Science by Marilyn: Alternative Biochemistries – Non-carbon

[Samantha York]

Alternative Biochemistries – Non-carbon based

Science fiction has long speculated on life forms based on something other than our known ones of carbon and hydrogen along with water as a solvent. The first one was speculated on way back in 1891 and of course uses the atom just under Carbon in the Periodic Table – Silicone. Star Trek used this concept in the ‘Devil in the Dark’ with the Horta. Even HG Wells wrote: “One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminum organisms – why not silicon-aluminum men at once? – wandering through an atmosphere of gaseous sculpture, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace."
However in reality there are several problems with the idea. The first is that the equivalent of carbon dioxide is silicone dioxide that is basically what quartz or most of your beach sand is made of. It is not soluble in water until you get to extremely high temperatures. Secondly, unlike carbon silicone does not form bonds well with several other atoms that are needed to form the necessary functions of life. Silanes, which are the chemical equivalent of hydrocarbon, are extremely reactive in water.
Now given that using a solvent say sulfuric acid and at temperatures much higher than given, some sort of chemistry could exist that mimics some of what can happen in carbon with water. So perhaps the Horta was exuding something of its own internal chemistry?
Still there are further problems. There are no known analogs of protein or RNA or DNA known to exist that utilize silicone rather than carbon. Though some silicone based material can form a sort of semi-permeable membrane they simply cannot form the diverse kind that carbon ones can. Rarely does silicone form double and triple bonds, which are needed for so many living functions.
Consider that silicone is much more abundant on the Earth than Carbon and yet nature built life out of carbon. Simply put if nature selects for the best fit and ithen it selected carbon rather than the much more abundant silicone even on an early and still relatively hot earth.
While an interesting idea the chemistry is not there to support the idea that silicone based life is possible. Sorry but for the Horta to exist at all it has to be something not completely silicone based.
Now there are other chemistries that have been considered in carbon grouped elements such as germanium, tin and lead but while polymers have been formed much of the same argument with silicone remains in other parts.
Boron on the other hand is a much more diverse element in forming bonds than even carbon. The equivalent of hydrocarbon molecule though boranes are reactive in the presence of oxygen. However they are stable in reducing environments. The ability of boron to form long chains is possible but has not really been observed outside of laboratory conditions. While boron offers a much more plausible alternative to carbon than silicone it is relatively scarce in the universe.
One last compound that may substitute in our known biological chemical universe is arsenic. Chemically similar to phosphorus it has been proposed that it could under certain conditions replace phosphorus in the structure of RNA and DNA. These arsenic type compounds that are so much less stable than that of their phosphorus equivalent. One bacterium called GFAJ-1 is claimed in extremely high arsenic conditions to incorporate arsenic rather than phosphorus into its RNA and DNA.
Other examples on Earth include substituting selenium or tellurium for sulfur making selenoproteins and tellerioproteins.
For skeletal replacement instead of calcium, strontium has been incorporated and some organism such as sponges and diatoms use silica.