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Is there already discovered non-carbon form of life?

Is there already discovered non-carbon form of life?


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I have heard of some rumors going around about a living organism based on non-carbon. Is Has research been done with real form of life based on non-carbon? Or carbon-based life is only type of life what humans know of?

Carbon chauvinism


In a word, no. There are some good chemical reasons to expect all life to be carbon-based. Of course, it is no longer a silly fantasy to imagine an artificial intelligence that might qualify as life. (Still FAAAARRRR in the future, though, if ever.)


Is metal-based, or other non-carbon-based, life realistic?

One science fiction short story I read (I don't remember the title or author, sorry!) featured some small creatures (unnamed) that were formed from metal, with the argument that "just like life developed on Earth with a carbon base, why shouldn't life be able to form elsewhere, with a metal base?"

Is this realistic?
Is there anything similar to this that already exists on Earth
(ie, life based on something other than carbon)?


9 Silicon-Based Life

Silicon-based life is perhaps the most common form of alternate biochemistry explored in popular science fiction, most notably in the case of the Horta from Star Trek. The concept is an old one, dating back to speculations by H.G. Wells in 1894: &ldquoOne is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms&mdashwhy not silicon-aluminium men at once?&mdashwandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace.&rdquo

Silicon is popular precisely because it is so similar to carbon and can form four bonds just like carbon, opening the possibility of an entirely silicon-based biochemical system. It is the most abundant element in Earth&rsquos crust other than oxygen. There is a form of algae on Earth which incorporates silicon into its growth process. Silicon suffers the disadvantage of playing second fiddle to carbon, which is capable of forming more stable and diverse complex structures necessary for life. Carbon molecules incorporate oxygen and nitrogen, which form extremely stable bonds. Complicated silicon-based molecules have an unfortunate tendency to fall apart. Carbon is also extremely common throughout the universe and has been for billions of years.

Silicon life is unlikely to emerge on an Earth-like environment, as most free silicon would be locked up in volcanic and igneous rocks made of silicate minerals. It is theorized that things might be different in a high-temperature environment, but no evidence has been found. An extreme world like Titan could support silicon-based life, perhaps making up the basis of the methanogens mentioned earlier, as silicon molecules such as silanes and polysilanes mimic Earth&rsquos organic chemistry. However, on Titan, the surface is dominated by carbon, while most of the silicon is deep beneath the surface.

NASA astrochemist Max Bernstein has speculated that silicon-based life could exist on a very hot planet with a hydrogen-rich and oxygen-poor atmosphere, allowing complex silane chemistry with reversible silicon bonds with selenium or tellurium, but he thought it unlikely or rare. On Earth, such organisms would replicate very slowly, and our respective biochemistries would be of no threat to each other. They could slowly consume our cities, but, &ldquoPresumably you could take a jackhammer to it.&rdquo


Answers and Replies

Quite possibly, but I am glad that you still mentioned "in addition".
Most of what you mentioned in your post is discussed in a book I've read 'The Future of the Mind" by Prof Michio Kaku. It is an extremely engaging book. Will answer your question.

Obviously, a car engine or database or computer program are not alive because they lack to self propagate their information. There is a lot of talk about future AI with consciousness, meaning that such entities will be self aware and self replicating. That would be "life" because it passes along "information" to future generations. On Earth we have biological life based on carbon. In other worlds, there may well be non-carbon life, but still biological (meaning that it could evolve like life on Earth evolved). AI "life", on the other hand, would be of non-biological origin, but still able to pass on its information base to future generations.

The reason I posted this is to seek views and answers to my question: "wouldn't it be possible that our search may well be for extraterrestrial AI in addition to search for biological life? And if so, what would we look for, if not for biochemical signature of life?".

Modern textbooks of Life Science define life on Earth as the property of an entity that:

(1) Has inherent information base (DNA) that is self propagating and able to pass its characteristics to future generations.
(2) It is able to accumulate raw material from its environment
(3) Generate energy from its environment
(4) Use that energy to reassemble raw material and build more copies of itself based on its information base (DNA in case of carbon-based life on Earth).


