By Nura Abboud
28/6/2008
Archaea discovery by Woese et al in 1977 has opened a new age of research in the life sciences, as it produced for the first time the three armed tree of life [15, 21] Archaea are ubiquitous, gaining the nickname of extremophiles [21, 22] that can be classified according to their extreme habitats: Thermophiles, Methanogens and halophiles [15]. An example of a well-adapted and widely distributed halophilic microorganism is Halobacterium [5]. Formally, the species Halobacterium salinarum (HS), Halobacterium halobium (that was discovered by D. Oesterhelt et al in 1971) [10], and Halobacterium cutirubrum were considered separate species, but recently it was decided that they are similar enough to be regarded as one species named (HS). A microorganism that has features similar to HS has been described more than 80 years ago [11].
Despite its name, this marine microorganism is not a bacterium, but a gramnegative, rod-shaped archaeon that reproduces by binary fission and does not form spores. It is a single celled motile archaeon. [2, 4, 64]. HS is a good archaeal model for the study of archaeal genetics [18], as it has interesting properties and genetic variability [2]. HS is an obligatory halophile that can be found in bodies of salt water, such as salt lakes, the Dead Sea, and in evaporation ponds [3, 5]. Moreover, it has been occasionally isolated from salt-heavy food such as salt pork and sausages. HS plays an important role in the spoilage of many products preserved by the addition of salt. Studies showed that it is possible to grow cultures of HS in a chemically defined media [4].
HS exhibits a highly acidic cytoplasm saturated with KCl, potassium acts as an antifreeze/coolant to keep the cell metabolism functioning [4, 16] the metabolism of this chemoorganotrophic archaeon is largely unknown [17] Further Studies shown that the most important factor in determining halobacterial growth is NaCl concentration. Generally, it is assumed that HS requires a high concentrations of NaCl, a solute that cannot be replaced, to maintain their glycoprotein cell wall structure that is negatively charged , as it is contains a high number of acidic amino and is stabilize only in the presence of a high sodium cation concentration. The positive charge of sodium prevents the negative charges from repelling each other and lysing the cell [30]. HS requires more than three M mol of NaCl for growth and grows best at 30°C. NaCl concentration reduction causes structural deformations of halobacterial cells. A recent study done by C. Zenget et al has shown that the optimum NaCl concentrations is 3.9 M mol and not 5.5 M mol, as mentioned in most previous literature. Further investigation of the influence of NaCl concentration on HS growth in the same study found that increasing NaCl concentration above 3.9 M mol, the cell start to lyse [17].
In terms of energy production, HS has diverse ways of generating energy. HS
has the ability to oxidize various metabolites under aerobic conditions. HS is also a facultative phototrophe that can carry out photosynthesis. When respiration and nutrients are plentiful, HS will seek darkness. It can also grow in the absence of both respiration and photorespiration through the fermentation of arginine that can be only substituted by serine [29 ]
HS is capable of both chemo- and phototaxis. It contains four different retinal photosynthetic proteins that are structurally similar involved in light energy conversion and signal transduction. Bacteriorhodopsin (BR) and Halorhodopsin (HR) are the light-driven ion pumps. BR is a proton pump that converts light energy into a proton gradient, while HR is a chloride pump that allows HS to maintain the high internal salt concentration. Sensory rhodopsin I and II are photoreceptor pigments that act as light sensors that control the swimming behavior of the cell. SRI enables both a photophilic response to orange light and a photophobic response to UV light, while SRII enables a photophopic response to blue light. HtrII and HtrI are the accessory transducer proteins of SRII and SRI, respectively. The function of these proteins is to transmit photosignals from the receptor to proteins in the cytoplasm that control the action of the flagellar motor. Moreover, HtrII from HS acts as a chemotransducer to sense serine [14, 22, 24, 26, 28, and 29]. According to Mironova, O et al, “HtrII is the only archaeal transducer that accepts signals from two different sources.” [24].
Under constant environmental conditions, HS swims by means of rotating flagella, and in the absence of a stimulus performs a random walk. HS reverses its swimming direction about every 09-29 seconds, based on the spontaneous switching of the flagella motor from clockwise (CW, forward swimming) to counterclockwise (CCW, reverse swimming) or vice-versa. This switching causes a reversal of the swimming direction. CW movement of flagella is a response toward photophilic signals from SRI that helps HS to move toward the orange light signals where HR and BR are most active. CCW movement is a photophobic response from SRI to avoid harmful UV light, and SRII to avoid blue light. HS seeks darkness, where there are plenty of nutrients [28] HS is responsible for the bright pink or red appearance of the Dead Sea and other bodies of salt water due to BR. BR was discovered by D. Oesterhelt et al in 1973 [30], and is apoprotein encoded by bop gene and linked to retinal proteins. It serves as the active photosynthetic pigment that allows the archaeon to live with light as the only energy source [6, 29]. BR is expressed under anaerobic growth conditions [13]. Although the detailed molecular mechanism of proton translocation of BR is not completely understood [22], BR itself has structural similarities with the seven proteins that are found in nerve cells of higher animals.
Recently, a new homologue of ferritin, DpsA, has been found in HS as a
DNA-protecting protein under starvation conditions. DpsA is a true ferritin, but differs from the function and regulation of other ferritins. These findings strongly suggest that not all functions of ferritins are yet known. This is of special interest, because the Dps-ferritin of HS appears to operate with typical modulating regulators of bacteria, while the activity of the transcription apparatus is like that of eukarya [16, 19].
What can HS do? Scientists are working to blend the genes of Halophiles, including HS, with crop genes to make plants more tolerant to soil with a higher than average salinity [5, 23]. Halophiles are studied by astrologists at NASA, who propose that Halophiles may be representatives of life forms that may exist elsewhere in the universe, and HS may be a representative of life in the deep ocean of Europa, one of Jupiter’s moons [9]. HS is also one of the few reported organisms which can use large potassium gradients in a battery-like manner to serve as long term energy storage. Moreover, HS contains enzymes such as amylase, lipase and protease that are both stable and active under high salt concentrations, but have not been studied for related applications [20].
HS has received much interest as it is the key organism for producing BR, which is currently the only known structure that allows non-chlorophyll based photosynthesis. BR is currently being developed for applications in optical security, [7] and optical data storage [8]. In a recent study by B. Zabut et al, HS has been used in hydrogen gas production as a key organism that produces BR, but lacks both the system and the enzymes that can reduce protons into molecular hydrogen. To produce hydrogen gas, packed cells of HS or its plasma membrane combined with Rhodobacter sphaeroides (RS) are used in a photobioreactor. RS is the most promising photosynthetic bacteria of the several species of microorganisms that have been found to produce hydrogen, due to its high activity in hydrogen production under anaerobic conditions. Photobiological hydrogen could be an environmentally acceptable energy production method due to the fact that hydrogen gas is a renewable energy source. However, the photobioreactor process still needs serious improvements to become feasible for energy production [7].
Industrially-produced highly saline environments are frequently contaminated by toxic organic compounds. Microorganisms that are able to degrade organic compounds under high saline conditions would be valuable for their ability to “clean” out these environments. Based on the fact that Haloarchaea possesses some degree of organic degrading capacity, Dong-Jin Ha et al suggest the usage of HS as a biological treatment tool for highly saline industrial waste effluents that contaminate the environment. Their study evaluated the usage of HS in order to degrade the IPA (isopropyl alcohol) that is used in a number of industries, including pharmaceuticals, textile production, and cosmetics. Their results indicate that the GAPDH isolated from HS may be valuable in industries involving IPA processing [20].
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