Re: In photosynthesis, how do CO2 levels affect the light dependent stage

Hi Jonathon,

I apologize for not getting back to you sooner on this excellent question, but it is a complicated issue that has not yet been fully resolved. It has been observed that lowered CO2 levels do cause a decrease in the quantum efficiency of the light reactions, but the exact molecular mechanism of this inhibition is still up for debate.

One obvious consequence of lowered CO2 levels would be a decrease in the rate of carbohydrate production. The reducing power needed to reduce CO2 to carbohydrate is provided by the light reactions in the form of NADPH. This molecule is used as a currency for reducing power in biological systems. It exists in two forms, an oxidized NADP+ form, and a reduced NADPH form. Lowered efficiency of the Calvin cycle (the "dark reactions") would lead to an increase in the reduced NADPH form, because it would be produced faster than it was used. This would cause a corresponding decrease in the NADP+ form. Lack of NADP+, which is a cofactor of the light reactions, would leave the electrons (current) generated in the light reactions nowhere to go. They would back up, leaving the redox components of the light reaction machinery fully reduced. Because of this, an excited electron in the Photosystem II reaction center would have nowhere to go. It could be forced backwards and given off as fluorescence. It could also react destructively with other components of the system, or it could react with nearby oxygen to produce superoxide. Either of these last two products would be detrimental to the plant, and all would lead to lower quantum efficiency for the light reactions (a measure of the amount of captured light energy that is converted to chemical energy).

Also, Govindjee and others have found evidence for use of a carbonate anion as a cofactor in the transfer of an electron from quinone A to quinone B in the Photosystem II reaction. Since dissolved CO2 is converted to carbonate anion in a reversible fashion in aqueous solutions, lowered atmospheric CO2 would lead to lowered carbonate anion. This would presumably lead to lower quantum efficiency for Photosystem II. They also found that mutants lacking a residue thought to hold the carbonate anion in place led to a destabilization of the entire Photosystem II complex center (Hutchison, Biochimica et Biophysica Acta 1277 (1996) 83-92).

Other people have proposed that there must be some kind of regulation of PSII activity, because of the detrimental effects of backup in the photosynthetic electron chain (mentioned in paragraph 2). One is A mathematical model of electron transport: Thermodynamic necessity for Photosystem II regulation: "light stomata", by Laisk in PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON BIOLOGICAL SCIENCES 237(1289): 417-444.

There are lots of good resources for photosynthesis on the web. Here are a few links:

http://www.life.uiuc.edu/bio100/Link_page/photosyn.htm

Beginning of a particularly informative hypertext series on photosynthesis:
http://esg-www.mit.edu:8001/esgbio/ps/psdir.html

Cyclic photosynthesis used in bacteria when reducing power is not needed:
http://esg-www.mit.edu:8001/esgbio/ps/light.html

The dark reactions and how they are tied to the light reactions:
http://esg-www.mit.edu:8001/esgbio/ps/dark.html

Excellent site with lots of great pictures of photosynthetic process diagrams and molecules involved:
http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.htm

The photosynthesis center at Arizona State University:
http://photoscience.la.asu.edu/photosyn/default.html

That is the best answer that I can give you. Others who are experts on this specific topic might be able to better answer it. If I hear of anything else I'd be glad to let you know of it, if you'd write me an e-mail requesting that. Thanks for the question. I learned myself by searching for an answer to it.

Sincerely,
Todd Holland
Mad Scientist