Arsenic biochemistry
Encyclopedia
Arsenic biochemistry refers to biochemical processes that can use arsenic
or its compounds, such as arsenate
. Arsenic is a moderately abundant element
on the earth's crust, and although many arsenic compounds are often considered highly toxic
, a wide variety of organoarsenic compound
s are produced biologically and various organic and inorganic arsenic compounds are metabolized by numerous organism
s. This pattern is general for other related elements, including selenium
, which can exhibit both beneficial and deleterious effects. Arsenic biochemistry has become topical since many arsenic compounds are highly toxic and it is found in some aquifer
s, potentially affecting many millions of people via processes that are intrinsically biochemical.
induced by arsenic. Arsenic's high toxicity naturally led to the development of a variety of arsenic compounds as chemical weapons, e.g. dimethylarsenic chloride. Some were employed as chemical warfare agents, especially in World War I
. This threat led to many studies on antidotes and an expanded knowledge of the interaction of arsenic compounds with living organisms. One result was the development of antidotes such as British anti-Lewisite
. Many such antidotes exploit the affinity of As(III) for thiolate ligand
s, which convert highly toxic organoarsenicals to less toxic derivatives. It is generally assumed that arsenates bind to cysteine
residues in proteins.
, once known as Gosio's gas is an intensely malodorous organoarsenic compound that is commonly produced by microbial action on inorganic arsenic substrates. A topical source of this compound are the green pigments once popular in wallpapers, e.g. Paris green
. A variety of illness have been blamed on this compound, although its toxicity has been exaggerated.
Other organoarsenic compounds found in nature are arsenobetaine
and arsenocholine, both being found in many marine organisms. Some As-containing nucleoside
s (sugar derivatives) are also known. Several of these organoarsenic compounds arise via methylation
processes. For example, the mold Scopulariopsis brevicaulis produce significant amounts of trimethylarsine
if inorganic arsenic is present. The organic compound arsenobetaine
is found in some marine foods such as fish and algae, and also in mushrooms in larger concentrations. The average person's intake is about 10–50 µg/day. Values about 1000 µg are not unusual following consumption of fish or mushrooms; however, there is little danger in eating fish since this arsenic compound is nearly non-toxic.
s, are contaminated with arsenic, which is released in roasting
(burning in air). In such processing, arsenide
is converted to arsenic trioxide
, which is volatile at high temperatures and is released into the atmosphere. Poultry and swine farms make heavy use of the organoarsenic compound roxarsone as an antibiotic in feed. Some wood is treated with copper arsenates as a preservative. The mechanisms by which these sources affect "downstream" living organisms remains uncertain but are probably diverse. One commonly cited pathway involves biomethylation.
, are progressively metabolised (detoxified) through a process of methylation
. The methylation occurs through alternating reductive and oxidative methylation reactions, that is, reduction of pentavalent to trivalent arsenic followed by addition of a methyl group
(CH3).
In mammals, methylation occurs in the liver by methyltransferase
s, the products being the (CH3)2AsOH (dimethylarsinous acid) and (CH3)2As(O)OH (dimethylarsinic acid), which have the oxidation states As(III) and As(V), respectively. Although the mechanism of methylation of arsenic in humans has not been elucidated, the source of methyl is methionine
, which suggests a role of S-adenosyl methionine
. Exposure to toxic doses begin when the liver's methylation capacity is exceeded or inhibited.
Studies in experimental animals and humans show that both inorganic arsenic and methylated metabolites cross the placenta
to the fetus
, however, there is evidence that methylation is increased during pregnancy and that it could be highly protective for the developing organism.
. The biological half-life
of inorganic arsenic is about 4 days, but is slightly shorter following exposure to arsenate than to arsenite. The main metabolites excreted in the urine of humans exposed to inorganic arsenic are mono- and dimethylated arsenic acid
s, together with some unmetabolized inorganic arsenic.
and quackery
that continues into the 21st century. Starting in the early 19th century and continuing into the 20th century, Fowler's solution
, a toxic concoction of sodium arsenite, was sold. The organoarsenic compound Salvarsan was the first synthetic
chemotherapeutic agent, discovered by Paul Ehrlich
. The treatment, however, led to many problems, causing long lasting health complications. Around 1943 it was finally superseded by penicillin
.
