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Halobacterium halobium is a member of Archea, which can live in saline condition
of 4M (24% w/v). It is a habitant of salt lakes, solar salt evaporation ponds
and artificial saline habitats such as the surfaces of heavily salted foods like
certain fish and meats. It is an aerobic chemoheterotroph with complex
nutritional requirements. It does not utilize typical carbon sources such as glucose and
sucrose, and its growth relies on the complex carbon/nitrogen sources such as
peptone and yeast extract (Lee et al., 1998).
It seems to be well-equipped with metabolic machinery for the production of ATP through oxidative
phosphorylation, photophosphorylation and substrate level phosphorylation. Thus, the
organism has the ability to grow under aerobic, semiaerobic and anaerobic
conditions (Hartmann et al., 1980). This organism under aerobic
conditions respires, using respiratory chain and oxygen as a terminal electron acceptor. However,
when subjected to illumination and under oxygen limitation conditions, the
organism synthesizes Purple Membrane (PM) (Rodriguez-Valera et al., 1983).
The PM is the differential domain of plasma membrane consisting of
retinal, bacterio-opsin, and lipid molecules (Hartmann et al., 1980). The retinal fused with bactero-opsin is known as bacteriorhodopsin.
This bacteriorhodopsin, which is stabilized by lipid molecules in the PM,
is capable of using light energy to create proton gradient, which in turn is used
by ATPase for ATP synthesis (Lorber and DeLucas, 1990). Apart from these
molecules, Halobacterium has two major carotenoids—bacterioruberin and b-carotene (Sumper and Herrmann, 1976a). The interest in bacteriorhodopsin has arisen due to
its excellent thermodynamic and photochemical stability. The applications
comprise holography, spatial light modulators, artificial retina, neural network
optical computing and volumetric and associative optical memories (Margesin
and Schinner, 2001).
The literature pertaining to the production of PM is rather scanty. The
biogenesis of PM is induced by limiting the oxygen supply, which turns on the synthesis
of both bacterio-opsin and of retinal simultaneously. In contrast, most of the
lipid molecules necessary for the PM formation were drawn from the pool of the
cell membrane. Under the condition of sufficient aeration, low PM synthesis
was observed in the absence of illumination (Sumper and Herrmann, 1976b; and
El-Sayed et al., 2002). Oesterhelt and Stoeckenius (1973) have found that little
PM was formed when the medium was saturated with air. Rodriguez-Valera et al., (1983) too have reported that microaerophilic condition supported the
synthesis of PM. |
