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Re: Carbon uptake

Dave Gomberg wrote:
> A controversy has arisen over the following question:
> If the reaction:   Ca++ + 2HCO3-  ->  CO2 + CaCO3 + H2O (1)
> could be facilitated, would the reaction be to the benefit or detriment of
> aquatic plants trying to take up carbon from the water?

This would depend if the plant were only able to utilize CO2 or if it
were able to utilize carbonates.

> The reason for asking this question is that it appears there is a device on
> the market which may do this (CarboPlus).

If it did, the result would be a loss of CO2 over time as a result of
outgassing, if the CO2 were not replaced. pH would also rise quite
drastically (unless other acids were present especially organic acids of
natural origin (think peat)) and there would need to be a net input of
acid to maintain pH within the most desirable range of 6-7.5 Instead, it
is most common to add CO2 as dissolved gas. This balances any CO2
utilization by plants which "crack" bicarbonate for it and keeps the pH
constant (barring other effects).

The question is, is CarboPlus actually adding CO2 into solution by some
other reaction? There's been mention of some carbon electrodes which are
allegedly supposed to do this HOWEVER, it is much more likely that other
reactions will take place on those electrodes such as the electrolysis
of oxygen, chlorine, hydrogen or the formation of various deposits and
compounds. I say this not as an authority on the subject, but having
read these comments made by knowledgeable people on this forum. (please
correct me if I've said anything misleading or in error)

> I attempted to answer this question by consulting Aquatic Photosynthesis by
> Falkowski and Raven, a recent tome with a whole chapter devoted to uptake
> of carbon by aquatic plants.
> The bottom line appears to be this:   Since at the leaf surface carbonic
> anyhdrase greatly facilitates the reaction:   CO2 + H20 <-> H2CO3 which is
> otherwise a pretty slow reaction and since the bicarbonate ion is dominant
> at most aquarium pHs (6.5-9) (with increasing importance of the carbonate
> at lower pHs) the binding of carbon into CaCO3 is to the net detriment of
> plants, especially aquatics.  This point is discussed in some detail on
> page 135.
> If you are surprised by this conclusion, I invite you to consult the above
> reference and form your own conclusion.  I believe you will agree with
> mine, but welcome debate on the subject with appropriate references.   

I agree that if conditions are such that CO2 is being extracted from
bicarbonate which results in CaCO3 precipitation, then the pH must be
rising and that many species of plants will grow optimally in these pH
conditions. There are several species of plants however, which do not
mind alkaline conditions and are adapted to this method of gaining
carbon (so called biogenic decalcification) They do not need any help in
cracking bicarbonate so other reactions which artificially did so would
be competing for bicarbonate and raising the energy investment needed to
perform the trick.

As for the April 1 article on magic Amazonian laterite, I guess not
everybody realized that was an April Fool's prank in addition to the
transmutation report! George, good work on your heating coil experiment!
This certainly sets my mind at rest on the subject! ;-)

Steve Pushak                              Vancouver, BC, CANADA 

Visit "Steve's Aquatic Page"      http://home.infinet.net/teban/
 for LOTS of pics, tips and links for aquatic gardening!!!