[Photosynthetic oxidation of sulphide by green plants. I]

Hydrogen sulphide is an essential electron donor in the photosynthesis of sulphur bacteria. In addition, it has been known to be an inhibitor of photosynthesis in green plants. More specifically, it is assumed that sulphide interacts with the oxygen-evolving system. If the view is accepted that there is a more or less uniform concept of photosynthesis which is valid for all assimilating organisms, the role of sulphide as an inhibitor in green plant photosynthesis hardly seems to be acceptable.Our experiments show that sulphide indeed depresses photosynthetic oxygen evolution. However, during the period of decreased oxygen liberation, sulphide at a concentration up to 10(-3) M is oxidized by the living cells. During the oxidation of sulphide, photosynthetic oxygen evolution simultaneously increases. Oxidation of sulphide proceeds only in the light and in the presence of CO2. This new photosynthetic reaction has characteristics in common with CO2 assimilation. The rate of the sulphide oxidation reaction is comparatively low: when there is normal activity, 10 to 15 molecules of O2 are liberated while one molecule of S(--) is oxidized.Organisms tested thus far were blue-green, green and red algae as well as the flowering plants Lemna and Spirodela. Each of these plants, when photosynthetically active, was capable of oxidizing sulphide photosynthetically, indicating that S(--) may also play the role of a (secondary) electron donor in the photosynthesis of green plants.