Oxygen Isotope Fractionation during Photosynthesis in a Blue-Green and a Green Alga

Abstract

Oxygen isotope fractionation (180/160) 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 02 before and during the experiments, then the photosynthetic 02 evolved, as compared to the water 02, had an average difference of -0.36%0 for a blue-green alga and -0.80%0 for a green alga. These experiments suggest that there is no reason to invoke an inverse isotope effect in photosynthesis as part of the ex- planation for the 180 enrichment in atmospheric 02 relative to 02 in oceanic waters. In addition, in an indirect way, the experiments also support the argument that the bulk of 02 evolved during photosynthesis comes fronm water. A 10%s contribution of 02 arising froni CO2 would have been de- tectable in the present work.