Archaeobacteria and Eubacteria

ARCHAEOBACTERIA AND EUBACTERIA

It has been observed that ‘all cells’ categorically fall into either of the two groups, namely : the eukaryotes, and the prokaryotes. Besides, the multicellular plants and animals are invariably eukaryotic in nature and character, and so are the numerous unicellular organisms. The only prokaryotes are the organisms, such as : cyanobacteria (Gr. hyanos = dark blue). In the recent past this very classification has undergone a considerable change. It has been duly established and observed that there exists another ‘group of organisms’ amongst the bacteria that do not seem to fall into either of the two aforesaid categories. These organisms have been termed as the archaeobacteria, which essentially designate an altogether new primary kingdom having an entirely different status in the history and the natural order of life.

The enormous volume of informations based on experimental evidences gathered from studies of ribosomal RNA suggests that archaeobacteria and eubacteria strategically got separated at a very early stage in the pioneer process of evolution of life on this planet (earth). Importantly, the phylogenetic* distance that critically prevails between the two above mentioned categories of bacteria is reflected by some phenotypic** differences prominently, which may be summarized in the following Table : 2.4.

Archaeobacteria, in reality, do not represent a perfect homogeneous group. One may, however, observe a substantial degree of heterogeneity amongst the eubacteria, so do the different types of archaeobacteria specifically differ from each other with respect to morphology, metabolism, chemical composition, and habitat.

The ‘archaeobacteria’ are unusual organisms by nature, and this particular category is known to comprise essentially of three different types of bacteria, namely :

(1) Methanogenic bacteria,

(2) Extreme halophiles, and

(3) Thermoacidophiles

These three groups of organisms shall now be treated individually in the sections that follows :

1. Methanogenic Bacteria [Methanogens]

The methanogenic bacteria are considered to be the hard-core anaerobes which, invariably possess the capability of deriving energy for their progressive growth by certain particular oxidizing chemical entities, for instance : hydrogen (H₂), formic acid (HCOOH) ; and actually exert their ‘action’ by making use of the electrons thus produced to reduce ultimately carbon-dioxide (CO₂) to give rise to the formation of methane (CH₄) gas :

It has been observed that certain genera specifically may grow as autotrophs* — thereby utiliz-ing hydrogen and carbon dioxide as exclusive sources of carbon as well as energy ; whereas some others do need several additional components, for instance : organic-sulphur compounds, amino acids, acetic acid, and vitamins. Interestingly, a plethora of species actually grow quite abundantly and aggressively in a complex media viz., comprising of yeast extract in comparison to inorganic-salts containing media.

Coenzymes** : There are at least two uncommon coenzymes that invariably occur in all meth-anogenic bacteria (methanogens) that have not been noticed in other varieties of microorganisms.

Examples : Following are two typical examples of methanogenic coenzymes :

(a) Coenzyme M — directly involved in methyl transfer reactions, and

(b) Coenzyme F₄₂₀ — a flavin-like chemical entity intimately involved in the anaerobic elec-tron transport system of these microorganisms. It has the ability to fluoresce when exposed to UV light ; and, therefore, its presence may be detected by visualizing the organisms via a fluorescence microscope conveniently (also used for its critical identification and examina-tion).

Differentiation of Methanogens : The genera of methanogens i.e., the methane-producing bac-teria may be clearly differentiated exclusively based upon their morphology*** and Gram reaction****. However, the glaring distinct differences occurring in the cell-wall composition have been duly observed to correlate specifically with these genera.

Table 2.5 : records the morphology, motility, and wall composition of several methanogenic organisms with specific ‘genus’.

Importantly, the cell walls of two genera essentially consist of pseudomurein, that prominently differs from eubacterial peptidoglycan by the following two distinct structural features, namely :

(a) substitution of N-acetyltalosaminuronic acid for N-acetylmuramic acid, and

(b) presence of tetrapeptide composed totally of L-amino acids, having glutamic acid attached duly at the C-terminal end.

Habitats : Interestingly, the methanogenic bacteria most commonly found in anaerobic habi-tats that are eventually rich organic matter which ultimately produced by nonmethanogenic bacteria via fermentation to yield H₂ and CO₂. A few such common as well as vital habitats are, namely : marine sediments, swamps, marshes, pond and lake mud, intestinal tract of humans (GIT) and animals, rumen of cattle (e.g., cow, buffalow, sheep, pig, goat etc.), and anaerobic sludge digesters in sewage-treatment plants.

Figure. 2.6 [A and B] depicts the diagramatic sketch of the cells commonly observed in various kinds of methanogenic organisms (viz., methane-producing bacteria).

Figure 2.6 [A] evidently shows the typical cells of Methanosarcina barkeri and Methanospirillum hungatei representing ideally the methane-producing bacteria.

Figure 2.6 [B] likewise illustrates the characteristic cells of Methanobacterium thermo-autotrophicum and Methanobacterium ruminantium designating the methanogens.

