Halobacterium volcanii
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Also known as Haloferax volcanii
Description and Significance
Haloferax volcanii is an archaeon that can survive in environments of extreme salt concentrations. Halobacteria are located throughout the world in salt ponds and lakes, and may exist in the form of dormant or living cells, biopolymers in rocks, salt crystals, or as evaporates in desert regions.7 Haloferax volcanii in particular resides largely and predominantly in the bottom sediment of the Dead Sea, and are distinct from other organisms in their class in a few ways.2
H. volcanii are presumed to have been among the first living organisms on Earth when environmental conditions were much harsher than they are now. Though this is still highly debated, such a notion would correlate with the suggestion that H. volcanii may be present on and able to withstand the harsh conditions of Mars.7 Along with their ability to withstand harsh conditions is H. volcanii’s extensive ability for DNA repair. H. volcanii are also the first kind of archaeon to show horizontal gene transfer involving phage-mediated transduction, assimilation of naked DNA and conjugation, demonstrating importance and implications of surface proteins.6
Genome structure
Only one strain of Haloferax volcanii (Haloferax volcanii DS2) has a mapped genome. DS2 contains 4.01 million base pairs with approximately 4209 predicted genes. The most recent genome sequence draft was performed in April of 2007 by the Institute for Genomic Research. DS2 is found to have one chromosome and four plasmids. The chromosome has 2847757 base pairs and is 66.64% GC. Plasmid pHV4 has 635786 base pairs and is 61.67% GC. pHV 3 is 437906 base pairs and is 65.56% GC. pHV2 is 6359 base pairs and is 56.06% GC. Lastly, plasmid pHV1 is 85092 base pairs and is 55.5% GC.4
Cell structure and metabolism
There are two distinct properties in which Haloferax volcanii differ from other halobacteria. First, the cells of H. volcanii are disc shaped and cupped when grown under optimum conditions (highly saline, and a temperature of 42 degrees Celsius, although they can grow at 37 degrees Celsius). Secondly, optimum requirements for NaCl are 1.7 to 2.5M which is twice the range value that is normally seen for other halobacteria. Their tolerance for MgCl2 is also much higher than other halobacteria.2
In general, Haloferax volcanii are gram positive, irregularly shaped rods, discs, or cups. They contain no endospores, are usually motile, and are at the minimum moderately halophilic. They are facultative anaerobes, meaning that they prefer to be in oxygenic environments but can survive in the lack of oxygen and always in aquatic environments and are chemoorganotrophs which means that in addition to using oxygen for respiration, they need carbon as their energy source.5 Cellular potassium ions help H. volcanii cells to survive lysis in high salt concentrations, while their pigments serve as a shield against ultraviolet light and also as a way to increase temperature by absorbing sunlight.7 Their highly negatively charged surfaces (due to a negatively charged amino terminus of amino acids) makes them more soluble and flexible at extreme salt concentrations.6
All archea have chaperonins that are similar to type II chaperonins found in eukaryotic organisms. Most archea also contain several heat shock proteins (Hsp), and those that do not usually contain the genes that encode for them. H. volcanii were used to determine that only one of these three genes are required for the organism to grow, though each does allow for functional specialization when present.9
H. volcanii are also the species of halobacteria and microorganisms that have helped to establish the importance of surface proteins. Seen under a resolution of 2nm, it has been determined that H. volcanii have dome-shaped, morphological complexes with pores at their apex which open into a funnel in the direction of the cell membrane. These complexes (cell envelopes) are also capable of establishing lateral connectivity, also important to the cell’s structure.3
Ecology
Haloferax volcanii have many features that allow them to interact in a unique way with their environment. As suggested by their preponderance at the bottom of the Dead Sea where other organisms cannot survive, they have adapted to living in extreme environmental conditions. Among these is their ability to lie dormant for long periods of time in the event that environmental conditions are too unfavorable; once the conditions have moderated some, they return to their free-living state.7 The ability to lie dormant, specialized Hsp’s, and unparalleled DNA repair mechanisms give H. volcanii the ability to experience life and environments unattainable by almost all other microorganisms. Research has demonstrated that when the DNA of H. volcanii has been completely fragmented via exposure to extreme radiation not tolerated by any other microbe, H. volcanii reassembles completely and functionally in a matter of hours. This archaeon has also survived lethal amounts of UV exposure, extreme dryness, and a simulated vacuum (like that of outer space). It has further been determined that extreme salt concentrations lead to the same DNA mutations that radiation does. In this regard, H. volcanii already has available DNA repair mechanisms when exposed to radiation.8 Their extremophilic nature has lead scientists to experiment and believe that H. volcanii would be able to withstand environmental conditions on Mars that are so harsh no other organism tested has yet been able to survive.
Pathology
H. volcanii has no known pathogenic traits to any other microorganisms, animals, plants or humans.5