''Halobacterium'' (common abbreviation ''Hbt.'') is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
in the family
Halobacteriaceae
In taxonomy, the Halobacteriaceae are a family of the Halobacteriales in the domain Archaea.See the NCBIbr>webpage on Halobacteriaceae Data extracted from the ''Halobacteriaceae'' represent a large part of halophilic Archaea, along with member ...
.
The
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
''Halobacterium'' ("salt" or "ocean bacterium") consists of several species of
Archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
with an
aerobic
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cellu ...
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
which requires an environment with a high concentration of
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
; many of their
proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
will not function in low-salt environments. They grow on
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s in their aerobic conditions. Their
cell walls are also quite different from those of
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
, as ordinary
lipoprotein
A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, sur ...
membranes
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
fail in high salt concentrations. In shape, they may be either rods or
cocci
A coccus (plural cocci) is any bacterium or archaeon that has a spherical, ovoid, or generally round shape. Bacteria are categorized based on their shapes into three classes: cocci (spherical-shaped), bacillus (rod-shaped) and spiral ( of whi ...
, and in color, either red or purple. They reproduce using
binary fission
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two digits (0 and 1)
* Binary function, a function that takes two arguments
* Binary operation, a mathematical operation that t ...
(by constriction), and are
motile
Motility is the ability of an organism to move independently, using metabolic energy.
Definitions
Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
. ''Halobacterium'' grows best in a 42 °C environment. The genome of an unspecified ''Halobacterium'' species, sequenced by
Shiladitya DasSarma, comprises 2,571,010 bp (base pairs) of DNA compiled into three circular strands: one large chromosome with 2,014,239 bp, and two smaller ones with 191,346 and 365,425 bp. This species, called ''Halobacterium'' sp. NRC-1, has been extensively used for postgenomic analysis. ''Halobacterium'' species can be found in the
Great Salt Lake
The Great Salt Lake is the largest saltwater lake in the Western Hemisphere and the eighth-largest terminal lake in the world. It lies in the northern part of the U.S. state of Utah and has a substantial impact upon the local climate, particula ...
, the
Dead Sea
The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Bank ...
,
Lake Magadi
Lake Magadi is the southernmost lake in the Kenyan Rift Valley, lying in a catchment of faulted volcanic rocks, north of Tanzania's Lake Natron. During the dry season, it is 80% covered by soda and is well known for its wading birds, including f ...
, and any other waters with high salt concentration. Purple ''Halobacterium'' species owe their color to
bacteriorhodopsin
Bacteriorhodopsin is a protein used by Archaea, most notably by haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting ...
, a light-sensitive protein which provides chemical energy for the cell by using sunlight to pump protons out of the cell. The resulting proton gradient across the cell membrane is used to drive the synthesis of the energy carrier
ATP. Thus, when these protons flow back in, they are used in the synthesis of ATP (this proton flow can be emulated with a decrease in pH outside the cell, causing a flow of H
+ ions). The bacteriorhodopsin protein is chemically very similar to the light-detecting pigment rhodopsin, found in the vertebrate retina.
Species of ''Halobacterium''
![Halobacterium salinarum NRC-1](https://upload.wikimedia.org/wikipedia/commons/8/8d/Halobacterium_salinarum_NRC-1.png)
* ''Halobacterium cutirubrum'' > ''Halobacterium salinarum''
* ''Halobacterium denitrificans'' > ''Haloferax denitrificans''
* ''Halobacterium distributum'' > ''Halorubrum distributum''
* ''Halobacterium halobium'' > ''Halobacterium salinarum''
* ''
Halobacterium jilantaiense''
* ''Halobacterium lacusprofundi'' > ''Halorubrum lacusprofundi''
* ''Halobacterium mediterranei'' > ''Haloferax mediterranei''
* ''
Halobacterium noricense''
* ''Halobacterium pharaonis'' > ''Natronomonas pharaonis''
* ''
Halobacterium piscisalsi''
* ''Halobacterium saccharovorum'' > ''Halorubrum saccharovoru''
* ''
Halobacterium salinarum
''Halobacterium salinarum'', formerly known as ''Halobacterium cutirubrum'' or ''Halobacterium halobium'', is an extremely halophilic marine obligate aerobic archaeon. Despite its name, this is not a bacterium, but a member of the domain Archaea ...