Contents

Overview of hypothetical types of biochemistry
Type Basis Synopsis Remarks
Alternative-chirality biomolecules Alternative biochemistry Different basis of biofunction Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules using D amino acids or L sugars may be possible molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules.
Ammonia biochemistry Non-water solvents Ammonia-based life The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J. B. S. Haldane raised the topic at a symposium about life's origin.
Arsenic biochemistry Alternative biochemistry Arsenic-based life Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms.
Borane biochemistry (Organoboron chemistry) Alternative biochemistry Boranes-based life Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. Boron, however, is exceedingly rare in the universe in comparison to its neighbours carbon, nitrogen, and oxygen. On the other hand, structures containing alternating boron and nitrogen atoms, being similar to carbon compounds, provide another possible substitute for hydrocarbons.
Dust and plasma-based biochemistry Nonplanetary life Exotic matrix life In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.
Extremophiles Alternative environment Life in variable environments It would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.
Heteropoly acid biochemistry Alternative biochemistry Heteropoly acid-based life Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds [ citation needed ] the heteropoly acids are one such family.
Hydrogen fluoride biochemistry Non-water solvents Hydrogen fluoride-based life It has been considered as a possible solvent for life by scientists such as Peter Sneath.
Hydrogen sulfide biochemistry Non-water solvents Hydrogen sulfide-based life Hydrogen sulfide is the closest chemical analog to water, but is less polar and a weaker inorganic solvent.
Methane biochemistry (Azotosome) Non-water solvents Methane-based life Methane (CH4) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos: hydrogen and carbon. Methane life is hypothetically possible.
Non-green photosynthesizers Other speculations Alternate plant life Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth. In particular, retinal is capable of, and has been observed to, perform photosynthesis. [4] Bacteria capable of photosynthesis are known as microbial rhodopsins. A plant or creature that uses retinal photosynthesis is always purple.
Shadow biosphere Alternative environment A hidden life biosphere on Earth A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life.
Silicon biochemistry (Organosilicon) Alternative biochemistry Silicon-based life Like carbon, silicon can create molecules that are sufficiently large to carry biological information however, the scope of possible silicon chemistry is far more limited than that of carbon.
Silicon dioxide biochemistry Non-water solvents Silicon dioxide-based life Gerald Feinberg and Robert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium.
Sulfur biochemistry Alternative biochemistry Sulfur-based life The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones.
Alternative nucleic acids Alternative biochemistry Different genetic storage Xeno nucleic acids (XNA) may possibly be used in place of RNA or DNA. XNA is the general term for a nucleic acid with an altered sugar backbone. Examples of XNA include TNA, which uses threose, HNA, which uses 1,5-anhydrohexitol, GNA, which uses glycol, CeNA, which uses cyclohexene, LNA, which utilizes a form of ribose that contains an extra linkage between its 4' carbon and 2' oxygen, FANA, which uses arabinose but with a single fluorine atom attached to its 2' carbon, and PNA, which uses, in place of sugar and phosphate, N-(2-aminoethyl)-glycine units connected by peptide bonds. [5] In comparison, Hachimoji DNA changes the base pairs instead of the backbone. These new base pairs are P (2-Aminoimidazo[1,2a][1,3,5]triazin-4(1H)-one), Z (6-Amino-5-nitropyridin-2-one), B (Isoguanine), and S (rS = Isocytosine for RNA, dS = 1-Methylcytosine for DNA). [6] [7]

A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. [8] [9] Although life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms.

Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules using D amino acids or L sugars may be possible molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Amino acids whose chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicist Paul Davies speculates that some of them might be products of "anti-chiral" life. [10]

It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal) organisms such as in comparisons between members of Archaea and other domains, [ citation needed ] making it an open topic whether an alternative stereochemistry is truly novel.

On Earth, all known living things have a carbon-based structure and system. Scientists have speculated about the pros and cons of using atoms other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe. [11] Sagan used the term "carbon chauvinism" for such an assumption. [12] He regarded silicon and germanium as conceivable alternatives to carbon [12] (other plausible elements include but are not limited to palladium and titanium) but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos. [13] Norman Horowitz devised the experiments to determine whether life might exist on Mars that were carried out by the Viking Lander of 1976, the first U.S. mission to successfully land an unmanned probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. [14] He considered that there was only a remote possibility that non-carbon life forms could exist with genetic information systems capable of self-replication and the ability to evolve and adapt.

Silicon biochemistry Edit

The silicon atom has been much discussed as the basis for an alternative biochemical system, because silicon has many chemical properties similar to those of carbon and is in the same group of the periodic table, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information. [15]

However, silicon has several drawbacks as an alternative to carbon. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms as is necessary for the chemical versatility required for metabolism, and yet this precise inability is what makes silicon less susceptible to bond with all sorts of impurities from which carbon, in comparison, is not shielded. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms. [15] Moreover, where it does interact with other atoms, silicon creates molecules that have been described as "monotonous compared with the combinatorial universe of organic macromolecules". [15] This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds (the double-bonded carbon is part of the carbonyl group, a fundamental motif of carbon-based bio-organic chemistry).

Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations. [16]

Of the varieties of molecules identified in the interstellar medium as of 1998 [update] , 84 are based on carbon, while only 8 are based on silicon. [17] Moreover, of those 8 compounds, 4 also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation on which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets. Also, even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (the relative abundance of silicon to carbon in Earth's crust is roughly 925:1), terrestrial life is carbon-based. The fact that carbon is used instead of silicon may be evidence that silicon is poorly suited for biochemistry on Earth-like planets. Reasons for which this may be that silicon is less versatile than carbon in forming compounds, that the compounds formed by silicon are unstable, and that it blocks the flow of heat. [18]

Even so, biogenic silica is used by some Earth life, such as the silicate skeletal structure of diatoms. According to the clay hypothesis of A. G. Cairns-Smith, silicate minerals in water played a crucial role in abiogenesis: they replicated their crystal structures, interacted with carbon compounds, and were the precursors of carbon-based life. [19] [20]

Although not observed in nature, carbon–silicon bonds have been added to biochemistry by using directed evolution (artificial selection). A heme containing cytochrome c protein from Rhodothermus marinus has been engineered using directed evolution to catalyze the formation of new carbon–silicon bonds between hydrosilanes and diazo compounds. [21]

Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon. Polysilanols, the silicon compounds corresponding to sugars, are soluble in liquid nitrogen, suggesting that they could play a role in very-low-temperature biochemistry. [22] [23]

In cinematic and literary science fiction, at a moment when man-made machines cross from nonliving to living, it is often posited, [ by whom? ] this new form would be the first example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as computers (or computer-guided robots) and filed under "silicon-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".

