Mike F.
With the right enzymes
Tue May 29 23:46:56 2001

With the right enzymes, H+ Ions could be encouraged to give you C + H2O. I'm not sure, but methinks that would be
exothermic, also.

With the right cell structure life forms may be able to make diamonds as a waste product!

______________________

OK:-

The solution for Venus is to seed the cloud decks with a microbe engineered to convert CO2 to diamond + O2. In a short span of time one would end up with a crunchy and durable surface deposit, plus an 87 bar oxygen atmosphere. Hopefully the rocks have got some unoxidised metals left to soak up the oxygen (a la BIF?) or the planet ends up unusable, but sparkly.

Back to your chemistry, if you supply a source of H2, most things are exothermic. Trouble is, where does the H2 come from? (and how much energy does it take to create molecular hydrogen, and what processes achieve this?) [Deep dark biosphere answers will only be accepted if they flesh out their model, rather than asserting the case!]