On the O/O isotope effect associated with photosynthetic O production.

While photosynthetically evolved O has been repeatedly shown to have nearly the same oxygen isotope composition as source water so that there is no corresponding O/O isotope effect, some recent O-enrichment studies suggest that a large isotope effect may occur, thus feeding a debate in the literature. Here, the classical theory of isotope effects was applied to show that a very small isotope effect is indeed expected during O production. Explanations of the conflicting results are briefly discussed.