Other exotic element-based biochemistries Edit

    are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing atmosphere. However, boron's low cosmic abundance makes it less likely as a base for life than carbon.
  • Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds [citation needed] the heteropoly acids are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond-like crystals (such as cubic zirconia). Titanium, aluminium, magnesium, and iron are all more abundant in the Earth's crust than carbon. Metal-oxide-based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon-based life would be unlikely. The Cronin group at Glasgow University reported self-assembly of tungsten polyoxometalates into cell-like spheres. [24] By modifying their metal oxide content, the spheres can acquire holes that act as porous membrane, selectively allowing chemicals in and out of the sphere according to size. [24] is also able to form long-chain molecules, but suffers from the same high-reactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an electron acceptor is widespread and can be traced back 3.5 billion years on Earth, thus predating the use of molecular oxygen. [25]Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)

Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. [26] Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). [27] Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy.

It has been speculated that the earliest life forms on Earth may have used arsenic biochemistry in place of phosphorus in the structure of their DNA. [28] A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic is poorly suited for this function. [29]

The authors of a 2010 geomicrobiology study, supported in part by NASA, have postulated that a bacterium, named GFAJ-1, collected in the sediments of Mono Lake in eastern California, can employ such 'arsenic DNA' when cultured without phosphorus. [30] [31] They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate or other means to reduce the effective concentration of water and stabilize its arsenate esters. [31] This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls. [32] [33] Science writer Carl Zimmer contacted several scientists for an assessment: "I reached out to a dozen experts . Almost unanimously, they think the NASA scientists have failed to make their case". [34] Other authors were unable to reproduce their results and showed that the study had issues with phosphate contamination, suggesting that the low amounts present could sustain extremophile lifeforms. [35] Alternatively, it was suggested that GFAJ-1 cells grow by recycling phosphate from degraded ribosomes, rather than by replacing it with arsenate. [36]

In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner, [37] and by the astrobiological committee chaired by John A. Baross. [38] Solvents discussed by the Baross committee include ammonia, [39] sulfuric acid, [40] formamide, [41] hydrocarbons, [41] and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid. [42]

Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist [43] however, on another occasion he said that he was a carbon chauvinist but "not that much of a water chauvinist". [44] He speculated on hydrocarbons, [44] : 11 hydrofluoric acid, [45] and ammonia [44] [45] as possible alternatives to water.

Some of the properties of water that are important for life processes include:

  • A complexity which leads to a large number of permutations of possible reaction paths including acid–base chemistry, H + cations, OH − anions, hydrogen bonding, van der Waals bonding, dipole–dipole and other polar interactions, aqueous solvent cages, and hydrolysis. This complexity offers a large number of pathways for evolution to produce life, many other solvents [which?] have dramatically fewer possible reactions, which severely limits evolution.
  • Thermodynamic stability: the free energy of formation of liquid water is low enough (−237.24 kJ/mol) that water undergoes few reactions. Other solvents are highly reactive, particularly with oxygen.
  • Water does not combust in oxygen because it is already the combustion product of hydrogen with oxygen. Most alternative solvents are not stable in an oxygen-rich atmosphere, so it is highly unlikely that those liquids could support aerobic life.
  • A large temperature range over which it is liquid.
  • High solubility of oxygen and carbon dioxide at room temperature supporting the evolution of aerobic aquatic plant and animal life.
  • A high heat capacity (leading to higher environmental temperature stability).
  • Water is a room-temperature liquid leading to a large population of quantum transition states required to overcome reaction barriers. Cryogenic liquids (such as liquid methane) have exponentially lower transition state populations which are needed for life based on chemical reactions. This leads to chemical reaction rates which may be so slow as to preclude the development of any life based on chemical reactions. [citation needed]
  • Spectroscopic transparency allowing solar radiation to penetrate several meters into the liquid (or solid), greatly aiding the evolution of aquatic life.
  • A large heat of vaporization leading to stable lakes and oceans.
  • The ability to dissolve a wide variety of compounds.
  • The solid (ice) has lower density than the liquid, so ice floats on the liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds), then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.

Water as a compound is cosmically abundant, although much of it is in the form of vapour or ice. Subsurface liquid water is considered likely or possible on several of the outer moons: Enceladus (where geysers have been observed), Europa, Titan, and Ganymede. Earth and Titan are the only worlds currently known to have stable bodies of liquid on their surfaces.

