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BIOENERGETICS AND PHOTOSYNTHESIS

Fractionation of the Three Stable Oxygen Isotopes by Oxygen-Producing and Oxygen-Consuming Reactions in Photosynthetic Organisms^1,[w]

Department of Plant and Environmental Sciences and Minerva Avron-Evenari Center of Photosynthesis Research Center (Y.H., D.E., A.K.), and Institute of Earth Sciences (E.B., B.L.), Hebrew University of Jerusalem, Jerusalem 91904, Israel

The triple isotope composition (¹⁷O and ¹⁸O) of dissolved O₂ in the ocean and in ice cores was recently used to assess the primary productivity over broad spatial and temporal scales. However, assessment of the productivity with the aid of this method must rely on accurate measurements of the ¹⁷O/¹⁶O versus ¹⁸O/¹⁶O relationship in each of the main oxygen-producing and -consuming reactions. Data obtained here showed that cleavage of water in photosystem II did not fractionate oxygen isotopes; the ¹⁸O and ¹⁷O of the O₂ evolved were essentially identical to those of the substrate water. The fractionation slopes for the oxygenase reaction of Rubisco and respiration were identical (0.518 ± 0.001) and that of glycolate oxidation was 0.503 ± 0.002. There was a considerable difference in the slopes of O₂ photoreduction (the Mehler reaction) in the cyanobacterium Synechocystis sp. strain PCC 6803 (0.497 ± 0.004) and that of pea (Pisum sativum) thylakoids (0.526 ± 0.001). These values provided clear and independent evidence that the mechanism of O₂ photoreduction differs between higher plants and cyanobacteria. We used our method to assess the magnitude of O₂ photoreduction in cyanobacterial cells maintained under conditions where photorespiration was negligible. It was found that electron flow to O₂ can be as high as 40% that leaving photosystem II, whereas respiratory activity in the light is only 6%. The implications of our findings to the evaluation of specific O₂-producing or -consuming reactions, in vivo, are discussed.

² These authors contributed equally to the paper.

^[w] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.063768.

^* Corresponding author; e-mail aaronka{at}vms.huji.ac.il; fax 972–2–6584463.

Received April 3, 2005; returned for revision May 1, 2005; accepted May 2, 2005.

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