In vitro
studies suggest that arsenic trioxide
(As2O3) inhibits the proliferation of myeloma cells via cell cycle arrest as well as triggering cell death. These results suggest that arsenic trioxide may be a clinically useful treatment in patients with multiple myeloma
or leukemia
.
and RNA
contain phosphate
in their structure. A hypothetical analog would use arsenate
to partially or fully replace phosphate. A 2010 paper in Science
reports that GFAJ-1
, a proteobacterium from Mono Lake
, is facultatively capable of incorporating a small percentage of arsenate
into its DNA under conditions of phosphorus starvation. However, other scientists have expressed doubts that arsenate has replaced phosphate in the DNA of this organism and have suggested that procedural errors and interpretative errors may have been responsible.
providing evidence for the biogeochemical cycling and availability of arsenic (V) as an electron acceptor on primordial earth. A modern day example would be Searles
and Mono Lake
s, in the Western United States, which have been established to harbor an arsenic respiring biosphere
. Each of these lakes contain a substantial amount of dissolved inorganic arsenic that is exploited by prokaryotic life through arsenic (III) and arsenate (V) based metabolic reactions.
While arsenic tolerance and exploitation has been demonstrated to exist in these modern extreme biospheres, it has not yet been demonstrated to exist phosphorus-free. While some of the key properties of phosphate, such as its thermodynamic instability as well as its kinetic
stability, make it advantageous for Earth's biological life, arsenic may be more thermodynamically realistic for life on other planets or moons such as Saturn
's moon Titan
.
All known life forms use adenosine phosphate compounds (AMP
, ADP
and ATP
) to transfer chemical energy
. It is known that arsenic-bearing analogs of these compounds can form; however, it is unclear if they are ever utilized by living organisms. It has been observed that some microorganisms of genera Bacillus
and the unrelated family Halanaerobacteriales can use arsenic in its pentavalent oxidation state [arsenic (V)] as an electron acceptor
. The isolated microorganisms are observed to use lactate
, sulfide
, or hydrogen
as electron donors. In this role, arsenic effectively functions as an alternative to oxygen
. This form of chemolithotrophy does not require that the arsenic be incorporated into any of the organism's essential compounds.
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
or its compounds, such as arsenate
Arsenate
The arsenate ion is AsO43−.An arsenate is any compound that contains this ion. Arsenates are salts or esters of arsenic acid.The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As[V]....
. Arsenic is a moderately abundant element
Abundance of elements in Earth's crust
The table shows the abundance of elements in Earth's crust. Numbers show percentage or parts per million in mass; 10,000 ppm = 1%.Note that numbers are estimates, and they will vary depending on source and method of estimation. Order of magnitude of data can roughly be relied upon.The table shows...
on the earth's crust, and although many arsenic compounds are often considered highly toxic
Arsenic toxicity
Arsenic and many of its compounds are especially potent poisons. Arsenic disrupts ATP production through several mechanisms. At the level of the citric acid cycle, arsenic inhibits pyruvate dehydrogenase and by competing with phosphate it uncouples oxidative phosphorylation, thus inhibiting...
, a wide variety of organoarsenic compound
Organoarsenic compound
Organoarsenic chemistry is the chemistry of compounds containing a chemical bond between arsenic and carbon. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as insecticides, herbicides, and fungicides. In general these applications are declining in...
s are produced biologically and various organic and inorganic arsenic compounds are metabolized by numerous organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s. This pattern is general for other related elements, including selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
, which can exhibit both beneficial and deleterious effects. Arsenic biochemistry has become topical since many arsenic compounds are highly toxic and it is found in some aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
s, potentially affecting many millions of people via processes that are intrinsically biochemical.
Arsenic toxicity
Arsenic is a cause of mortality throughout the world; associated problems include heart, respiratory, gastrointestinal, liver, nervous and kidney diseases. Genotoxicity involves inhibition of DNA repair and DNA methylation. The carcinogenic effect of arsenic arises from the oxidative stressOxidative stress
Oxidative stress represents an imbalance between the production and manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage...
induced by arsenic. Arsenic's high toxicity naturally led to the development of a variety of arsenic compounds as chemical weapons, e.g. dimethylarsenic chloride. Some were employed as chemical warfare agents, especially in World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
. This threat led to many studies on antidotes and an expanded knowledge of the interaction of arsenic compounds with living organisms. One result was the development of antidotes such as British anti-Lewisite
Dimercaprol
Dimercaprol or British anti-Lewisite , is a compound developed by British biochemists at Oxford University during World War II. It was developed secretly as an antidote for lewisite, the now-obsolete arsenic-based chemical warfare agent. Today, it is used medically in treatment of arsenic,...
