[APD] pH's at the leaf's surface, bicarb uptake vs use, CO2/non CO2, algae etc

["Thomas Barr" <tcbiii at earthlink_net>]

For the curious:

Hydrillia can have huge pH differentials between the top adaxial surface of
it's leaves(pH 8-10) and the lower underneath abaxial surfaces(pH4) and the
plant's leaf is only two cells thick.It also has no stomata(take a leaf
under a scope and see for yourself). You can also see the chloroplast move
around quite fast under strong lighting(Cytoplasmic streaming). 

So why the pH differentials? The plant pumps out protons on the abaxial
surface lowering the pH. So if you lived in an environment that was CO2
poor, and there was some HCO3, bicarbonate around, what would the carbon
species be if the pH was reduced to 4? All of it would be in the form of
CO2 which would available for uptake.

This is termed bicarbonate USE and not bicarbonate UPTAKE. The use is
indirect and active but the HCO3 is not taken to the cell's surface and
converted to CO2. 
Myriophyllum spictatum will do HCO3 _uptake_ as will most
algae.............. but both algae and plants prefer the CO2. 

If you remove the high CO2 enrichment, many plants such as mermaid weed,
Myriophyllum, hornwort, AND ALGAE take about week to gear up and start
using HCO3 at high rate. Why have the enzymes hanging around for HCO3
uptake if there's plenty of CO2 around? It's a wasteful process to do that
so they adapt to their environment. If there's enough CO2, they just don't
need all these enzymes around.  

So adding CO2 seems that it would help both algae and plants grow better. 
CO2 helps the plants grow much better as they need MORE CO2 than the algae
in relative terms. Similarly, like the higher NO3, PO4, K+, trace values we
also keep, this gives an advanatage to the plants, rather the notion of
___limitation___. 
 
This is a plausible arguement for the reason why you see good plant growth
in non CO2 tanks as well as CO2 tanks.
The algae and plants have the same issue in a non CO2 tank and both adjust
after as little while the new conditions.Growth is slower for both, but
many of the principles are the same. Bicarb plants do very well in these
tanks, some wimpy plants don't but I think much of it has to do with if you
have  ablance of floating/bicarb using plants, then...........you can also
have some of those wimpy plants like Gloss, hairgrass(a C3-C4 intermediate
plant), Alternathera reinickii etc.  

Much of the issues go back to light and plants being able to outcompete
algae for light. Having less light helps plants dominate more. They are
already low light plants generally adapt for good light competition. 

But going back to the CO2/HCO3 issue with the enzymes: having CO2 levels
that bounce around daily or weekly, is BAD for plant growth.
It "confuses" them. And you see bad algae issues in such tanks as some
algae are quick to respond to changes while a larger plant generally is not.
Some algae will die off if you change the non CO2 tank to a CO2 enriched
tank. 
 
Hydrilla also has C4 metabolism to concentrate carbon around Rubisco
spacially to prevent photorespiration from occuring at higher temps, high
growth rates, and high O2 levels. There are few others that are C4 plants
but this ability is facultative in aquatic plants and is obligate in
terrestrials plants.

Proserpinaca palustris will change it's leaf shape and become
"skeletonized" in low CO2 environments and regain it's emergent and CO2
rich leaf shape when you add CO2.I've seen this in my non CO2 tanks and
also in the wild. I thought it was simply emergent growth but it appears to
be a direct response to increased CO2 concentrations in the water after
growing in both locations. This would be an interesting project for someone
to look into. 


Regards, 
Tom Barr


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