For life on Earth, carbon is king. All organisms build their cells from carbon-based molecules. Scientists and science fiction authors have long speculated that because silicon atoms bond to other atoms in a manner similar to carbon, silicon could form the basis of an alternative biochemistry of life. Yet even though silicon is widely available on Earth and makes up 28% of the planet’s crust (versus 0.03% for carbon), the element is almost entirely absent from life’s chemistry.
That may soon change. Researchers reported in San Diego, California, this week at the semiannual meeting of the American Chemical Society that they have evolved a bacterial enzyme that efficiently incorporates silicon into simple hydrocarbons—a first for life. Down the road, organisms able to incorporate silicon into their cells could lead to a novel biochemistry for life, although for now creating actual silicon-based creatures (like the Horta from Star Trek, pictured) remains a long way off.
To get biology to adopt silicon, Frances Arnold, a chemist at the California Institute of Technology (Caltech) in Pasadena, along with postdoctoral assistant Jennifer Kan and graduate student Rusty Lewis, started by isolating a so-called thermophilic bacterium, which grows in hot springs. Like many organisms, the bacterium contains an enzyme called cytochrome c, which shuttles electrons to other proteins, making it widely useful in biochemistry. In some cases, however, enzymes in thermophilic bacteria expand their roles to carry out other reactions on the side. So the Caltech researchers tested their microbe and found that in rare cases its cytochrome c also added silicon to hydrocarbons.
In nature, Arnold notes, cytochrome c’s silicon-adding ability is so feeble that it’s probably just a byproduct of the enzyme’s function—not even close to its primary role. To try to beef it up, the team incubated the bacteria with silicon and carbon compounds and selected the organisms that produced the most hydrocarbons that incorporated silicon. After only three rounds of this artificial selection, the enzymes had evolved to churn out silicon-containing hydrocarbons 2000 times as readily as natural cytochrome c. “The power of evolution really shows up when a new function appears and then is forced to adapt via directed evolution,” Arnold says.
For now, the silicon-spiked hydrocarbon compounds, called organosilanes, probably aren’t useful either to the bacteria or to industry. They’re short and stubby, unlike the long chainlike versions that chemical companies make for uses such as adhesives, caulks, and sealants.
Nevertheless, Joseph DeSimone, a chemist at the University of North Carolina, Chapel Hill, who saw Arnold present her results at the meeting, called the results “amazing” and said they open new vistas for chemistry. Arnold notes that living organisms specialize in carrying out complex chemistry at moderate temperatures and pressures, making them ideal for use as green chemical factories. So someday, evolved microbes may be able to produce complex silicon-based materials, such as those used in adhesives, using only a fraction of the energy chemical companies require today.
Taking the bigger leap to produce silicon-based life like Star Trek’s Horta remains a far more distant prospect. But now that scientists have a toehold, Arnold says, it will be fun to see what they can do with it. “Now, we have the opportunity to bring silicon into life.”