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Water is involved in the photosynthesis. How much water are we talking about compared with the total amount on water on Earth? Is it enough to have an effect on the average age of water molecules?

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I would guess that fraction of water involved in photosynthesis is rather small, but since this is by far the largest natural phenomena that "destroys" water molecules, it has the predominant effect on their average age. – Pygmalion May 6 '12 at 7:23
One could calculate an upper limit: find the biomass mass and assume it is all a result of photosynthesis ( since it has organic molecules) and compare with the mass of water in the oceans. It will be a small number – anna v May 6 '12 at 7:27
@annav The other posibility would be an (slightly wrong) assumption that photosynthesis and burning are in equilibrium. Then you can estimate the quantity of water from total CO$_2$ emissions. – Pygmalion May 6 '12 at 7:32
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+1 for an interesting question. Who'd have thought life could recycle the entire water content of the Earth in only 10 million years? – John Rennie May 6 '12 at 7:55

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According to http://ga.water.usgs.gov/edu/earthhowmuch.html the total volume of water on the earth is $1.386\times 10^9$km$^3$, which is about $1.4 \times 10^{21}$kg (I'm rounding because I don't know the average temperature and therefore density of the water).

According to http://en.wikipedia.org/wiki/Biomass_(ecology)#Global_rate_of_production the annual photoautotrophic production of biomass is 104.9 billion tonnes C/yr. Assuming this is all carbohydrate one carbon atom is associated with roughly one water molecule (monosaccharides are C6H12O6) so the weight of water associated with that weight of carbon is $1.6 \times 10^{14}$kg.

So if you assumed all water gets cycled through the biosphere you'd conclude that the average life of a water molecule is 8.75 million years.

I must admit this is shorter than I expected.

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shorter because you have not subtracted how much of that biomass decays into carbon and hydrogen, which will end up as water again. Most biological mass has small lifetime . – anna v May 6 '12 at 11:30
also the world's supply might be replenished by meteorites continually.Have a look telegraph.co.uk/science/evolution/5422498/… – anna v May 6 '12 at 11:38

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