> I'm still pondering the phosphorous issue.  It seems like it all
> eventually ends up in the water, and could be recovered from algae
> growth or extraction of lake and sea floor sediments?

Perhaps.
It depends in part on how dilute it gets and what value is placed on 
it. If that's the sole remaining source then the value could be quite 
high.

On average, and this varies rather widely, there is notionally several 
kg of Uranium within digging depth on a "1/4 acre" residential 
section. Extracting it, let along profitably, is another matter.

Gold is perhaps easier. But not by a lot. Sea water contains vast 
amounts of gold all up, but totally uneconomic quantities per given 
volume. The level is so low that it is exceptionally difficult to 
assess it accurately but a\n average figure of around 10 parts per 
trillion seems about right. May be 5+ times high in places such as the 
Bering Sea which are fed by high gold areas.

Reasonable reference here

        http://www.goldfever.com/gold_sea.htm

Best hope for economic extraction is probably to engineer a 
microorganism that retains gold and drinks a lot.

At say 10 ppt by volume that's about 140 microgram per ton(ne). At 
current gold prices of about $US650/gram

        http://goldprice.org/news/uploaded_images/gpsentiment240507sm-700412.gif

that about $US1 of gold per tonne of seawater.

It's sobering (to me) that the total expected gold content of seawater 
is only about 8 times more than all the gold that mankind has 
extracted from various sources throughout history - ie sea water is 
rather low grade ore. Total gold extracted in history is thought to be 
about 5000 cubic metres! 'Quite a lot'.



        Russell





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