Living in a physical world IX. Making and maintaining liquid water

Abstract

Metabolically active organisms contain water in its liquid phase – I believe no exceptions are known. Life’s domain consists of the intersection of the circumstances under which liquid water will persist and those at the earth’s surface – except as we artifi cially maintain some bit of that domain elsewhere. No single phase of a single compound so characterizes the conditions necessary for life. Yes, a few ice crystals can often be tolerated, usually if extracellular. And water vapour can play some useful roles – in reducing evaporation, as a condensable resource, perhaps for producing the density variations that permit free convection. But liquid water is crucial to life as we know it. In a once well-known book, The Fitness of the Environment, Lawrence Henderson (1913) assigns a defi nitional rather than merely a facilitating role to the particular and peculiar physical and chemical properties of liquid water. The specifi cs of his argument still, one might say, hold water, even if their context now strikes the reader as offensively teleological and tautological and fails to persuade – at least as we contemporary biologists understand the idea of fi tness. Water abounds on earth, and most of that water will be in the liquid phase under conditions typical of its surface. But life perpetually pushes against environmental limits – in particular, against its abiotic barriers. Managing where water is minimally accessible can provide temporary refuges, open otherwise unexploited regions, and so forth. In all too many places, temperatures sometimes fall to levels at which water prefers its solid phase. Nor can life ignore its gaseous phase. While in few habitable places do temperatures exceed water’s normal boiling point, vaporization occurs in virtually all terrestrial habitats. So familiar is that last point that a subtle peculiarity of our immediate world can escape our notice. Enclose a dish of some volatile liquid in an air-fi lled container and keep the whole thing in a dark place at a constant temperature. Evaporation will proceed until the gas phase contains vapour at its saturation partial pressure, at 100% relative humidity for that substance. But atmospheric air, even over large bodies of fresh water, rarely reaches 100% humidity (or 97%, when equilibrated above seawater). Temperature variation, convection, and wind create innumerable opportunities for condensation. And thus we hang out our washing even on overcast days, confi dent that it will dry in the air. Nonetheless, equilibration can and does occur. For instance, the air within soil most often contains water at full saturation pressure, even in all but the few top centimeters of the soil of deserts (Schmidt-Nielsen and Schmidt-Nielsen 1950).