. Many such antidotes exploit the affinity of As(III) for thiolate ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s, which convert highly toxic organoarsenicals to less toxic derivatives. It is generally assumed that arsenates bind to cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
residues in proteins.
Organoarsenic compounds in nature
TrimethylarsineTrimethylarsine
Trimethylarsine is the chemical compound with the formula 3As, commonly abbreviated AsMe3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building block to other organoarsenic compounds, and serves as a ligand in coordination...
, once known as Gosio's gas is an intensely malodorous organoarsenic compound that is commonly produced by microbial action on inorganic arsenic substrates. A topical source of this compound are the green pigments once popular in wallpapers, e.g. Paris green
Paris Green
Paris Green is an inorganic compound more precisely known as copper acetoarsenite. It is a highly toxic emerald-green crystalline powder that has been used as a rodenticide and insecticide, and also as a pigment, despite its toxicity. It is also used as a blue colorant for fireworks...
. A variety of illness have been blamed on this compound, although its toxicity has been exaggerated.
Other organoarsenic compounds found in nature are arsenobetaine
Arsenobetaine
Arsenobetaine is an organoarsenic compound that is the main source of arsenic found in fish. It is the arsenic analog of trimethylglycine, commonly known as betaine...
and arsenocholine, both being found in many marine organisms. Some As-containing nucleoside
Nucleoside
Nucleosides are glycosylamines consisting of a nucleobase bound to a ribose or deoxyribose sugar via a beta-glycosidic linkage...
s (sugar derivatives) are also known. Several of these organoarsenic compounds arise via methylation
Methylation
In the chemical sciences, methylation denotes the addition of a methyl group to a substrate or the substitution of an atom or group by a methyl group. Methylation is a form of alkylation with, to be specific, a methyl group, rather than a larger carbon chain, replacing a hydrogen atom...
processes. For example, the mold Scopulariopsis brevicaulis produce significant amounts of trimethylarsine
Trimethylarsine
Trimethylarsine is the chemical compound with the formula 3As, commonly abbreviated AsMe3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building block to other organoarsenic compounds, and serves as a ligand in coordination...
if inorganic arsenic is present. The organic compound arsenobetaine
Arsenobetaine
Arsenobetaine is an organoarsenic compound that is the main source of arsenic found in fish. It is the arsenic analog of trimethylglycine, commonly known as betaine...
is found in some marine foods such as fish and algae, and also in mushrooms in larger concentrations. The average person's intake is about 10–50 µg/day. Values about 1000 µg are not unusual following consumption of fish or mushrooms; however, there is little danger in eating fish since this arsenic compound is nearly non-toxic.
Anthropogenic arsenic compounds
Anthropogenic (man-made) sources of arsenic, like the natural sources, are mainly arsenic oxides and the associated anions. Man-made sources of arsenic, include wastes from mineral processing swine and poultry farms. For example, many ores, especially sulfide mineralSulfide mineral
The sulfide minerals are a class of minerals containing sulfide as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, the sulfarsenides and the sulfosalts...
s, are contaminated with arsenic, which is released in roasting
Roasting (metallurgy)
Roasting is a step in the processing of certain ores. More specifically, roasting is a metallurgical process involving gas–solid reactions at elevated temperatures with the goal of purifying the metal component. Often before roasting, the ore has already been partially purified, e.g. by froth...
(burning in air). In such processing, arsenide
Arsenide
Arsenide is an arsenic anion with the charge −3. The trianion is formed by the reduction of arsenic by three electrons. For example heating arsenic powder with excess sodium gives sodium arsenide . The anions have no existence in solution since they are extremely basic...
is converted to arsenic trioxide
Arsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...
, which is volatile at high temperatures and is released into the atmosphere. Poultry and swine farms make heavy use of the organoarsenic compound roxarsone as an antibiotic in feed. Some wood is treated with copper arsenates as a preservative. The mechanisms by which these sources affect "downstream" living organisms remains uncertain but are probably diverse. One commonly cited pathway involves biomethylation.