Not all properties of water are necessarily advantageous for life, however. [46] For instance, water ice has a high albedo, [46] meaning that it reflects a significant quantity of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased. [46]

There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Because boiling points vary with the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid-phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 92 bars (91 atm) of pressure, it can indeed exist in liquid form over a wide temperature range.

Ammonia Edit

The ammonia molecule (NH3), like the water molecule, is abundant in the universe, being a compound of hydrogen (the simplest and most common element) with another very common element, nitrogen. [47] The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J. B. S. Haldane raised the topic at a symposium about life's origin. [48]

Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water. [47] [49] Ammonia can dissolve most organic molecules at least as well as water does and, in addition, it is capable of dissolving many elemental metals. Haldane made the point that various common water-related organic compounds have ammonia-related analogs for instance the ammonia-related amine group (−NH2) is analogous to the water-related hydroxyl group (−OH). [49]

Ammonia, like water, can either accept or donate an H + ion. When ammonia accepts an H + , it forms the ammonium cation (NH4 + ), analogous to hydronium (H3O + ). When it donates an H + ion, it forms the amide anion (NH2 − ), analogous to the hydroxide anion (OH − ). [39] Compared to water, however, ammonia is more inclined to accept an H + ion, and less inclined to donate one it is a stronger nucleophile. [39] Ammonia added to water functions as Arrhenius base: it increases the concentration of the anion hydroxide. Conversely, using a solvent system definition of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium. [49] The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogous imine group (C=NH) could be used instead. [39]

However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be a third, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. Gerald Feinberg and Robert Shapiro have questioned whether ammonia could hold prebiotic molecules together well enough to allow the emergence of a self-reproducing system. [50] Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable for aerobic metabolism. [51]

A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth usually exists within the melting point and boiling point of water at normal pressure, between 0 °C (273 K) and 100 °C (373 K) at normal pressure ammonia's melting and boiling points are between −78 °C (195 K) and −33 °C (240 K). Chemical reactions generally proceed more slowly at a lower temperature. Therefore, ammonia-based life, if it exists, might metabolize more slowly and evolve more slowly than life on Earth. [51] On the other hand, lower temperatures could also enable living systems to use chemical species that would be too unstable at Earth temperatures to be useful. [47]

Ammonia could be a liquid at Earth-like temperatures, but at much higher pressures for example, at 60 atm, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K). [39]

Ammonia and ammonia–water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn's largest moon Titan. [52]

Methane and other hydrocarbons Edit

Methane (CH4) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos: hydrogen and carbon. It has a cosmic abundance comparable with ammonia. [47] Hydrocarbons could act as a solvent over a wide range of temperatures, but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane. [47] Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on the surface of Titan by the Cassini spacecraft.

There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia. [53] [54] [55] Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. [56] However, water is also more chemically reactive and can break down large organic molecules through hydrolysis. [53] A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way. [53] Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules. [46] Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules. [53] Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry. [53]

Astrobiologist Chris McKay has argued, on thermodynamic grounds, that if life does exist on Titan's surface, using hydrocarbons as a solvent, it is likely also to use the more complex hydrocarbons as an energy source by reacting them with hydrogen, reducing ethane and acetylene to methane. [57] Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 10 25 molecules per second and disappearance of hydrogen near Titan's surface. As Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present. [56] [57] [58] The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane. [56] While restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are to be considered more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a non-living surface catalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow. [59] He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery. [59]

Azotosome Edit

A hypothetical cell membrane termed an azotosome capable of functioning in liquid methane in Titan conditions was computer-modeled in an article published in February 2015. Composed of acrylonitrile, a small molecule containing carbon, hydrogen, and nitrogen, it is predicted to have stability and flexibility in liquid methane comparable to that of a phospholipid bilayer (the type of cell membrane possessed by all life on Earth) in liquid water. [60] [61] An analysis of data obtained using the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere. [62] [63]

Hydrogen fluoride Edit

Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is −84 °C, and its boiling point is 19.54 °C (at atmospheric pressure) the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbor molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath [64] and Carl Sagan. [45]

HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as paraffin waxes, are stable with it. [45] Like water and ammonia, liquid hydrogen fluoride supports an acid–base chemistry. Using a solvent system definition of acidity and basicity, nitric acid functions as a base when it is added to liquid HF. [65]

However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane. [66]

Hydrogen sulfide Edit

Hydrogen sulfide is the closest chemical analog to water, [67] but is less polar and a weaker inorganic solvent. [68] Hydrogen sulfide is quite plentiful on Jupiter's moon Io and may be in liquid form a short distance below the surface astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there. [69] On a planet with hydrogen-sulfide oceans the source of the hydrogen sulfide could come from volcanos, in which case it could be mixed in with a bit of hydrogen fluoride, which could help dissolve minerals. Hydrogen-sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live on sulfur monoxide, which is analogous to oxygen (O2). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.

