Oxygen isotope fractionation during photosynthesis in a blue-green and a green alga [Agmenellum quadruplicatum, Chlorella pyrenoidosa]

Oxygen isotope fractionation (18O/16O) at the natural abundance level has been measured during photosynthesis of a blue-green and a green alga. When sufficient attention is paid to removal of contaminating air O2 before and during the experiments, then the photosynthetic O2 evolved, as compared to the water O2, had an average difference of -0.36‰ for a blue-green alga and -0.80‰ for a green alga. These experiments suggest that there is no reason to invoke an inverse isotope effect in photosynthesis as part of the explanation for the 18O enrichment in atmospheric O2 relative to O2 in oceanic waters. In addition, in an indirect way, the experiments also support the argument that the bulk of O2 evolved during photosynthesis comes from water. A 10% contribution of O2 arising from CO2 would have been detectable in the present work.