''
* ''Halobacterium sodomense'' > ''Halorubrum sodomense''
* ''Halobacterium trapanicum'' > ''Halorubrum trapanicum''
* ''Halobacterium vallismortis'' > ''Haloarcula vallismortis''
* ''Halobacterium volcanii'' > ''Halobacterium volcanii''
Genome structure
The ''Halobacterium'' NRC-1 genome is 2,571,010 bp compiled into three circular replicons. More specifically, it is divided into one large chromosome with 2,014,239 bp and two 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 constituting 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 ''Halobacterium''. 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 An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria.
The S-layers of both archaea and bacteria consists of a monomolecular layer composed of only one (or, in a few cases, two) i ...
made from the cell-surface glycoprotein. They 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. Though the cytoplasm retains an osmotic equilibrium with the hypersaline environment, the cell maintains a high potassium concentration using many active transporters.
Many ''Halobacterium'' species possess proteinaceous organelles called gas vesicles.
Ecology
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 adds to its reddish hue as well. An optimal temperature for growth has been observed at 37 °C.
''Halobacterium'' may be a candidate for a life form present on Mars. One of the problems associated with the survival on Mars is the destructive ultraviolet light. 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). In addition, ''Halobacterium'' makes pigments called bacterioruberins that are thought to protect cells from damage by ultraviolet light. The obstacle they need to overcome is being able to grow at a low temperature during a presumably short time when a pool of water could be liquid.
Applications
Food Industry
There is potential for ''Halobacterium'' species to be used in the food industry.
Some examples of uses can include the production of
Beta-Carotene, a pigment in halophilic bacteria that contributes to their red coloration, is used in the food industry as a natural food dye. Halophiles also produce degradative enzymes such as
lipase
Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...
s,
amylase
An amylase () is an enzyme that catalyses the hydrolysis of starch (Latin ') into sugars. Amylase is present in the saliva of humans and some other mammals, where it begins the chemical process of digestion. Foods that contain large amounts of ...
s,
protease
A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
s, and
xylanase
Endo-1,4-β-xylanase (EC 3.2.1.8, systematic name 4-β-D-xylan xylanohydrolase) is any of a class of enzymes that degrade the linear polysaccharide xylan into xylose, thus breaking down hemicellulose, one of the major components of plant cell w ...
s that are used in various food processing methods. Notable applications of these enzymes include enhancing the fermentation process of salty foods, improving dough quality for the baking of breads, and contributing to the production of coffee.
Bioremediation
Many species of halophilic bacteria produce
exopolysaccharide
Extracellular polymeric substances (EPSs) are natural polymers of high molecular weight secreted by microorganisms into their environment. EPSs establish the functional and structural integrity of biofilms, and are considered the fundamental comp ...
s (EPS) which are used industrially as
bioremediation
Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluent ...
agents.
Biosurfactants Biosurfactant usually refers to surfactants of microbial origin. Most of the biosurfactants produced by microbes are synthesized extracellularly and many microbes are known to produce biosurfactants in large relative quantities. Some are of commerci ...
are also released by many halophilic bacteria and these amphiphilic compounds have been used for soil remediation. Many halophiles are highly tolerant of heavy metals making them potentially useful in the bioremediation of
xenobiotic
A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural compo ...
compounds and heavy metals that are released into the environment from mining and metal plating. Halophiles contribute to the bioremediation of contaminants by converting xenobiotics into less toxic compounds.
Some ''Halobacterium'' species have been shown to be effective in the bioremediation of pollutants including aliphatic hydrocarbons, such as those found in crude oil; and aromatic hydrocarbons such as
4-hydroxybenzoic acid
4-Hydroxybenzoic acid, also known as ''p''-hydroxybenzoic acid (PHBA), is a monohydroxybenzoic acid, a phenolic derivative of benzoic acid. It is a white crystalline solid that is slightly soluble in water and chloroform but more soluble in pola ...
, a contaminant in some high salinity industrial runoffs.
Pharmaceuticals
Some strains of ''Halobacterium'', including ''
Halobacterium salinarum
''Halobacterium salinarum'', formerly known as ''Halobacterium cutirubrum'' or ''Halobacterium halobium'', is an extremely halophilic marine obligate aerobic archaeon. Despite its name, this is not a bacterium, but a member of the domain Archaea ...
'', are being explored for medical applications of their radiation-resistance mechanisms. Bacterioruberin is a carotenoid pigment found in ''Halobacterium'' which decreases the bacteria’s sensitivity to γ-radiation and UV radiation.
It has been shown in knockout studies, that the absence of bacterioruberin increases the sensitivity of the bacterium to oxidative DNA-damaging agents. Hydrogen peroxide, for example, reacts with bacteroruberin which prevents the production of
reactive oxygen species
In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
The reduction of molecular oxygen () p ...
