In a new paper, Jennifer Kan of Caltech and her colleagues show that—with a little coaxing—microbes can make silicon-carbon bonds after several generations of evolution. Life on Earth relies heavily on carbon-carbon bonds, but scientists have long speculated that other chemical bonds—particularly involving silicon—could lead to other kinds of biology.
The researchers isolated an enzyme (a type of protein) from a bacterium (Rhodothermus marinus) found in hot springs in Iceland, then inserted the gene for it into another bacterium, E. coli. After a few rounds of mutations, the enzyme was able to build carbon-silicon bonds, and it did so many times more efficiently than these bonds can be produced synthetically in the lab. In other words, enzymes found naturally here on Earth can be “repurposed” to build new bonds not found in life as we know it.
Carbon-carbon bonds break apart and recombine easily at relatively low temperatures, which makes them ideal for biochemistry. It isn’t obvious what advantage carbon-silicon bonds would hold for life, as those bonds are very resistant to heating—similar to the silicon-oxygen bonds commonly found in rocks on Earth. In fact, most terrestrial rocks are made of silicates like quartz and feldspar, which have to be heated to very high temperatures before they become liquid and undergo reactions—an essential property for biology.
We know, however, that silicon is important to many living organisms. Diatoms, a type of algae that live in the oceans, are critically dependent on silicon for their growth, as their shells are composed of silicon dioxide. Plants also use silicon, in the form of silica within stem walls that allow them to stand erect. In animals, silicic acid is a major constituent of hair, nails, and skin. And when a bone is broken, high levels of silica are found around the break as it heals.
So silicon has its uses for life on Earth. The question is whether it could play an even larger role in organisms on other worlds. There are two principle possibilities: life based on silicone and life based on silanes.
Silicones are organo-silicon polymers that could possibly figure in alternative life forms existing at higher pressures and temperatures than occur commonly on Earth (although not high enough to melt the silicates). Any silicone-based life would require higher abundances of silicon than carbon, and a world devoid of oxygen. They would also need a suitable solvent other than water.
Silane is a molecule similar to methane (CH4), but with the carbon atom replaced by a silicon atom. Under certain environmental conditions, silanes can link together to form polysilanes and polymers. Silane-based life might be possible on a world with no oxygen and no liquid water, low temperatures and high pressures, and a hydrocarbon solvent in the form of liquid methane. This is all available on Saturn’s moon Titan. However, it is difficult to envision such a life form existing near that moon’s surface, because of the abundance of carbon and relative lack of silicon on Titan.
Which brings us back to the result reported by Kan and her colleagues. Silicon-carbon bonds, made by Earthly bacteria, may have great applications for pharmaceuticals and agricultural chemicals. They probably have less importance for life, at least on Earth. Yet it shows us life’s great flexibility and its ability to use any resources available in its environment. And that helps us appreciate how diverse biochemistry may be on other worlds.