Pioneering findings on the dual role of carbon dioxide in photosynthesis published in the latest issue of the scientific journal PNAS by Researchers at Umeå University.
It is well known that inorganic carbon in the form of CO2 (carbon dioxide) is reduced in a light driven process (photosynthesis) to organic compounds in the chloroplasts. Less known is the fact that inorganic carbon also affects the rate of the photosynthetic electron transport, as was published by the Nobel Prize winner Otto Warburg and his collaborator in the late 50's (Warburg and Krippdahl 1958).
Their explanation for the stimulating effect was logical at that time since they proposed that CO2 was the source of oxygen that plants produce. Their idea was proven to be incorrect many years later and instead we now know that H2O (water) is the source of oxygen in the atmosphere. The observed stimulating effect by inorganic carbon on photosynthetic electron transport, also observed by others later, has ever since Warburgs publication continued to cause an inflammatory debate among researchers around the world up till these days as indicated by the hundreds of papers published.
Our results will now put an end to this debate.

The Messinger group developed sensitive techniques based on "Membrane Inlet Mass Spectroscopy" (MIMS) that has been used in their research to measure isotopic exchange reactions in photosynthetic samples under analytically controlled conditions. Messinger and collaborators have among other things worked and published papers on the role of inorganic carbon on the donor side of photosystem 2.
In three papers in EMBO journal (1998, 2002 and 2008) the Samuelsson group (Karlsson, Shutova and Villarejo) published data that led to the hypothesis that HCO3- (bicarbonate) is acting as H+ proton acceptor on the luminal side of the thylakoid membrane, close to the water splitting site.
Recently the MIMS methods developed in Messingers laboratory became so sensitive that we were able to test the hypothesis put forward in the EMBO paper (Shutova et al 2008). The idea is that if HCO3- is acting as proton acceptor in photosystem 2 then, a light driven production of CO2 in addition to O2 should be detected.
The results that will be published in PNAS this week give strong support for the hypothesis that HCO3- react with the protons formed when water is split and as a result CO2 is formed and is detected in the mass spec experiments. So it seems as if two different carbon species, both derived from the carbonic acid cycle, has got the optimal chemical properties to be used as terminal electron acceptor (CO2) in the very end of the photosynthetic reaction and at the same time as proton acceptor (HCO3-) in the very beginning of the photosynthetic reaction.

Link to the publication
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