We have all heard the conventional wisdom that water is the essence of life. Organisms are made predominantly of water, so it seems rational that if we were to explain the origin of life on earth or speculate as to the likelihood of any given exoplanet having life, the presence of water here or there would seem like an essential component to life’s origin(s). Some astrobiologists, planetary scientists and astronomers have gone so far as to make claims that the discovery of water on another planet makes the presence of life inevitable there.
Nobel laureate and Harvard origin of life researcher Jack Szostak wrote a nice piece for a conference sponsored by the Templeton Foundation back in 2003. In it he questions the standard assumption that some have that the presence of water implies a strong probability that life would be present on a planet in which we find it.
But was water really ideal for the origin of life here or on any exoplanet? Szostak makes several points that are worth relating to those who are convinced that water is the miracle solvent for life’s chemistry and that the presence of water on any planet necessarily increases the probablility that life will be found there. “Stepping back from our parochial water-dominated viewpoint we can immediately see that water is really a noxious, toxic, corrosive and generally lethal environment for life.”
The chemical reactivity of water is damaging to genetic material. Hydrolytic reactions are not kind to biomolecules. Szostak gives the example of the effects of water on the nucleotide cytosine, which causes a spontaneous deamination (stripping away an amino group), and transforms the base to uracil. That’s a mutation, folks. Water breaks down sugars, and makes dehydration reactions necessary in order to polymerize proteins and nucleic acids. He elaborates, “Water restricts the domain of chemistry that is suitable for life to a very small fraction of the possibilities that might otherwise be open to exploitation by evolution.”
Life on our planet has evolved energetically costly repair mechanisms to protect genetic material. Complex cell membranes had to evolve to provide compartmental boundaries to maintain the integrity of biomolecules necessary for life’s chemistry. Metabolic intermediates are often unstable in water. Active sites of enzymes have in fact evolved to exclude water; a mutation in the genetic material that codes for the enzyme that allows water access in the active site often leads to the synthesis of undesirable side-products.
Also metabolic precursors need to stay soluble, but not diffuse across the plasma membrane.
Interestingly enough a universal mechanism to control cell diffusion of biomolecules that evolved was the use of phosphate esters in metabolic intermediates in spite of the fact that phosphate is the least abundant of the major elements of life, and in its reduced form is highly soluble in the presence of iron, so much of it disappeared during planetary differentiation with much of the remaining phosphorus precipitating into very insoluble salts (e.g. apatite).
Of course we know that water needs a narrow temperature range in order to remain stable. Ice crystal formation of freezing water destroys cellular membranes. In my own lab I use a -70C freezer to quickly freeze microbes, and have to use cryoprotectants like glycerol or milk to reduce ice crystal formation. Organisms have evolved elaborate strategies such as the synthesis of complex sugars like trehalose or peptides to bind and prevent crystallization.
Biochemistry relies on the production and especially the post-translational folding of proteins to shape them to do all sorts of regulatory jobs in the cell. The hydrogen bonding properties of water make it very difficult for proteins to maintain their critical three dimentional shape, because hydrogen bonding of water to unfolded proteins is more likely and inhibits intramolecular bonding necessary for folding. Szostak supposes that stable folded protein structures could be possible with simpler, shorter polymeric sequences (and easier to make) if water was not such a good hydrogen bond donor and acceptor.
The speculation that the presence of water is a necessary condition for the origin of life, or that the discovery of the presence of water makes it much more likely that life has originated on other planets may not…well, hold water.
