1. A method is described for the simultaneous monitoring of H2O, O2, CO2 and their 18O-labelled analogs in experiments on the photosynthesis both of aquatic and terrestrial plants. The apparatus consists of a Varian-MAT mass spectrometer with a modified inlet system equipped at the entrance of the ion source with a polyethylene membrane permeable for H2O, O2 and CO2.
2. Experiments have been performed with Chlorella pyrenoidosa Chick. using H218O and K2C18O3 as substrates while H2O and O2 and CO2 in solution as well as their 18O-labelled analogs were monitored during dark and light periods.
3. It has been observed that the isotopic composition of the photosynthetic oxygen corresponds to that of the surrounding water and differs grossly from that of the dissolved CO2 and K2CO3. It is concluded that either H2O is the precursor of the photosynthetic oxygen or that in case CO2 is involved a very fast and complete equilibration between the latter and H2O has to occur after CO2 enters the cell.
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