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Old 02/24/2016, 10:56 PM   #3192
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Originally Posted by 34cygni View Post
Note that the equation I gave is identical to the one at the beginning of the article you linked to. It's the standard simplified representation of photosynthesis, which is why I said CH2O represents sugar.
Yes, I was commenting for others.

Photosynthesis normally tends to raise alk because it consumes CO2, which when drawn down will cause HCO3- to dissociate into CO2 and hydroxide. That's why the persistently low alk and Ca levels reported by DNA and others demand an explanation -- like I said, this has been at the back of my mind for a while...
Consuming carbon dioxide does not change the alkalinity. Neither does adding it. Hydroxide has one unit of alkalinity, as does HCO3-, so it's a net zero.

I don't see where that link provides evidence that marine primary producers consume alk when they're CO2 limited and running on bicarb...
You are correct that running on bicarbonate does not consume net alkalinity. That's not what I was trying to say. I was referring to the period between the uptake of bicarbonate and the release of the OH-, which is going to be fairly small, but might affect the alkalinity. There's also some possible consumption into organics into the cell itself, which is going to be small.

Subtract the OH- common to each step...
HCO3- + H+ ----> H2CO3 ----> CO2 + H2O ----> CH2O + O2
This is the equation that shows that photosynthesis doesn't change alkalinity. The HCO3- is balanced by the H+.


The definition of total alkalinity:

TA = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]

Jonathan Bertoni

Last edited by bertoni; 02/24/2016 at 11:15 PM.
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