PHOTOSYNTHETIC FRACTIONATION OF THE STABLE ISOTOPES OF OXYGEN AND CARBON

Isotope discrimination during photosynthetic exchange of O2 and CO2 was measured using enzyme, thylakoid, and whole cell preparations. Evolved oxygen from isolated spinach thylakoids was isotopically identical (within analytical error) to its source water. Similar results were obtained with Anacystis nidulans Richter and Phaeodactylum tricornutum Bohlin cultures purged with helium. For consumptive reactions, discrimination (DELTA, where 1 + DELTA/1000 equals the isotope effect, k16/k18 or k12/k13) was determined by analysis of residual substrate (O2 or CO2). The DELTA for the Mehler reaction, mediated by ferredoxin or methylviologen, was 15.3 parts per thousand. Oxygen isotope discrimination during oxygenation of ribulose-1,5-bisphosphate (RuBP) catalyzed by RuBP carboxylase/oxygenase (Rubisco) was 21.3 parts per thousand and independent of enzyme source, unlike carbon isotope discrimination: 30.3 parts per thousand for spinach enzyme and 19.6 to 23 parts per thousand for Rhodospirillum rubrum and A. nidulans enzymes, depending on reaction conditions. The DELTA for 02 consumption catalyzed by glycolate oxidase was 22.7 parts per thousand. The expected overall DELTA for photorespiration is about 21.7 parts per thousand. Consistent with this, when Asparagus sprengeri Regel mesophyll cells approached the compensation point within a sealed vessel, the deltaO-18 of dissolved O2 came to a steady-state value of about 21.5 parts per thousand relative to the source water. The results provide improved estimates of discrimination factors in several reactions prominent in the global O cycle and indicate that photorespiration plays a significant part in determining the isotopic composition of atmospheric oxygen.