Silicon dioxide and silicates Edit

Silicon dioxide, also known as silica and quartz, is very abundant in the universe and has a large temperature range where it is liquid. However, its melting point is 1,600 to 1,725 °C (2,912 to 3,137 °F), so it would be impossible to make organic compounds in that temperature, because all of them would decompose. Silicates are similar to silicon dioxide and some have lower melting points than silica. Gerald Feinberg and Robert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium. [70]

Other solvents or cosolvents Edit

Other solvents sometimes proposed:

    : supercritical carbon dioxide and supercritical hydrogen. [71]
  • Simple hydrogen compounds: hydrogen chloride. [72]
  • More complex compounds: sulfuric acid, [40]formamide, [41]methanol. [72]
  • Very-low-temperature fluids: liquid nitrogen[42] and hydrogen. [42]
  • High-temperature liquids: sodium chloride. [73]

Sulfuric acid in liquid form is strongly polar. It remains liquid at higher temperatures than water, its liquid range being 10 °C to 337 °C at a pressure of 1 atm, although above 300 °C it slowly decomposes. Sulfuric acid is known to be abundant in the clouds of Venus, in the form of aerosol droplets. In a biochemistry that used sulfuric acid as a solvent, the alkene group (C=C), with two carbon atoms joined by a double bond, could function analogously to the carbonyl group (C=O) in water-based biochemistry. [40]

A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent. [74] A 61.2% (by mass) mix of water and hydrogen peroxide has a freezing point of −56.5 °C and tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment. [75] [76]

Supercritical carbon dioxide has been proposed as a candidate for alternative biochemistry due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes and because "super-Earth"- or "super-Venus"-type planets with dense high-pressure atmospheres may be common. [71]

Non-green photosynthesizers Edit

Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth. [77] [78] These studies indicate that blue plants would be unlikely however yellow or red plants may be relatively common. [78]

Variable environments Edit

Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a spore or hibernation state that can be sustained for years or even millennia between more active life stages. [79] Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.

For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state, [79] whereas desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods. [80] Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.

Alanine world and hypothetical alternatives Edit

The genetic code evolved during the transition from the RNA world to a protein world. [81] The Alanine World Hypothesis postulates that the evolution of the genetic code (the so-called GC phase [82] ) started with only four basic amino acids: alanine, glycine, proline and ornithine (now arginine). [83] The evolution of the genetic code ended with 20 proteinogenic amino acids. From a chemical point of view, most of them are Alanine-derivatives particularly suitable for the construction of α-helices and β-sheets – basic secondary structural elements of modern proteins. Direct evidence of this is an experimental procedure in molecular biology known as alanine scanning. The hypothetical "Proline World" would create a possible alternative life with the genetic code based on the proline chemical scaffold as the protein backbone. Similarly, "Glycine" and "Ornithine" worlds are also conceivable, but nature has chosen none of them. [84] Evolution of life with Glycine, Proline or Ornithine as the basic structure for protein-like polymers (foldamers) would lead to parallel biological worlds. They would have morphologically radically different body plans and genetics from the living organisms of the known biosphere. [85]

Dust and plasma-based Edit

In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space. [86] [87] Computer models showed that, when the dust became charged, the particles could self-organize into microscopic helical structures, and the authors offer "a rough sketch of a possible model of. helical grain structure reproduction".

Life on a neutron star Edit

Frank Drake suggested in 1973 that intelligent life could inhabit neutron stars. [88] Physical models in 1973 implied that Drake's creatures would be microscopic. In 1980, Robert L Forward wrote the science fiction novel Dragon's Egg using Drake's suggestion as a thesis. [89]

Scientists who have considered possible alternatives to carbon-water biochemistry include:


Discovering new species

Scientists and conservationists are regularly updating the inventory of life with the discovery of new species. Last week, scientists at the Smithsonian Institution reported the discovery of a primitive eel in a reef off the coast of the South Pacific island nation of Palau. The new species, Protoanguilla palau, bore little relation to 19 other forms of eel currently in existence and some of its characteristics – such as a second upper jaw – were more in line with fossils from 65m years ago.

Other recent highlights, as compiled by the International Institute for Species Exploration (IISE) at Arizona State University, include the eternal light mushroom, or Mycena luxaeterna, which emits bright yellowish light. The new species was collected from forests near Sao Paulo, Brazil. Another highlight was the golden spotted monitor lizard (Varanus bitatawa), a two-metre long beast discovered on Luzon Island in the Philippines. It has evaded earlier discovery by spending most of its time in the trees.

But most scientists expect the next rush of discovery to come from even smaller organisms, such as bacteria. The IISE also highlighted the discovery of a new bacteria growing on the shipwrecked hull of the Titanic. Halomonas titanicae is an iron oxide-eating bacteria, that could eventually eat the wreck up.


Is there life on other planets?

The ultimate goal of NASA's exoplanet program is to find unmistakable signs of current life on a planet beyond Earth. How soon that can happen depends on two unknowns: the prevalence of life in the galaxy and how lucky we get as we take those first, tentative, exploratory steps.

Our early planet finding missions, such as NASA&rsquos Kepler and its extended incarnation, K2, or the coming James Webb Space Telescope, could yield bare bones evidence of the potentially habitable worlds. James Webb, designed in part to investigate gas giants and super Earths, might find an outsized version of our planet. NASA's Nancy Grace Roman Space Telescope or the Wide-Field Infrared Survey Telescope, could zero in on a distant planet&rsquos reflected light to detect the signatures of oxygen, water vapor, or some other powerful indication of possible life.

