If the truth be told the presence of a bacterium that uses arsenic isn't exactly unique. Other organisms, bacteria, algae and fungus use arsenic is various ways in their metabolisms instead of phosphorus.
http://en.wikipedia....of_biochemistryArsenic as an alternative to phosphorus
Arsenic, which is chemically similar to phosphorus, while poisonous for most Earth life, is incorporated into the biochemistry of some organisms.[10] 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).[11] 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 on Earth may have used arsenic in place of phosphorus in the backbone of its DNA.[12] A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic would not be suitable for this function.[13] However, a geomicrobiology study released by NASA has revealed that a bacterium named GFAJ-1, collected in the sediments of Mono Lake in eastern California, appears to employ such 'arsenic DNA' when cultured without phosphorus.[14][15][16][17][18] The bacterium may employ high levels of poly-β-hydroxybutyrate to stabilize its arsenate esters.[16]
Other exotic element-based biochemistries
Boron's chemistry is possibly even more variable than that of carbon, since it has the ability to form polyhedral clusters and three-center two-electron bonds. Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. 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; 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, aluminum, 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.
Sulfur 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 theoretical, especially since 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.[9] Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)
More on boron life.
http://www.daviddarl...based_life.htmlBoron is one of the few elements that seems to offer a plausible alternative to carbon as a basis for life elsewhere in the universe. Like carbon and silicon, boron has a strong tendency to form covalent molecular compounds. Being a group III element, however, it has one less valence electron than the number of valence orbitals, which makes its chemistry noticeably different from that of carbon.
There are no direct analogs to hydrocarbons in boron chemistry because, although boron forms a lot of different structural varieties of hydride, in these the boron atoms are linked indirectly through hydrogen bridges. Boron forms bonds with nitrogen that are somewhat like the carbon-carbon bond – two electrons from the nitrogen being donated in addition to the covalent electron sharing. Boron-nitrogen compounds largely match the chemical and physical properties of alkanes (such as methane and ethane) and aromatic hydrocarbons (such as benzene) but with higher melting and boiling points. Borazole especially is both chemically and physically similar to benzene. However, the fact that borazole and its derivatives are more reactive than their benzene counterparts would make any boron-based biochemistry more feasible within the lower temperatures at which ammonia is a liquid solvent since the reactions would then be more controllable. Interestingly, boron has an affinity ammonia as a solvent, which would suit a low-temperature biological scheme.
One of the biggest drawbacks to boron as a basis for life it is scarcity. On Earth, its abundance in the continental crust is only about 10 parts per million, so that any biology would seem to depend on their being present some mechanism for bringing about greater local concentrations of the element.
Then there are the clouds...
http://www.npr.org/b...louds-are-nicer
Edited by Moontanman, 5 December 2010 - 01:40 AM.
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