At what total rate is water used in metabolic processes over the entire biosphere?

Question

A recent [column on "The Straight Dope" website](http://www.straightdope.com/columns/read/3300/do-we-drink-dinosaur-urine) discussed the idea that water molecules are largely cycled around through the water cycle, and that therefore the water molecules that come out of your tap could have been drunk (and excreted) by the dinosaurs. Cecil Adams (the author) points out a few flaws with this argument, namely that we've been burning fossil fuels for a couple of centuries and releasing water into the atmosphere, and that liquid water is constantly dissociating and reforming; so it's not strictly true to say that every water molecule on Earth has been around since the time of the dinosaurs. One thing that occurred to me in reading this article is that biological processes have also been consuming and producing water for many millions of years now; this would be another mechanism by which water molecules can be "reshuffled". **Is is possible to estimate how much water participates in these processes (per annum)?** I know that there are estimates out there about how much carbon dioxide is produced/consumed by the biosphere, for example, but I'm not sure whether one can straightforwardly extrapolate from "carbon consumed" to "water consumed," and I don't know if there are other significant metabolic processes that consume/produce water molecules.

Answer 1

Dear Asked,

Photosynthesis and (aerobic) respiration are certainly major biological "metabolic" cyclers. You would have to research anaerobic routes to see how much H2O they're cycling. The process of coal consolidation (eliminating some of the H and O) could be said to cycle some water, and more as the coal is exposed and/or burned to liberate the remaining H's. However: water is also tied up in "hydrated" mineral formation at diverging oceanic plate boundaries, and liberated much later by convergent plate subduction and volcanic eruption. And lastly, what exactly do you want to consider a water molecule? Is it the association of a particular oxygen atom with two particular hydrogen atoms? If so, because of the dynamic self-dissociation of water, recycling is very rapid indeed. There are other reactions kind of intermediate between these: bicarbonate/carbonate precipitation/re-dissolution is one that immediately comes to mind. Again, dynamic, hence, ubiquitously re-cycling, if you want to to think of it as that.

And once you think you have a handle on the definitions, then consider how you might obtain data, i.e. by isotopic differentiation (.delta. values). It's fascinating work to delve into!

Hope this helps you think of some of the issues involved, -- Mr. d.