The
extreme halophiles
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 electrontransport
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 produced
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 appreciable
protonmotive force that in turn carries out the ATP synthesis strategically.
(3) Conclusively,
halobacteria essentially follows the mechanism of light-monitored synthesisof ATP. Furthermore, these are actually devoid of bacteriochlorophyll.