Biomethylation of arsenic
Inorganic arsenic and its compounds, upon entering the food chainFood chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
, are progressively metabolised (detoxified) through a process of methylation
Methylation
In the chemical sciences, methylation denotes the addition of a methyl group to a substrate or the substitution of an atom or group by a methyl group. Methylation is a form of alkylation with, to be specific, a methyl group, rather than a larger carbon chain, replacing a hydrogen atom...
. The methylation occurs through alternating reductive and oxidative methylation reactions, that is, reduction of pentavalent to trivalent arsenic followed by addition of a methyl group
Methyl group
Methyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...
(CH3).
In mammals, methylation occurs in the liver by methyltransferase
Methyltransferase
A methyltransferase is a type of transferase enzyme that transfers a methyl group from a donor to an acceptor.Methylation often occurs on nucleic bases in DNA or amino acids in protein structures...
s, the products being the (CH3)2AsOH (dimethylarsinous acid) and (CH3)2As(O)OH (dimethylarsinic acid), which have the oxidation states As(III) and As(V), respectively. Although the mechanism of methylation of arsenic in humans has not been elucidated, the source of methyl is methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
, which suggests a role of S-adenosyl methionine
S-Adenosyl methionine
S-Adenosyl methionine is a common cosubstrate involved in methyl group transfers. SAM was first discovered in Italy by G. L. Cantoni in 1952. It is made from adenosine triphosphate and methionine by methionine adenosyltransferase . Transmethylation, transsulfuration, and aminopropylation are the...
. Exposure to toxic doses begin when the liver's methylation capacity is exceeded or inhibited.
Studies in experimental animals and humans show that both inorganic arsenic and methylated metabolites cross the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
to the fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
, however, there is evidence that methylation is increased during pregnancy and that it could be highly protective for the developing organism.
Excretion
In humans, the major route of excretion of most arsenic compounds is via the urineUrine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
. The biological half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of inorganic arsenic is about 4 days, but is slightly shorter following exposure to arsenate than to arsenite. The main metabolites excreted in the urine of humans exposed to inorganic arsenic are mono- and dimethylated arsenic acid
Arsenic acid
Arsenic acid is the chemical compound with the formula H3AsO4. More descriptively written as AsO3, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but only found in solution where it...
s, together with some unmetabolized inorganic arsenic.
Arsenic-based drugs
Despite, or possibly because of, its long-known toxicity, arsenic-containing potions and drugs have a history in medicineMedicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
and quackery
Quackery
Quackery is a derogatory term used to describe the promotion of unproven or fraudulent medical practices. Random House Dictionary describes a "quack" as a "fraudulent or ignorant pretender to medical skill" or "a person who pretends, professionally or publicly, to have skill, knowledge, or...
that continues into the 21st century. Starting in the early 19th century and continuing into the 20th century, Fowler's solution
Fowler's solution
Fowler's solution is a solution containing potassium arsenite that once was prescribed as a remedy or a tonic. A Dr. Fowler of Stafford, England proposed its use in 1786 as a substitute for a patent medicine, "tasteless ague drop." It was prescribed in the United States until the late 1950s for a...
, a toxic concoction of sodium arsenite, was sold. The organoarsenic compound Salvarsan was the first synthetic
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
chemotherapeutic agent, discovered by Paul Ehrlich
Paul Ehrlich
Paul Ehrlich was a German scientist in the fields of hematology, immunology, and chemotherapy, and Nobel laureate. He is noted for curing syphilis and for his research in autoimmunity, calling it "horror autotoxicus"...
. The treatment, however, led to many problems, causing long lasting health complications. Around 1943 it was finally superseded by penicillin
Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....
.
In vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
studies suggest that arsenic trioxide
Arsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...
(As2O3) inhibits the proliferation of myeloma cells via cell cycle arrest as well as triggering cell death. These results suggest that arsenic trioxide may be a clinically useful treatment in patients with multiple myeloma
Multiple myeloma
Multiple myeloma , also known as plasma cell myeloma or Kahler's disease , is a cancer of plasma cells, a type of white blood cell normally responsible for the production of antibodies...
or leukemia
Leukemia
Leukemia or leukaemia is a type of cancer of the blood or bone marrow characterized by an abnormal increase of immature white blood cells called "blasts". Leukemia is a broad term covering a spectrum of diseases...