, and thus protects the bacterium by reducing the oxidative capacity of the DNA-damaging agent.
''H. salinarum'' also exhibits high intracellular concentrations of potassium chloride which has also been shown to confer radiation resistance. ''Halobacterium'' are also being explored for the pharmaceutical applications of bioactive compounds they produce, including anticancer agents, antimicrobial biosurfactancts, and antimicrobial metabolites.
Significance and applications
Halobacteria are halophilic microorganisms that are currently being studied for their uses in scientific research and biotechnology. For instance, genomic sequencing of the ''Halobacterium'' species NRC-1 revealed their use of eukaryotic-like RNA polymerase II and translational machinery that are related to ''
Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'' and other Gram-negative bacteria. In addition, they possess genes for DNA replication, repair, and recombination that are similar to those present in bacteriophages, yeasts, and bacteria. The ability of this ''Halobacterium'' species to be easily cultured and genetically modified allows it to be used as a
model organism
A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
in biological studies. Furthermore, ''Halobacterium'' NRC-1 have also been employed as a potential vector for delivering vaccines. In particular, they produce gas vesicles that can be genetically engineered to display specific epitopes. Additionally, the gas vesicles demonstrate the ability to function as natural adjuvants to help evoke stronger immune responses. Because of the requirement of Halobacteria for a high-salt environment, the preparation of these gas vesicles is inexpensive and efficient, needing only tap water for their isolation.
Halobacteria also contain a protein called
Bacteriorhodopsins which are light-driven proton pumps found on the cell membrane. Although most proteins in halophiles need high salt concentrations for proper structure and functioning, this protein has shown potential to be used for biotechnological purposes because of its stability even outside of these extreme environments. Bacteriorhodopsins isolated from ''Halobacterium salinarum'' have been especially studied for their applications in electronics and optics. Particularly, bacteriorhodopsins have been used in holographic storage, optical switching, motion detection, and
nanotechnology
Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal o ...
. Although numerous uses of this protein have been presented, there are yet to be any high-scale commercial applications established at this time.
Recombination and mating
UV irradiation of ''Halobacterium'' sp. strain NRC-1 induces several gene products employed in
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
.
For instance, a
homolog
In biology, homology is similarity due to shared ancestry between a pair of structures or genes in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats and birds, the arms of prima ...
of the ''
rad51
DNA repair protein RAD51 homolog 1 is a protein encoded by the gene ''RAD51''. The enzyme encoded by this gene is a member of the RAD51 protein family which assists in repair of DNA double strand breaks. RAD51 family members are homologous to th ...
''/''
recA
RecA is a 38 kilodalton protein essential for the repair and maintenance of DNA. A RecA structural and functional homolog has been found in every species in which one has been seriously sought and serves as an archetype for this class of homolog ...
'' gene, which plays a key role in recombination, is induced 7-fold by UV. Homologous recombination may rescue stalled replication forks, and/or facilitate recombinational repair of DNA damage.
In its natural habitat, homologous recombination is likely induced by the UV irradiation in sunlight.
''Halobacterium volcanii'' has a distinctive mating system in which cytoplasmic bridges between cells appear to be used for transfer of DNA from one cell to another.
In wild populations of ''Halorubrum'', genetic exchange and recombination occur frequently.
This exchange may be a primitive form of sexual interaction, similar to the more well studied bacterial transformation that is also a process for transferring DNA between cells leading to
homologous recombination
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
al repair of DNA damage.
Further reading
Scientific journals
*
DasSarma, S., B.R. Berquist, J.A. Coker, P. DasSarma, J.A. Müller. 2006. Post-genomics of the model haloarchaeon Halobacterium sp. NRC-1
Saline Systems 2:3.
*
*
Scientific books
*
DasSarma, S. 2004. Genome sequence of an extremely halophilic archaeon, in Microbial Genomes, pp. 383–399, C.M. Fraser, T. Read, and K.E. Nelson (eds.), Humana Press, Inc., Totowa, NJ.
*
Lynn Margulis
Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolutionary biologist, and was the primary modern proponent for the significance of symbiosis in evolution. Historian Jan Sapp has said that "Lynn Ma ...
, Karlene V.Schwartz, ''Five Kingdoms. An Illustrated Guide to the Phyla of Life on Earth'' (W.H.Freeman, San Francisco, 1982) pp. 36–37
*
*
*
References
External links
The ''Halobacterium'' Genome
{{Taxonbar, from=Q139014
Archaea genera
Halophiles
Phototrophic bacteria
Euryarchaeota