Isotope ratios in photosynthetic oxygen

Axenic suspensions of the fresh water green alga Ankistrodesmus braunii were illuminated under aerobic conditions. The released gas mixture was introduced into the ion source of an isotope mass spectrometer, which recorded the 18O/16O ratio. The 18O content of the photosynthetic oxygen (approximately 0.199%) exceeded that of the cell water (approximately 0.197%) significantly.