered in a paracrystalline protein layer, which makes them resistant to all antibiotics that target the bacterial sell wall synthesis (Albers, 2011). Some of the scientists, like Valas (2011) believe, that archaea might have evolved from the Gram-positive bacteria, trying to find a habitat free from...
References: Brooker, R. J., Widmaier, E. P., Graham, L. E., Stiling, P. D. 2014 Biology 3rd edition, New York: McGraw-Hill Education
Cary, N. C. 2009 New Foundations of Evolution: On the Tree of life, New York: Oxford University Press
Kato, S. Y. 2011 Microbiology Research Advances: Archaea: Structure, Habitats and Ecological Significance, New York: Nova Science Publishers
Woese, C. R., Fox, G. E. 1977 Phylogenetic structure of the prokaryotic domain: the primary kingdoms. Proc Natl Acad Sci, 74(11), 5088-90, available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC432104/, [accessed on 19/02/2015]
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