2. Extreme Halophiles

The extreme halophiles are aerobic organisms and chemoorganotrophic* in nature that essen-tially need nearly 17 to 23% (w/v) sodium chloride (NaCl) for their normal and good growth. These extreme halophiles invariably stain Gram-negative organisms that specifically vary from the rod or disk-shaped cells (i.e., the genus Halobacterium) to spherical or ovoid cocci (i.e., the genus Halococcus).

Habitat : They are most commonly found in ‘salt lakes’, such as :

·        The Great Salt Lake ; the Dead Sea,

·        Industrial plants generating salt by solar evaporation of sea-water, and

·        Salted proteinaceous substances e.g., salted fish.**

In usual practice, the colonies are found to range from red to orange colouration by virtue of the presence of carotenoids*** that particularly appear to cause adequate protection to the ensuing cells against the damaging effect of the sunlight (having UV radiation).

Salient features : The salient features of the Halobacterium and the Halococcus cells are as stated below :

(1) The cells do resist ‘dehydration’ particularly at high sodium chloride (NaCl) concentration due to the adequate maintenance of a high intracellular osmotic concentration of potassium chloride (KCl).

(2) Both ribosomes and the cytoplasmic membrane are found to be fairly stable only at relatively high concentrations of KCl, whereas the corresponding enzymes are observed to be active only at high concentrations of either NaCl or KCl.

(3) Importantly, the Halobacterium cell walls are invariably made up of ‘certain protein subunits’ which are held together only in the presence of NaCl ; and, therefore, if the critical level of NaCl happens to fall below approximately 10% (w/v), the cells undergo break up.

(4) Interestingly, the Halococcus cell walls are usually comprised of a complex heteropolysaccharide which is found to be stable reasonably at comparatively lower NaCl concentrations.

Adenosine Triphosphate (ATP) Synthesis. It is worthwhile to mention here that generally the ‘halobacteria’ are ‘aerobic’ in nature. It is amply established that in aerobic organisms, an electron-transport chain invariably gives rise to a specific protonmotive force that in turn helps to carry out the desired ATP-Synthesis.

Salient Features : There are several salient features that are associated with the ATP-synthesis, namely :

(1) ATP-synthesis may alternatively be accomplished by halobacteria via fermentation of arginine (an amino acid), which permits them to grow in an anaerobic environment.

(2) The third method of ATP formation is rather unique and extraordinary to the ‘halobacteria’. Predominently distinct patches of a purple pigment, known as bacteriorhodopsin*, are pro-duced in the cell membrane particularly at reasonably low O₂ levels. Subsequently, when these cells containing the said pigments are exposed to the UV-light—the pigment gets bleached gradually. In the course of the ‘bleaching phenomenon’, the resulting protons** get duly extruded right into the outside portion of the membrane, thereby exerting an appre-ciable protonmotive force that in turn carries out the ATP synthesis strategically.

(3) Conclusively, halobacteria essentially follows the mechanism of light-monitored synthesis of ATP. Furthermore, these are actually devoid of bacteriochlorophyll.

3. Thermoacidophiles

The thermoacidophiles are generally the aerobic Gram –ve archaeobacteria prominently char-acterized by a remarkable tendency and capability to attain growth not only under extremely high acidic conditions, but also at considerably elevated temperatures.

There are two most prominent genera that belong to this particular category, namely :

(a) Thermoplasma, and

(b) Sulfolobus.

3.1 Thermoplasma

These chemoorganotrophic microorganisms very much look alike the mycoplasm (i.e., a group of organisms that lack cell walls and are highly pleomorphic), and obviously varying from spherical in shape to filamentous in nature. The ideal and optimum temperature for their progressive growth ranges between 55 and 59 °C (minimum, 44 °C ; maximum, 62 °C), whereas the optimum pH is 2 (minimum, 1 ; maximum, 4). It has been duly observed that the cells of these thermoplasmas undergo abundant lysis virtually at a neutral pH. In actual practice, the thermoplasmas have been duly isolated from the re-sidual heaps of burning coal refuse.

3.2 Sulfolobus

The cells of this particular genus are more or less lobe-shaped or spherical in shape and appear-ance. They have the definite cell walls that are essentially made up of protein. However, the optimum temperature and optimum pH of different species of sulfolobus are as given below :

Optimum temperature : 70–87 °C ;

Optimum pH : 2 [Min. 1 ; Max. 4].

Nevertheless, the sulfolobus are established to be autotrophic* facultatively. In fact, sulfolobus may be grown in two different manners as stated under :

Method ‘A’ — as ‘chemolithotrophs’ when adequately provided with ‘S’ as an element and an electron donor, and

Method ‘B’ — as ‘chemoorganotrophs’ in the respective media comprising of organic substrates.

Interestingly, the natural occurrence of the sulfolobus species are prominently and predominently found in sulphur (acidic) hot springs around the world.