See also ''Natronococcus''
Higher order taxa:
Archaea; Euryarchaeota; Halobacteria; Halobacteriales; Halobacteriaceae
Halobacterium sp. NRC-1, H. salinarum
Description and Significance
Halobacteria are halophilic microorganisms, which means they grow in extremely high salinity environments. This archaeon can act as a good model for some aspects of eukaryotic biology, such as DNA replication, transcription, and translation. Comparing a halophile genome to that of other prokaryotes should give insight into microbial adaptation to extreme conditions.
The Halobacterium NRC-1 genome is 2,571,010 bp compiled into 3 circular replicons. More specifically, it is divided into one large chromosome with 2,014,239 bp and 2 small replicons pNRC100 (191,346 bp) and pNRC200 (365,425 bp). While much smaller than the large chromosome, the two plasmids account for most of the 91 insertion sequences and include genes for a DNA polymerase, seven transcription factors, genes in potassium and phosphate uptake, and cell division. The genome was discovered to contain a high G+C content at 67.9% on the large chromosome and 57.9% and 59.2% on the two plasmids. The genome also contained 91 insertion sequence elements comprised of 12 families, including 29 on pNRC100, 40 on pNRC200, and 22 on the large chromosome. This helps explain the genetic plasticity that has been observed in Halobacteria. Of the archaea, Halobacteria are viewed as being involved in the most lateral genetics (gene transfer between domains) and a proof that this transfer does take place.
Cell Structure and Metabolism
Halobacterium species are rod shaped and enveloped by a single lipid bilayer membrane surrounded by an S-layer made from the cell-surface glycoprotein. Halobacteria grow on amino acids in aerobic conditions. Although Halobacterium NRC-1 contains genes for glucose degradation as well as genes for enzymes of a fatty acid oxidation pathway, it does not seem able to use these as energy sources. Even though the cytoplasm retains an osmotic equilibrium with the hypersaline environment, the cell maintains a high potassium concentration. It does this by using many active transporters.
Halobacteria can be found in highly saline lakes such as the Great Salt Lake, the Dead Sea, and Lake Magadi. Halobacterium can be identified in bodies of water by the light-detecting pigment bacteriorhodopsin, which not only provides the archaeon with chemical energy, but gives it a reddish hue as well. An optimal temperature for growth has been observed at 37oC.
On an interesting note, however, Halobacteria are a candidate for a life form present on Mars. One of the problems associated with the survival on Mars is the destructive ultraviolet light. Halobacteria have an advantage here. These microorganisms develop a thin crust of salt that can moderate some of the ultraviolet light. Sodium chloride is the most common salt and chloride salts are opaque to short-wave ultraviolet. Their photosynthetic pigment, bacteriorhodopsin, is actually opaque to the longer wavelength ultraviolet (its red color). The obstacle Halobacteria would need to overcome is being able to grow at a low temperature during a presumably short time span when a pool of water could be liquid.
Ng et al. 2000. Genome sequence of Halobacterium species NRC-1. PNAS vol 92, no 22; 12176-12181.
Landis, Geoffrey A. 2001. Martian Water: Are There Extant Halobacteria on Mars?. Astrobiology vol. 1 no. 2: 161-164.
Koonin, E.V., K.S. Makarova, and L. Aravind. 2001. Horizontal Gene Transfer in Prokaryotes: Quantification and Classification. Annu. Rev. Microbiol. 55:709-42.