pH effect on Photosynthesis

[randy holmes-farley]

Charles:

In the review of the Invers et al paper, you say that

"It was found that at lower pH’s (5-6) the seagrass could photosynthesize at a greater rate due to larger amounts of CO2 being present. "

That caught my eye as the pH should not impact the amount of CO2 and H2CO3 present in seawater in equilibrium with air (or close to it, like a reef tank).

Indeed, what they did was control the total inorganic carbon at 2.2 mM. At pH 5.5, there is much more CO2/H2CO3 given that limitation.

Consequently, lowering the pH in seawater would not have this effect, because the tank will just blow off all that excess CO2 to the air, and not be able to maintain such a high DIC.

So I have no quibble with what you wrote: it is all quite accurate. But it should not be construed by readers to mean that lowering the pH of a reef tank will help seagrass grow or to increase the level of CO2.

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[jamesw]

Fascinating! Randy, how did they control the total inorganic carbon?

Cheers
James Wiseman

 

[randy holmes-farley]

James:

I don't know

I've only read their abstract. Here's a quote from it:

"Photosynthetic inorg. carbon utilization by the seagrasses Posidonia oceanica (L.) Delile, Cymodocea nodosa (Ucria) Aschers., Zostera marina (L.) and Phyllospadix torreyi S. Watson was studied by manipulating the concns. of aq. dissolved CO2 and HCO-3 in seawater. At const. dissolved inorg. concn. (ca. 2.2 mM), photosynthetic rates were higher at low pH (i.e. 6-7; [CO2] = 1.2-0.21 mM) than at "normal" pH (i.e. 8.2; [CO2] = 0.016 mM) in all four species."

Charles presumably can tell us how they did it, but by lowering the pH and taking a measurement before the water had a chance to degas could work.

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[delbeek]

Randy: Unfortunately that was one of the articles I took notes on at the library and not one I photocopied, so I can't comment much on how they performed the experiment. For what its worth, I did note that the CO2 they refered to was aqueous. Next time to go to the library I'll look it up if you haven't done so already yourself.

For what it's worth, I only mentioned the importance of maintaining alkalinity, I did not advise adding CO2, mainly because the pH's at which photosynthesis increased (5-6) are much too low for a marine tank anyway and you'd just end up nuking your tank. Reminds me of the time a friend of mine was playing around with a CO2 bottle from a pop machine and an airstone in the late 80's. He turned it on then got a phone call ... 1/2 hr later he came back to a tank with a pH of 3 and most things dead!

Aloha!
Charles

 

[randy holmes-farley]

Charles:

No, I agree that you didn't recommend CO2, and wasn't intending to criticize the review. It was interesting and I'm glad you brought it up. I only wanted to make sure that we understood what must be happening. One of the reasons this came to mind is that a poster on reefcentral once responded to an algae problem by claiming that low pH increased CO2 and thereby might drive algae growth. It is likely that the source of that advice was an experiment similar to this one, where CO2 is added, and as a result, the pH drops, not the other way around.

If they continually added CO2 in water to lower the pH, that would be one way to maintain high CO2 in the water.

FWIW, I once added a small chunk of dry ice to my sump. I became rather concerned as the pH dropped rapidly (into the sixes, maybe lower) but the tank was fine.

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[Anonymous]

"It was found that at lower pH’s (5-6) the seagrass could photosynthesize at a greater rate due to larger amounts of CO2 being present. "

That caught my eye as the pH should not impact the amount of CO2 and H2CO3 present in seawater in equilibrium with air (or close to it, like a reef tank).

Indeed, what they did was control the total inorganic carbon at 2.2 mM. At pH 5.5, there is much more CO2/H2CO3 given that limitation.

Consequently, lowering the pH in seawater would not have this effect, because the tank will just blow off all that excess CO2 to the air, and not be able to maintain such a high DIC.

I have not read the article, but I think that I can answer the question anyway.

Consider the carbonate equilibria:

As I understand it, when the pH is lowered (by any method) this will drive the reaction towards the left, and by that I mean that carbonate (CO3^2-) molecules are converted to bicarb (HCO3), and bicarb molecules are converted to carbonic acid (H2CO3). The carbonic acid then equilibriates with the partial pressure of CO2 in the atmosphere. The result is a greater concentration of CO2 in the water.

In addition, consider a graph of alkalinity on the Y axis and total concentration of inorganic carbon on the X axis. If you then take the a data point for any water sample for these two values, and draw a line through it to the origin, the slope of the line produced is a function of the pH. So, if you increase the alkalinity and thereby increase the slope, the pH also increases. And if you increase the total inorganic carbon (w/o increasing ALK) and thereby decrease the slope of the line, the pH will also decrease. (as seen in the graph below)

I did my best to find an image of this, but alas, the internet came up short. So, please bear with me on the crude computer sketch. There is a very good graphic, as well as a very thorough explanation in Stumm & Morgan's Aquatic Chemistry.