Extreme Halophiles

2.5.2. 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.

* Having chemical affinity for tissues or certain organs. ** Wherein they may cause spoilage. *** One of a group of pigments (e.g., carotene) ranging in colour from light yellow to red, widely distributed in

plants and animals e.g., βββββ-carotene (in carrots) ; lycopene (in tomatoes) ; and lukein (in spinach).

STRUCTURE AND FUNCTION : BACTERIAL CELLS

(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 O 2 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.