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Re: H ion modelling

Richard Sexton wrote:
> What happens to your model if you add a few drops of
> say hydrochloric acid?

Well, with moderate additions not much.  In all of the simulations the
plant population grows to a point where senescence, pruning and so on just
balances the growth rate and after that the population stays mostly
constant.  Small additions of hydrochloric acid lowered alkalinity and pH
slightly and increased the size of the final plant population by only a
few percent.  Additional increments of acid also decreased alkalinity and
pH and increased the final population, but only slightly.  The hydrogen
limitation wasn't lifted until the pH got down to 5.5 or so, just before
the pH crashed.

I have been comparing net growth rates and haven't tried comparing the
growth rate unadjusted for mortality etc.

This is a smaller effect than I anticipated and as yet I don't have a good
explanation for it.  Quite possibly there's still a gaff in the model

The model is quite independent of John Raven's speculation about possible
hydrogen limitation.

> ------------------------------

Dan Drake wrote:

> > 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.

He concluded specifically that there was a need for hydrogen ions when
plants grow on nitrate.  Conversely there was also a need for hydroxyl
ions when plants grow on ammonium.

His conclusion is based on balancing all the various reactions necessary
for a plant to assimilate nitrate and on specific experiments with
Hydrodictyon africanum, which I believe is a freshwater macroalgae.

> > 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.

Raven made this point, as well.  Quoting:

"We may note that, while protons and hydroxyl ions mentioned above
as 'conditional' nutrients depending on the nitrogen source used are
present at relatively low concentrations as the  free ions (about 10 umols
m-3 and 1 mmol m-3, respectively in seawater), the buffer capacity of most
waters makes these ions relatively abundant within the context of a pH
change of a few fractions of a unit, so protons or hydroxyl ions are
unlikely to become growth-limiting."

Yes, that really is one sentence.  Yes, the whole book is like that.

We are growing aquatic plants in mostly closed culture and many of us are
using water with relatively small buffering capacities.  The closed nature
of the culture means that changes "of a few fractions of a unit" can
accumulate over time to become significant.  The low buffering capacity of
the water some of us use may also make limitations possible that won't
happen under the conditions that Raven considers.

But you're right that under most conditions there shouldn't be a large
effect on pH or alkalinity in the tank - in many cases the effect probably
wouldn't be observable.  But I have to question whether the availability
of the hydrogen ions are sufficient to prevent growth limitation.

Raven estimated that plants needed about 1 mole of hydrogen ions for each
mole of nitrate they assimilated.  This is half what you might get by
estimating from the nitrate to amino conversion.  Our planted tanks
commonly have nitrate concentrations in the range of 0.08 to 0.8
mmoles/liter, while hydrogen ion is there at only 0.00001 to 0.001
mmoles/liter.  This big difference suggest that plants may have a hard
time getting hydrogen ions at a rate equal to the rate they can get
nitrate.  If they can't match the rates, then the hydrogen ion may become
growth limiting.

Roger Miller