But unless we get lucky, the search for signs of life could take decades. Discovering another blue-white marble hidden in the star field, like a sand grain on the beach, will probably require an even larger imaging telescope. Designs are already underway for that next-generation planet finder, to be sent aloft in the 2030s or 2040s.

MIT physics professor Sara Seager looks for possible chemical combinations that could signal the presence of alien life. She and her biochemistry colleagues first focused on the six main elements associated with life on Earth: carbon, nitrogen, oxygen, phosphorous, sulfur and hydrogen.

&ldquoWe&rsquore going to have so few planets, we have to get lucky,&rdquo Seager said. &ldquoI don&rsquot want to miss anything. I don&rsquot want to miss it because we weren&rsquot smart enough to think of some molecule.&rdquo

To find out how about the advanced, space-based telescope technology being developed at NASA to search for life among the stars, read Inventing the Future


Scientists discover first multicellular life that doesn't need oxygen

Light microscopy image of the undescribed species of Spinoloricus, stained with Rose Bengal. The scale bar is 50 micrometers. Image credit: Danovaro, et al.

(PhysOrg.com) -- Oxygen may not be the staple of modern complex life that scientists once thought. Until now, the only life forms known to live exclusively in anoxic conditions were viruses, bacteria and Archaea. But in a new study, scientists have discovered three new multicellular marine species that appear to have never lived in aerobic conditions, and never metabolized oxygen.

The discovery of the new species, which live buried in sediment under the Mediterranean seafloor, is significant in that it marks the first observation of multicellular organisms, or metazoans, that spend their entire lifecycle under permanently anoxic conditions. A few metazoans have been known to tolerate anoxic conditions, but only for limited periods of time.

The team of Italian and Danish researchers, Roberto Danovaro, et al., that discovered the new life forms has identified the creatures as belonging to the animal phylum Loricifera, the most recently described animal phylum. Loriciferans, which have a length of less than one millimeter, typically live in sediment. The three new organisms belong to different genera (Spinoloricus, Rugiloricus, and Pliciloricus), although their species have not yet been named.

Despite belonging to previously known taxonomic groups, the new species possess some radical differences compared with other metazoans. Most significantly, the new species do not have mitochondria, the cellular organelles that use oxygen and sugar to generate the cell’s energy. Instead, the new loriciferans have organelles that resemble hydrogenosomes, which are used by some single-celled eukaryotes to generate energy without oxygen. However, this is the first time that these organelles have been observed in multicellular organisms. Previous research has indicated that hydrogenosomes may have evolved from mitochondria, while other research suggests they evolved independently.

To find the new species, the researchers carried out three oceanographic expeditions from 1998 to 2008 to search for life in the extreme environments located more than 3,000 meters (about two miles) under the Mediterranean Sea. The researchers focused on an area called the L’Atalante basin, which is located off the southern coast of Greece. As the scientists explain, this type of “deep hypersaline anoxic basin” was created by the flooding of mineral sediments from 5.5 million years ago. For the past 50,000 years, the basin has possessed a dense hypersaline brine layer up to 60 meters thick. The brine serves as a physical barrier that prohibits oxygen exchange between the water and sediment, making the basin completely oxygen-free. In addition, the basin is rich in methane and hydrogen sulphide, and is also home to a diverse assembly of prokaryotes that have adapted to these conditions.

Because previous studies have reported the presence of cadaverous metazoans that had sunk to anoxic deep-sea sediments in the Black Sea, the researchers here stained the newly collected specimens with Rose Bengal, a protein binding stain that colors living organisms with a much greater intensity than deceased organisms, demonstrating that the new species were indeed alive. In addition, the scientists observed specimens of the undescribed species of both genera Spinoloricus and Rugiloricus that had a large oocyte in their ovary, which showed a nucleus containing a nucleolus, providing evidence of reproduction.

LM image of the undescribed species of Spinoloricus stained with Rose Bengal showing the presence of an oocyte. Image credit: Roberto Danovaro.

“The results reported here support the hypothesis that the loriciferans inhabiting the anoxic sediments of the L’Atalante basin have developed an obligate anaerobic metabolism and specific adaptations to live without oxygen,” the researchers conclude. “Although the evolutionary/adaptative mechanisms leading to the colonization of such extreme environments by these metazoans remain an enigma, this discovery opens new perspectives for the study of metazoan life in habitats lacking molecular oxygen.”

The work is financially supported by the EU within the framework of the HERMES (Hot Spot Ecosystem Research on the Margins of European Seas) and HERMIONE (Hotspot Ecosystem Research and Man's Impact On European Seas) projects.


New discoveries on deadly fungus—possibly a key for treatment

Aspergillus fumigatus growing on a petri dish. Credit: Jan-Peter Kasper, University of Jena

Aspergillus fumigatus kills as many people as malaria and tuberculosis, but is less known. It is found everywhere, for example in the soil or in our compost, but is not normally dangerous to healthy people.

