fish1

Samuel Ruben and his coworkers at the University of California were able to prepare specially "labeled" water in which the 0.85% of the molecules contained 18O atoms. When this water was supplied to a suspension of photosynthesizing algae, the proportion of 18O in the oxygen gas that was evolved was 0.85%, the same as that of the water supplied.

Why after photosynthesis, the % of oxygen atom are still the same? Shouldn't it be increased cause oxygen is a product of photosynthesis?