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Re: Hydrogen as the limiting nutrient?



> Date: Fri, 8 Oct 1999 11:22:43 -0400 (edt) 
> From: "Roger S. Miller" <rgrmill at rt66_com> 
> Subject: Hydrogen as the limiting nutrient? 
>  
> Folks, 
>  
> In one of John Raven's books he pointed out that when aquatic plants
> and algae use nitrate as their nitrogen source they must also consume
> hydrogen ions. (When plants use ammonia/um as their nitrogen source,
> the necessary amount of hydrogen is imported with the nitrogen.) 

Are you sure (Is he?  And why?) that they use hydrogen *ions*?  Certainly
hydrogen is needed since the NO3- that they're consuming is going mostly
into NH2 radicals. (*)  But the obvious way to perform this reduction
reaction is to use the hydrogen that abounds in all organic matter.  Fat
is mostly a long string of CH2, and sugars are CHOH, speaking crudely.
It would be pointless to make a balanced equation of this, but the result
of a lot of fancy chemistry is that you've got NO3 and CH2 reacting to
make NH2 and CO2.  

(*) Not speaking here of the infamous free radicals, much less making
political jokes, but just using radicals as convenient groupings for
descriptive purposes.


> Hydrogen ions are 
> usually present at rather low concentrations -- 0.0001 milligrams per 
> liter at pH 7 -- which gives rise to the theoretical possibility that 
> hydrogen ion availability could be growth limiting.

True, but not very important, because removing H+ will cause new ions to
pop up out of the woodwork, or rather out of the dominant buffering
system.  You'll normally have carbonate at a concentration several orders
of magnitude higher than the one named.  Taking out H+ will pull H+ out
of H2CO3 and will shift the buffer system up to a higher pH, as you've
noted.

Quantitatively, even if you start with a few mg/l of nitrate and reduce
it all to ammonia, somhow using H+ as the reducing agent, consider the
carbonate. There's surely more than 35 mg/l (KH of 2 -- too low!) in the
carbonic-bicarbonate system. So if you strip the H+ from a few milligrams
of H2CO3 you don't change the pH much. 


-- 
Dan Drake
dd at dandrake_com
http://www.dandrake.com/