.
Arsenic in nucleic acids
DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
contain phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
in their structure. A hypothetical analog would use arsenate
Arsenate
The arsenate ion is AsO43−.An arsenate is any compound that contains this ion. Arsenates are salts or esters of arsenic acid.The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As[V]....
to partially or fully replace phosphate. A 2010 paper in Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
reports that GFAJ-1
GFAJ-1
GFAJ-1 is a strain of rod-shaped bacterium in the family Halomonadaceae. The extremophile was isolated from the hypersaline and alkaline Mono Lake in eastern California by a research team led by NASA astrobiologist Felisa Wolfe-Simon...
, a proteobacterium from Mono Lake
Mono Lake
Mono Lake is a large, shallow saline lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in a basin that has no outlet to the ocean...
, is facultatively capable of incorporating a small percentage of arsenate
Arsenate
The arsenate ion is AsO43−.An arsenate is any compound that contains this ion. Arsenates are salts or esters of arsenic acid.The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As[V]....
into its DNA under conditions of phosphorus starvation. However, other scientists have expressed doubts that arsenate has replaced phosphate in the DNA of this organism and have suggested that procedural errors and interpretative errors may have been responsible.
Prebiotic arsenic
A common objection to arsenic (V) as a chemical surrogate to phosphorus (V) in ancient systems is that it is easily reduced to arsenic (III) metabolically and therefore would not have been as freely available. However, recent research into arsenic metabolism has demonstrated that it may have evolved during the Archean eonArchean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
providing evidence for the biogeochemical cycling and availability of arsenic (V) as an electron acceptor on primordial earth. A modern day example would be Searles
Searles Lake
Searles Lake is an endorheic dry lake in the Mojave Desert of San Bernardino County, California, with the mining community, Trona on its western shore. The evaporite basin is approximately long and at its widest point, yielding 1.7 million tons annually of industrial minerals within the basin to...
and Mono Lake
Mono Lake
Mono Lake is a large, shallow saline lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in a basin that has no outlet to the ocean...
s, in the Western United States, which have been established to harbor an arsenic respiring biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...
. Each of these lakes contain a substantial amount of dissolved inorganic arsenic that is exploited by prokaryotic life through arsenic (III) and arsenate (V) based metabolic reactions.
While arsenic tolerance and exploitation has been demonstrated to exist in these modern extreme biospheres, it has not yet been demonstrated to exist phosphorus-free. While some of the key properties of phosphate, such as its thermodynamic instability as well as its kinetic
Kinetic
-Art and entertainment:* Kinetic * Kinetic art* The 13th episode of the first season of the TV series Smallville* Kinetic, a comic by Allan Heinberg and Kelley Pucklett-Companies:...
stability, make it advantageous for Earth's biological life, arsenic may be more thermodynamically realistic for life on other planets or moons such as Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
's moon Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
.
All known life forms use adenosine phosphate compounds (AMP
Adenosine monophosphate
Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...
, ADP
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
and ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
) to transfer chemical energy
Chemical energy
Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction or, to transform other chemical substances...
. It is known that arsenic-bearing analogs of these compounds can form; however, it is unclear if they are ever utilized by living organisms. It has been observed that some microorganisms of genera Bacillus
Bacillus
Bacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species...
and the unrelated family Halanaerobacteriales can use arsenic in its pentavalent oxidation state [arsenic (V)] as an electron acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
. The isolated microorganisms are observed to use lactate
Lactate
Lactate may refer to:*The act of lactation*The conjugate base of lactic acid...
, sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
, or hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
as electron donors. In this role, arsenic effectively functions as an alternative to oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. This form of chemolithotrophy does not require that the arsenic be incorporated into any of the organism's essential compounds.
See also
Arsenic compounds- ExtremophileExtremophileAn extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...
- GeomicrobiologyGeomicrobiologyGeomicrobiology is the result of the combination of geology and microbiology. The field of geomicrobiology concerns the role of microbe and microbial processes in geological and geochemical processes and vice-versa...
- Hypothetical types of biochemistry
- Organoarsenic chemistry