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].

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 intercellular DNA exchange via conjugation. In 1984, the species was the first archaeon ever to be observed undergoing genetic transfer [6].

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; if placed in distilled water, H. volcanii cells will lyse [3].

Because of the high osmolarity of its habitat, H. volcanii must be able to counteract osmotic pressures which would rupture non-halophilic microorganisms. Most other halophiles maintain a high cytoplasmic concentration of organic solutes (e.g., sugars) to halt this process; however, members of order Halobacteriales (including H. volcanii) instead maintain a high internal concentration of K⁺ ions to counterbalance the hypertonicity of the surroundings, making them unique among the halophiles of all 3 domains of life [3].

Additionally, H. volcanii contains large amounts of C₅₀-carotenoid compounds in its cell membrane. These pigments have a red color, which becomes visually apparent when the cells are present in sufficient concentrations. One of these carotenoids, 3′,4′-epoxymonoanhydrobacterioruberin, is unique to H. volcanii [9]. It is theorized that the membrane-embedded pigments protect internal cellular structures from damaging solar radiation [10].

Metabolism

A chemooganotroph, H. volcanii is facultatively anaerobic. As a carbon source, polysaccarides, glucose, other sugars, or amino acids can be catabolized. Oxygen is preferred as a terminal electron acceptor; however, oxygen solubility in brine is much reduced, necessitating the inclusion of anaerobic pathways in cellular respiration [3]. In the absence of sufficient oxygen, fumarate, nitrate, DMSO, or TMAO can be employed as alternative electron acceptors [11].

Glucose catabolism utilizes a modified Entner-Douderoff pathway, delaying the phosphorylation step until after glucose is oxidized. The Embden-Meyerhoff pathway is not employed, though H. volcanii possesses EMP-like enzymes for other purposes [3].

As a result of the high internal K⁺ ion gradient, metabolic enzymes must be able to function in the highly saline cytoplasmic environment. H. volcanii possesses halophilic enzymes specifically adapted for functioning in high salt concentrations. [12].

Life Cycle

H. volcanii reproduces exclusively by binary fission. Under ideal growth conditions, doubling time is approximately 4 hours [3].

Ecology (including pathogenesis)

H. volcanii is the predominant microorganism in the sediment of the Dead Sea. A few other related halophiles, such as Haloarcula marismortui and Halorubrum sodomense, are its primary competitors [13]. Owing to the harsh hypersaline environment of the sea, however, few organisms exist which can survive in the magniesium-rich brine. The carotenoid pigments of H. volcanii and the other extremophilic archaea living in the Dead Sea are known to occasionally imbue the waters with a reddish hue, when conditions favor blooms of microbial growth [7].

Increasingly, the Dead Sea is becoming less and less hospitable to even extremophiles such as H. volcanii. The salinity of the sea is increasing over time, due to both natural factors and human activity. As a result, microbial activity and diversity in the Dead Sea has diminished sharply since 1980. Though it is still able to survive, levels of H. volcanii in the sea are currently at a minimum relative to the time span over which measurements have been recorded. Blooms of H. volcanii are increasingly rare, and usually coincide with dilution of the upper strata of the sea by a temporary influx of fresh water [13].

H. volcanii is not a known pathogen of any plant or animal species.

Interesting feature

The conditions in which Haloferax volcanii is able to survive are very similar to that found on the Martian surface (i.e, high salt and high radiation.) As a result, H. volcanii is being evaluated as a model organism to test the survivability of Earth-borne extremophiles on the surface of Mars [14].