A new look at life as we know it

Life as we know it cannot exist without six vital elements from the periodic table — carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorous.

Lots of other elements are also used such as iron, sodium, magnesium and calcium, but these six are a must.

This was thought to be the absolute unbreakable golden rule of biochemistry — until recently. Research just reported in the journal Science has shown that some bacterial species can cope without phosphorous and use another element as a replacement for the same jobs.

Nature uses a plethora of enormously complicated molecules that contain thousands of atoms.

Carbon atoms make up the skeletal back bone of these molecules, along which are attached hydrogen atoms. Chemical groups decorate these large molecules which often contain oxygen and nitrogen atoms.

Sulphur is a vital ingredient occurring in most proteins and phosphorous is a similarly vital element found in DNA, the molecules that holds the genetic code, and in energy carrying molecules called ATP for example.

But can any of these elements be replaced with others taking over the same role?

Science fiction writers have long told tales of life on other worlds where silicon forms the basis of biological molecules instead of carbon.

From our knowledge of chemistry, however, this isn’t all that likely. Silicon isn’t able to form the long chains that carbon can, allowing it to take on the skeletal function in molecules on Earth.

We know that the carbon- based building blocks that life uses form relatively easily through naturally occurring processes. The corresponding silicon ones don’t.

Life is lazy and so will take the easy route and use chemistry available to it. We can therefore be confident that life found elsewhere will be carbon based as it is here.

As another example, the least said about the scientific merits of the proposed nitrogen-based life forms in the Hollywood movie “Evolution” the better.

So can any of these special six elements be replaced? Until recently, the answer was a clear no.

However, a group of scientists led by Dr Felisa Wolfe-Simon, of NASA and the US Geological Survey, have shown that a species of bacteria called GFAJ-1, found in Mono Lake, California, is able to use the element arsenic, which occurs in relatively high concentrations in the lake, in place of phosphorous.

This may sound strange since we are all used to considering arsenic to be poisonous, which, for humans, it most certainly is.

Phosphorous is taken up by organisms primarily in the form of oxygen containing compounds called phosphates. Arsenic sits below phosphorous and is therefore in the same group of the periodic table. The corresponding arsenic compounds, arsenates, have very similar properties to phosphates as a result.

The reason why arsenic is ordinarily toxic is that the biochemical pathways that have evolved to take up and use phosphates for building molecules will also readily take up and use arsenates.

But these arsenate-containing products are much less stable than the phosphate containing versions and the molecules can much more readily break up, leading to the serious adverse consequences and death.

But these special bacteria have evolved the ability to cope with this instability of arsenate containing compounds.

Ordinarily they prefer and will grow far more quickly in the presence of phosphorous.

But when times are hard and phosphates aren’t available, they are able to feed on arsenates instead and can still thrive, contrary to what we would have expected.

Studies of these bacteria have shown that when fed on a diet where phosphorous is absent and only arsenic is available, they still grow and that arsenic is incorporated into their DNA and ATP molecules.

While many of the musings of science fiction writers may remain firmly in the realms of fantasy, what these exciting results show us is that life as we know may be far more adaptable and flexible than we had previously imagined.

We may still be able to dismiss the more far-fetched suggestions out there, but life elsewhere in the universe may be waiting to be discovered thriving in far more chemically-diverse environments than we could have imagined possible.



Dr Elliott is lecturer in chemistry at the University of Huddersfield