Duane Ager & William Pope
February 2017

Does silicon-based life exist?

Our February 2016 edition of the environmental SCIENTIST Growing in space: Science beyond the biosphere sparked our members’ interest. On a related issue, we recently received a question which we thought was worth exploring in more detail: “Does silicon-based life exist – and if not, why?” Dr Duane Ager, IES member and Chartered Environmentalist, and Professor William Pope, one of our Vice Presidents, Chartered Environmentalist and Chartered Scientist, stepped in to answer.

Of course, we saw Star Trek as children, where there is a silicon-based life form from Janus VI (the Horta), which eats through rock like we walk through air. There's also a great Marvel character, the Sandman, who can transform into glass and back, and can manipulate the earth - a marvellous ability. But, the short answer is that silicon-based life is probably restricted to the realms of Sci-Fi. 

The idea appears to be broadly based on silicon's apparent similarity to carbon. Silicon is close to carbon in the periodic table, it has a valency of 4 (the same as carbon) and so each atom could in principle form bonds with up to 4 other atoms and could also therefore apparently be a building block a bit like carbon - and so may in principle be able to form similar structures, one might think. It is also widely available in the biosphere (indeed, in the universe) as that potential building block, in quantity.

However it all falls apart from there. 

Life is driven by some form of an energy source or sources, and needs an agent(s) and process(es) to release and use the energy available in its energy source(s). For life to survive, the agent(s) must be universally and continuously available, in quantity, and be a replenishing not a diminishing resource. In our environment we have oxygen available to be that agent, and during cellular respiration glucose (as a carbon based energy source) is broken down, producing CO2 and H2O, and energy is released. One could go on about ATP, and O2/CO2/various cycles and so on, but that's not the main point here, really.

The main point is that for life as we know it, when carbon reacts with oxygen it can form gasses - for example CO2 - and water, both of which are easily eliminated from a carbon-based organism in the process of its metabolism.

In comparison, when silicon reacts with oxygen, it forms a stable lattice which is then not possible to easily eliminate from the "organism" (if the organism did indeed exist), and so a silicon-based life form would not be able to get rid of the waste products arising from its own metabolism. It would therefore be a severely self-limiting life form! 

And, that's all ignoring the implausibility of the necessary oxygen replenishment processes and the other various cycles required for sustaining the silicon-based life if there is no CO2 and water release, and from which we benefit with carbon-based life.

The theoretical biochemistry of silicon is also problematic since it does not form many chiral compounds (despite considerable efforts in the laboratory); with carbon compounds, energy is released in a series of tightly controlled, interlocking steps whereby "right handed" carbohydrates are oxidised by enzymes composed of "left handed" amino acids. (Interestingly, it has been hypothesised that the handed-ness is a result of the first carbohydrates forming on a silicon surface, thus the handed carbon system that evolved was perhaps initially dictated by silicon, which one would think might be a bit perverse in all the circumstances. But that's life!)

However, the overall summary of all this is that energy-release from a hypothetical silicon-based life form is a massively problematical theoretical issue, which one would think would preclude the life form itself existing. 

In practical terms, however, and thinking about our (human) ability to experience and interact with a silicon-based life form, even if it could exist in theory, it is even more problematical. Silicon is inert at the moderate temperatures in the environments in which life as we know it exists, leading to an idea that silicon based life - if it could exist at all - could perhaps only exist at very high temperatures; these potential theoretical "organisms" have been called "lavolobes" and "magmobes". Unfortunately, the reality is that at such very high temperatures the chemistry of the other compounds required to interact with silicon would also be very different to that which has evolved for carbon-based organisms at standard temperatures and pressures - if indeed it's possible at all - so such a theoretical life form, if it did exist at all, would not be something which we could encounter, as we could not (and do not) exist in its high temperature and new-chemistry environment.

But, what about something which we could not even imagine, never mind just contemplate existing? Thinking off the planet Earth, astronomers looking at meteorites, comets and other planets have identified silicon dioxides and silicates - but no evidence of silicones or silanes which are the better potential biochemical precursors. So, at the moment, there is also no evidence of a new silicon-based life form which is outwith our experience or imagination.  

But, also and interestingly, from an environmental science point of view: in addition, the study of planet atmospheres has not as yet found any evidence of gasses that are "out-of-chemical-equilibrium" such as we observe on earth (with oxygen and methane which are constantly replenished by living organisms). This lack of "out-of-chemical-equilibrium" would therefore suggest that there is no life form out there at all, as yet found anyway (whether carbon or silicon-based). Maybe that exists somewhere yet to be found, but it seems a long shot. 

We also don't know of any theories as to how a silicon-based organism would carry its version of a genetic code. Last year Frances Arnold et al. published a paper which showed that the cytochrome c enzyme of thermophilic bacteria isolated from hot springs is able to incorporate silicon into carbohydrates - but (a) this only happens rarely and (b) required the carbon in the carbohydrate anyway, so it couldn't be viewed as providing any evidence let alone proof that silicon-only-based life is likely.

So, the answer, at least for now, is no – although silicon can sometimes be used biologically as a sort of structural support (and there are some examples claiming silicon as an essential trace element) for carbon-based life - silicon-based life itself does not exist, as far as we know, because of the chemical and physical properties of the element itself. It does make great Sci-Fi though!