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Oxygen Isotope Fractionation during Photosynthesis in a Blue-Green and a Green Alga ^1,2

^a University of Texas Marine Science Institute, Port Aransas, Texas 78373

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.

⁴ Present address: Gulf Coast Hydroscience Center, Bay St. Louis, Miss. 39520.

¹ This work was supported in part by National Science Foundation Grant GA 11414 (P.L.P.). C.L.R.S. was supported by National Institutes of Health Training Grant GM 00836.

² Combined data taken from the Masters theses of the first two authors.