Halobacterium salinarum

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A Microbial Biorealm page on the genus Halobacterium salinarum

Classification

Archaea; Euryarchaeota; Halobacteria; Halobacteriales; Halobacteriaceae; Halobacterium; H. salinarium (4)

Description and significance

Halobacterium salinarum is not a bacterium, but is a model organism from the halophilic branch of Archaea (2). It was discovered 80 years ago when isolated from salted fish, long before the proposal for a third domain was put forward in 1978. It is classified as an extremophile due to its ability to survive in environments with very high salt concentrations. It is found in high salt food such as salt pork, marine fish, and sausages. It is also present in hides, hyper-saline lakes, and salterns. Due to their high salinity, these salterns become purple or reddish color with the presence of halophilic Archaea. As a species that colonizes salines, Halobacterium is known for its distinct color and presence in mass cultures seen at Great Salt Lake, Yellowstone National Park, and other places with saline levels around 4M+ (3).

Genome structure

Two strains of H. salinarum, NRC-1 and R1, have been fully sequenced by Ng et al. and Oesterhelt et al. respectively. NRC-1 has 2,571,010 base pairs, one large chromosome, and two mini-chromosomes. The large chromosome is very G-C rich (68%) which increases its stability. This is vital for the extreme environments this halophile is found in. There are also a number of megaplasmids present with 58% G-C. It is reported that strain NRC-1 has 2 megaplasmids and R1 has 2 megaplasmids. The genome encodes for 2,360 predicted proteins for NRC-1 and 2,837 proteins encoded for R1. Proteome comparisons link H. salinarum to Archaea with some similarities to bacteria similar to Gram-positive Bacillus subtilis (5,7).

Cell structure, metabolism & life cycle

H. salinarum is a rod-shaped, single-celled, motile microorganism that can live with only light as an energy source due to its retinal protein bacteriorhodopsin (light driven proton pump). It is classified as gram-negative even though there is no cell wall, instead there is a single lipid bilayer surrounded by an S-layer. The S-layer, made from glycoprotein, accounts for about 50% of the cell surface proteins that form a lattice in the membrane. This lattice is stable in high-salt conditions due to sulfate residues in the glycan chains of the glycoprotein that give it a negative charge (3).

H. salinarum contains 4 retinal proteins, which are photosynthetic pigments involved in light energy conversion and signal transduction. The proton pump bacteriorhodopsin permits halobacterium to grow with only light as an energy source by utilizing the proton gradient, such as in the process of ATP generation. Halorhodopsin helps maintain the salt concentration internally during growth. Sensory rhodopsin I uses phototaxis which mediates response to orange and UV lights. It also makes a complex with the transducer protein htrI. Sensory rhodopsin II uses phototaxis in response to blue light and makes a complex with the transducer protein htrII (1).

Halophiles grow in aerobic and anaerobic condition using three distinct systems for energy gain. Under aerobic conditions Halophiles oxidizes pyruvate and channels it into the tricarboxylic acid cycle. Photosynthesis is conducted by the bacteriorhodopsin using a proton gradient to drive ATP production. Fermentation of argentine also generates ATP (3).

Ecology (including pathogenesis)

Halobacteria, with its indicative reddish hue produced by the presence of bacteriorhodopsin, is found in saline lakes such as the Dead Sea and Lake Magradi. Its optimal growth temperature is 37°C.

This archaeon can adapt to extreme conditions involving high salt, low oxygen, and the presence of high amounts of UV radiation. It can survive high salt concentrations through utilizing compatible solutes such as potassium chloride in order to reduce osmotic stress. It has multiple active transporters to balance potassium levels and highly acidic proteins to prevent protein precipitation (6). H. salinarum can manage low oxygen due to its management of light-energy utilized by bacteriorhodopsin. They have also developed a DNA repair method to deal with high exposure to UV radiation. It also absorbs the UV light using bacterioruberin which produces the observed red color (3).

Interesting feature

DNA from between 121 and 419 million years ago was discovered in ancient salt deposits. This is the oldest genetic material ever found (9). This organism would have survived several mass extinctions. Six segments of the DNA had never been seen before in science. H. salinarum is a close genetic relative. Other previous discoveries had been made of ancient halophilic bacteria but contamination was an issue (8).

References

1. Andersson, M., Structural dynamics of light-driven proton pumps, Structure, 2009, 17(9):1265

2. DasSarma, S., Extreme Microbes, Am Sci, 2007, 95(3):224-231

3. Halobacterium salinarum. Membrane Biochemistry Dieter Oesterhelt. Max Planck Institute of Biochemistry.10/22/2011. http://mnphys.biochem.mpg.de/en/eg/oesterhelt/web_page_list/Org_Hasal/index.html

4. Halobacterium salinarum. NCBI taxonomy browser. 10/22/2011 http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=2242&lvl=3&lin=f&keep=1&srchmode=1&unlock

5. Ng,W. V., et al. Genome sequence of Halobacterium species NRC-1. Proc Natl Acad Sci USA. 2000. vol 92, no 22; 12176-12181

6. Pérez-Fillol, M., Rodrguez-Valera, F., Potassium ion accumulation in cells of different halobacteria, Microbiologia, 1986, 2(2):73-80.

7. Pfeiffer, F., Schuster, S.C., Broicher, A., Falb, M., Palm, P., Rodewald, K., et al., Evolution in the laboratory: The genome of Halobacterium salinarum strain R1 compared to that of strain NRC-1, Genomics, 2008, 91(4):335-346.

8. Reilly, Michael, "World's oldest known DNA discovered". The Discovery Channel. Updated 12/17/2009.

9. Vreeland, H; Rosenzweig, W D; Lowenstein, T; Satterfield, C; Ventosa, A (December 2006). "Fatty acid and DNA analyses of Permian bacteria isolated from ancient salt crystals reveal differences with their modern relatives".Extremophiles 10 (1): 71–8. doi:10.1007/s00792-005-0474-z