Those who die from it often have a poor immune system or are hospitalized for lung infections, such as COVID-19.

Aspergillus also constitutes an increasing problem in agriculture, because the fungus causes deadly infections in both plants and animals. In the same way that many bacteria are resistant to antibiotics, also this fungus is now becoming more and more resistant to the limited repertoire of treatments. It is therefore important to find new ways to fight fungal infections.

Researchers at the Department of Biomedicine at the Faculty of Medicine, UiB, together with researchers at the Faculty of Mathematics and Natural Sciences and a German research team, have now discovered an enzyme on the surface of the fungus.

The newly discovered enzyme breaks down a vital molecule that is important for the cells' energy metabolism: the NAD molecule, which is formed in our body from vitamin B3.

Without NAD, cells cannot survive. Therefore, the breakdown of NAD could affect immune cells and weaken our immune system to fight the fungal infection.

Recollected old scientific observations

Similar enzymes are found in bacteria that cause infections such as tuberculosis, streptococci or cholera. The idea that also fungi may have an NAD-degrading enzyme on their surface has been raised already in the 1950's:

"Enzyme activity that degrades NAD had been detected on the surface of a fungus, only the identity of the enzyme was never established. However, that fungus is commonly used in research laboratories and not known to be pathogenic. Probably, this discovery therefore went into the archive," says Professor Mathias Ziegler, leader of the study.

They want to re-examine the hypothesis that an enzyme, which breaks down NAD, may contribute to pathogenic mechanisms in fungi such as Aspergillus fumigatus.

"We measured strong enzyme activity on the surface of spores from Aspergillus fumigatus. It surprised us," says researcher and first author Øyvind Strømland.

Modern technology makes it possible to study the enzyme

"Using an elegant biochemical method, we identified fragments of the protein sequence from this enzyme. Since the entire genome of the fungus is known, we could then use these fragments to identify the gene that encodes the enzyme," says Ziegler.

"The next step was to use the genetic information and create a version of this gene that can be used by laboratory cell lines "trained" to produce sufficient amounts of the protein for detailed molecular studies," he continues.

In this way, researchers have been able to study how the enzyme breaks down NAD.

"There are two things that are central here. Highly sensitive analytical technology enabled identification of enzyme fragments. The other important element is that we now have the genome sequence and could easily identify the gene. That was not possible in the 50s," says Ziegler.

May design new drugs against the fungus

The researchers are clear that even though it is known from other diseases that similar bacterial enzymes break down NAD in infected cells, it cannot be said for sure that this is the case with the enzyme from Aspergillus fumigatus.

However, they know a lot more about the enzyme now through their research, and the hope is that the knowledge can help to discover new treatments for fungal infections.

Intriguingly, their bioinformatics analyses revealed that this type of enzyme is predominantly present in pathogenic fungi.

"If we could make a drug with molecules that resemble NAD, they might block the enzyme in our cells," the researchers suggest.


Possibility of Silicon-Based Life Grows

Science fiction has long imagined alien worlds inhabited by silicon-based life, such as the rock-eating Horta from the original Star Trek series. Now, scientists have for the first time shown that nature can evolve to incorporate silicon into carbon-based molecules, the building blocks of life on Earth.

Artist rendering of organosilicon-based life. Organosilicon compounds contain carbon-silicon bonds. Recent research from the laboratory of Frances Arnold shows, for the first time, that bacteria can create organosilicon compounds. This does not prove that silicon- or organosilicon-based life is possible, but shows that life could be persuaded to incorporate silicon into its basic components. Credit: Lei Chen and Yan Liang (BeautyOfScience.com) for Caltech

As for the implications these findings might have for alien chemistry on distant worlds, “my feeling is that if a human being can coax life to build bonds between silicon and carbon, nature can do it too,” said the study’s senior author Frances Arnold, a chemical engineer at the California Institute of Technology in Pasadena. The scientists detailed their findings recently in the journal Science.

Carbon is the backbone of every known biological molecule. Life on Earth is based on carbon, likely because each carbon atom can form bonds with up to four other atoms simultaneously. This quality makes carbon well-suited to form the long chains of molecules that serve as the basis for life as we know it, such as proteins and DNA.

Still, researchers have long speculated that alien life could have a completely different chemical basis than life on Earth. For example, instead of relying on water as the solvent in which biological molecules operate, perhaps aliens might depend on ammonia or methane. And instead of relying on carbon to create the molecules of life, perhaps aliens could use silicon.

Carbon and silicon are chemically very similar in that silicon atoms can also each form bonds with up to four other atoms simultaneously. Moreover, silicon is one of the most common elements in the Universe. For example, silicon makes up almost 30 percent of the mass of the Earth’s crust, and is roughly 150 times more abundant than carbon in the Earth’s crust.

Scientists have long known that life on Earth is capable of chemically manipulating silicon. For instance, microscopic particles of silicon dioxide called phytoliths can be found in grasses and other plants, and photosynthetic algae known as diatoms incorporate silicon dioxide into their skeletons. However, there are no known natural instances of life on Earth combining silicon and carbon together into molecules.

