BIOL/CSES 4684

HALOBACTERIUM

SOIL MICROBIOLOGY

This webpage was created by Amy Shober

1. IDENTIFYING CHARACTERISTICS

Strict aerobes (evidence that some may be facultatively anaerobic).
Irregular rod-shaped cells.
0.5-1.2 um x 1.0-6.0 um in size.
Cells stain Gram negative.
Nearly all are motile.
Chemoheterotrophic.
Can undergo photophosphorylation.
Lack peptidoglycan in cell wall.
May form gas vacuoles.
No spores or resting stages.
Oxidase positive.
The percent G+C ranges from 66 to 71%.
Colonies are usually red or purple in color.
Survive best in 12-15% salt concentration.

The above picture shows H. salinarum under phase contrast microscopy.

2. TAXONOMIC DESCRIPTION
Amino acids are commonly used as the carbon source for Halobacterium, however, some may utilize carbohydrates. Most species are motile by means of one or more polar flagella; they are rarely non-motile. For buoyancy regulation several species produce gas vacuoles. Membranes and proteins are well adapted to life in ionic environments and osmotic balance is maintained by accumulation of inorganic K⁺ ions inside the cell. Metabolism is aerobic with oxygen the primary electron acceptor. However, when oxygen levels in the highly saline environment diminish, Halobacteria can produce bacteriorhodopsin in the cell membrane. This bacteriorhodopsin colors the cell purple and uses light energy to produce ATP for energy. Most strains of Halobacteria grow best at neutral pH between 5 and 8.

Bergey’s Manual recognizes only one species in the genus Halobacterium. This species is H. salinarum. Although only one species is currently recognized, there are many strains that are currently acknowledged. These strains will most likely be defined as new species in the future.

The above picture is a scanning electron micrograph (150,000X) of H. salinarum.

3. ISOLATION AND ECOLOGY
There are many media that can be used for the isolation of Halobacterium. Typically these have high salt (NaCl and Mg salts) and amino acid concentrations. Some media that have been used include dried solar salts, salt from salt lakes, and nutrients extracted from fish juices or milk. Halobacterium colonies are often red in color due to the presence of carotenoids, but colonies producing bacteriorhodopsin are purple in color.

Halobacteria, which require approximately 12-15% salt concentration for growth, have been isolated from many types of saline environments. These include salt seas, salt lakes, rock salt, and saline soils. The red carotenoids of the bacteria are responsible for the red appearances of salt seas, such as the Red Sea. Halobacteria can also be found on salted fish, salted hides, bacon, and sausage. These microorganisms can often be attributed to the spoilage of these foods.

There are currently few applications for species in the genus Halobacterium. There is, however, research being conducted concerning the use of Halobacteria for clean up of Department of Defense sites. Research is also being conducted dealing with the use of Halobacterium enzymes to increase the amount of crude oil recovered from underground wells.

The above picture shows a bloom of Halobacteria in a saline pond at a salt works near San Quintin, Mexico.

4. ADDITIONAL SOURCES OF INFORMATION

Holt, J.G.; Kreig, N.R.; Sneath, P.H.A.; Staley, J.T.; Williams, S.T. 1994. Bergey’s Manual of Determinative Bacteriology, 9th ed. Williams and Wilkins. Baltimore, Maryland.

Madigan, M.T.; Martinko, J.M.; Parker, T. 1997. Brock Biology of Microorganisms. Prentice Hall. Upper Saddle River, New Jersey.

Javor, Barbara. 1989. Hypersaline Environments; Microbiology and Biogeochemistry. Springer-Verlag. Berlin.

Tortora, G.J.; Funke, B.R.; Case, C.L. 1996. Microbiology: An Introduction. The Benjamin/ Cummings Publishing Co. Redwood City, California.

Vreeland, R.H.; Hochstein, L.I. 1993. The Biology of Halophilic Bacteria. CRC Press. Boca Raton, Florida.

5. LINKS TO OTHER SITES ON HALOBACTERIUM.
Halophiles and Halophilic Environments. This site contains information on many halophiles including the genus Halobacterium.

The Univeristy of Wisconsin: Major Groups of Prokaryotes. This site contains information on Archea Bacteria including a discussion of Halophiles.