Photosynthetic fractionation of the stable isotopes of oxygen and carbon

Abstract

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 (Δ, where 1 + Δ/1000 equals the isotope effect, k16/k18 or k12/k13) was determined by analysis of residual substrate (O2 or CO2). The Δ for the Mehler reaction, mediated by ferredoxin or methylviologen, was 15.3.‰. Oxygen isotope discrimination during oxygenation of ribulose-1,5-bisphosphate (RuBP) catalyzed by RuBP carboxylase/oxygenase (Rubisco) was 21.3‰ and independent of enzyme source, unlike carbon isotope discrimination: 30.3‰ for spinach enzyme and 19.6 to 23%o for Rhodospirillum ruhrum and A. nidulans enzymes, depending on reaction conditions. The Δ for O2 consumption catalyzed by glycolate oxidase was 22.7‰. The expected overall Δ for photorespiration is about 21.7‰. Consistent with this, when Asparagus sprengen Regel mesophyll cells approached the compensation point within a sealed vessel, the δ18O of dissolved O2 came to a steady-state value of about 21.5‰ 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.