Still, chemists have artificially synthesized molecules comprised of both silicon and carbon. These organo-silicon compounds are found in a wide range of products, including pharmaceuticals, sealants, caulks, adhesives, paints, herbicides, fungicides, and computer and television screens. Now, scientists have discovered a way to coax biology to chemically bond carbon and silicon together.

“We wanted to see if we could use what biology already does to expand into whole new areas of chemistry that nature has not yet explored,” Arnold said.

The researchers steered microbes into creating molecules never before seen in nature through a strategy known as ‘directed evolution,’ which Arnold pioneered in the early 1990s. Just as farmers have long modified crops and livestock by breeding generations of organisms for the traits they want to appear, so too have scientists bred microbes to create the molecules they desire.

Scientists have used directed evolutionary strategies for years to create household goods such as detergents, and to develop environmentally-friendly ways to make pharmaceuticals, fuels and other industrial products. (Conventional chemical manufacturing processes can require toxic chemicals in contrast, directed evolutionary strategies use living organisms to create molecules and generally avoid chemistry that would prove harmful to life.)

Arnold and her team — synthetic organic chemist Jennifer Kan, bioengineer Russell Lewis, and chemist Kai Chen — focused on enzymes, the proteins that catalyze or accelerate chemical reactions. Their aim was to create enzymes that could generate organo-silicon compounds.

“My laboratory uses evolution to design new enzymes,” Arnold said. “No one really knows how to design them — they are tremendously complicated. But we are learning how to use evolution to make new ones, just as nature does.”

Researchers in Frances Arnold’s lab at Caltech have persuaded living organisms to make chemical bonds not found in nature. The finding may change how medicines and other chemicals are made in the future. Credit: Caltech

First, the researchers started with enzymes they suspected could, in principle, chemically manipulate silicon. Next, they mutated the DNA blueprints of these proteins in more or less random ways and tested the resulting enzymes for the desired trait. The enzymes that performed best were mutated again, and the process was repeated until the scientists reached the results they wanted.

Arnold and her colleagues started with enzymes known as heme proteins, which all have iron at their hearts and are capable of catalyzing a wide variety of reactions. The most widely recognized heme protein is likely hemoglobin, the red pigment that helps blood carry oxygen.

After testing a variety of heme proteins, the scientists concentrated on one from Rhodothermus marinus, a bacterium from hot springs in Iceland. The heme protein in question, known as cytochrome c, normally shuttles electrons to other proteins in the microbe, but Arnold and her colleagues found that it could also generate low levels of organo-silicon compounds.

After analyzing cytochrome c’s structure, the researchers suspected that only a few mutations might greatly enhance the enzyme’s catalytic activity. Indeed, only three rounds of mutations were enough to turn this protein into a catalyst that could generate carbon-silicon bonds more than 15 times more efficiently than the best synthetic techniques currently available. The mutant enzyme could generate at least 20 different organo-silicon compounds, 19 of which were new to science, Arnold said. It remains unknown what applications people might be able to find for these new compounds.

“The biggest surprise from this work is how easy it was to get new functions out of biology, new functions perhaps never selected for in the natural world that are still useful to human beings,” Arnold said. “The biological world always seems poised to innovate.”

In addition to showing that the mutant enzyme could self-generate organo-silicon compounds in a test tube, the scientists also showed that E. coli bacteria, genetically engineered to produce the mutant enzyme within themselves, could also create organo-silicon compounds. This result raises the possibility that microbes somewhere could have naturally evolved the ability to create these molecules.

“In the universe of possibilities that exist for life, we’ve shown that it is a very easy possibility for life as we know it to include silicon in organic molecules,” Arnold said. “And once you can do it somewhere in the Universe, it’s probably being done.”

It remains an open question why life on Earth is based on carbon when silicon is more prevalent in Earth’s crust. Previous research suggests that compared to carbon, silicon can form chemical bonds with fewer kinds of atoms, and it often forms less complex kinds of molecular structures with the atoms that it can interact with. By giving life the ability to create organo-silicon compounds, future research can test why life here or elsewhere may or may not have evolved to incorporate silicon into biological molecules.

In addition to the astrobiology implications, the researchers noted that their work suggests biological processes could generate organo-silicon compounds in ways that are more environmentally friendly and potentially much less expensive than existing methods of synthesizing these molecules. For example, current techniques for creating organo-silicon compounds often require precious metals and toxic solvents.

The mutant enzyme also makes fewer unwanted byproducts. In contrast, existing techniques typically require extra steps to remove undesirable byproducts, adding to the cost of making these molecules.

“I’m talking to several chemical companies right now about potential applications for our work,” Arnold said. “These compounds are hard to make synthetically, so a clean biological route to produce these compounds is very attractive.”

Future research can explore what advantages and disadvantages the ability to create organo-silicon compounds might have for organisms. “By giving this capability to an organism, we might see if there is, or is not, a reason why we don’t stumble across it in the natural world,” Arnold said.

The research was funded by the National Science Foundation, the Caltech Innovation Initiative program, and the Jacobs Institute for Molecular Engineering for Medicine at Caltech.