WITHOUT water, life could not exist. Or so the theory goes. But this basic tenet of science, which has dictated the search for living systems on other planets, is being challenged. Biologists, chemists and physicists will convene in the UK next week to think the unthinkable 鈥� that water may not, after all, be one of life鈥檚 essential ingredients.
鈥淚t is taken as axiomatic that life requires water,鈥� says Roy Daniel, an enzymologist at the University of Waikato in Hamilton, New Zealand, who helped to organise the meeting. 鈥淏ut if it鈥檚 true, we ought to know why. We don鈥檛.鈥�
The possible roles that water plays in living systems range from it simply being a passive medium in which the chemical reactions of life take place, to having one of several irreplaceable functions. These might be linked to liquid water鈥檚 well-known unique or anomalous properties, such as its ability to form unusual non-covalent bonds, or the fact that it conducts protons so readily.
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But no one has yet been able to produce conclusive evidence to show how water is vital for life, says John Finney, a physicist at University College London and another organiser of the meeting The Molecular Basis of Life: is Life Possible Without Water? to be held at the Royal Society in London.
The meeting will attempt to produce a definitive list of water鈥檚 life-supporting properties. 鈥淚 have asked the speakers to speculate, or give data, on how you could replace water in biological systems,鈥� says Finney. 鈥淚f you can鈥檛 replace it then it tells you something about its attributes.鈥�
However, evidence is mounting that some vital biological systems may be able to work in the absence of water. Much of the discussion will focus on enzymes, which can be considered to be the fundamental machinery that supports life.
The orthodox view is that for an enzyme to function, it needs around 20 per cent of its own mass of water to be present. But experiments suggest this is simply wrong, says Daniel.
In a bid to improve yields, biotechnology researchers routinely use solvents other than water as a medium for enzyme reactions. 鈥淭here are many cases where enzymes work successfully in very low water conditions,鈥� Daniel points out. 鈥淭hat tends to suggest water may not be as important as we once thought.鈥�
These experiments do not rule out the possibility that small amounts of water are still present and are necessary to keep the enzymes in good structural order. But there is a way of testing this, says Daniel. Biotech researchers have experimented with reactions where the substrates are in the gas phase and the enzymes are in dried, powdered form. This allows the amount of water added to the powdered enzyme to be tightly controlled.
Early experiments already suggest that enzyme activity is preserved at very, very low levels of water. So far, the enzyme requiring the least water has been a lipase or fat-digesting enzyme from the yeast Candida, which functions at less than 10 per cent water by mass.
Few enzymes have substrates that can enter the gas phase, though, and it is impossible to say whether an enzyme will be found that remains active in the complete absence of water. But 鈥渨e are beginning to think that the requirement for water may not be an absolute,鈥� says Daniel.
All terrestrial life needs water. However, the first life may have used water simply because it was available, says Daniel, not because it has any unique properties. If true, this has important implications for the search for life on other planets (麻豆传媒, 12 July, p 28). Life could have evolved on worlds where water is not available but some other solvent is, he speculates.