Haloferax volcanii

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A Microbial Biorealm page on the genus Haloferax volcanii

Classification

Higher Order Taxa

Domain: Archaea
Phylum: Euryarchaeota
Class: Halobacteria
Order: Halobacteriales
Family: Halobacteriaceae
Genus: Haloferax

Species

Taxonomy of Haloferax volcanii

Description and significance

Haloferax volcanii (formerly Halobacterium volcanii) was first identified in the 1930s by microbiologist Benjamin Elazari Volcani, for whom the species is named [1]. It is a moderate halophile and a mesophile, in addition to being mildly acidophilic, and can be found living in the sediment of the Dead Sea, a salt lake in Israel. The hypersaline water of the Dead Sea contains a high concentration of sodium, magnesium, and calcium salts, and is very slightly acidic; correspodingly, these conditions are ideal for growth of H. volcanii. It is one of only a small number of extremophiles adapted to survive in the harsh environment of the Dead Sea [7].

Haloferax volcanii is remarkable because it is an extremophile which can be cultured without much difficulty in vitro. It is increasingly finding use as a model organism in studies of archaeal genetics [5]. In culture, fastest growth occurs at NaCl concentrations of 1.5-2.5M, and Mg2+ concentrations of up to 1.5M, at a pH slightly below 7 and a temperature of 45 °C. It is incapable of surviving in NaCl solutions in excess of 5M [2]. Members of H. volcanii are pleotropic, and stain Gram-positive[2]. Like many halophiles, it maintains a high concentration of carotenoid pigments in its cell membrane, giving colonies of H. volcanii a reddish color [3]. H. volcanii is a chemoorganotroph, preferentially metabolizing sugars as a carbon source [3]. It is primarily aerobic, but is capable of anaerobic respiration under anoxic conditions [4].


Describe the disease caused by this organism if it is a pathogen, or the natural macroscopic "field guide" appearance and habitat of your organism if it is not. What is or has been the impact your organism on human history or our environment?. How does it do this? How have we harnessed this power, or tried to prevent it? In other words, how do you know it if you see it, and how does its presence influence humans in the present, and historically?

Genome structure

The genome of the DS2 strain of H. volcanii has been completely sequenced. The genome consists of one large circular chromosome approximately 2.85 Mb in length; three smaller circular chromosomes ranging in size between 85 kb and 636 kb; and a single plasmid of approximately 6.4 kb. In total, the complete genome length is approximately 4.01 Mb. It contains a predicted 4,063 protein-encoding genes [5].

The base pairing frequency in the genome of H. volcanii is heavily skewed in favor of GC pairing, comprising 65% of all base pairs. Atypically, the main chromosome possesses 2 origins of replication, a trait which is rare in archaea and almost nonexistent in bacteria. It also lacks any apparent genes which encode for RNA polyadenylation enzymes [5].

H. volcanii microbes are capable of plasmid exchange via conjugation. In 1984, the species was the first archaeon ever to be observed undergoing genetic transfer [6].

Describe the size and content of the genome. How many chromosomes and plasmids? Circular or linear? Other interesting features? What is known about its sequence?

Cell Structure, Metabolism & Life Cycle

Cell Structure

An individual H. volcanii archaeon can vary from 1-3 micrometers in diameter. Its variable pleomorphic appearance can resemble anything from curved discs to dome-like or cup-like shapes; this variation is observed even under ideal growth conditions [2]. H. volcanii possesses an S-layer cell wall composed of glycoprotein. Additionally, the cell membrane is associated with 4 novel glycosylated proteins, which are outwardly oriented but distinct from the S-layer [8]. The glycoprotein wall is stable only under high salt concentrations.

Because of the high osmolarity of its habitat, H. volcanii must be able to counteract osmotic pressures which would rupture non-halophilic microorganisms.

Ecology (including pathogenesis)

Describe its habitat, symbiosis, and contributions to environment. If it is a pathogen, how does this organism cause disease? Human, animal, plant hosts? Describe virulence factors and patient symptoms.

Interesting feature

Describe in detail one particularly interesting aspect of your organism or it's affect on humans or the environment.

References

[1] Elazari-Volcani, B. "Bacteria in the Bottom Sediments of the Dead Sea." Nature. 1943. Volume 152, p. 274-275.

[2] Garrity, G.M., Castenholz, R.W., and Boone, D.R. (Eds.) Bergey's Manual of Systemic Bacteriology, Volume One: The Archaea and the Deeply Branching and Phototrophic Bacteria. 2nd ed. New York: Springer. 2001. p. 316.

[3] Oren, Aharon. "The Order Halobacteriales." The Prokaryotes: A Handbook on the Biology of Bacteria. 3rd ed. New York: Springer. 2006. p. 113-164.

[4] Zaigler, A., Schuster, S.C., and Soppa, J. "Construction and usage of a onefold-coverage shotgun DNA microarray to characterize the metabolism of the archaeon Haloferax volcanii." Molecular Microbiology. 2003. Volume 48, issue 4, p. 1089–1105.

[5] Hartman, A.L., et al. "The Complete Genome Sequence of Haloferax volcanii DS2, a Model Archaeon." PLoS One. 2010. Volume 5, issue 3.

[6] Mevarech, M., and Werczberger, R. "Genetic Transfer in Haloferax volcanii." Journal of Bacteriology. 1985. Volume 162, No. 1, p. 461-462.

[7] Oren, Aharon. "Population dynamics of halobacteria in the Dead Sea water column." Limnology and Oceanography. 1983. Volume 28, issue 6, p. 1094-1103.

[8] Eichler, J. "Novel glycoproteins of the halophilic archaeon Haloferax volcanii." Archives of Microbiology. 2000. Volume 173, p. 445–448.

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.