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Articles

Light-driven Uptake of Oxygen, Carbon Dioxide, and Bicarbonate by the Green Alga Scenedesmus^1,2

Martin Marietta Laboratories, 1450 South Rolling Road, Baltimore, Maryland 21227

Mass spectrometric techniques were used to study several aspects of the competition between O₂ and species of inorganic carbon for photosynthetically generated reducing power in the green alga, Scenedesmus.

In contrast to wild type, no appreciable light-driven O₂ uptake was observed in a mutant lacking photosystem I. It is concluded that the carbon cycle-independent reduction of O₂ occurs at the expense of photosystem I-generated reducing equivalents.

The commonly observed differences between CO₂-grown and air-grown Scenedesmus with respect to CO₂ uptake and glycolate formation cannot be ascribed to differences in their capacity for light-driven O₂ uptake. There were no intrinsic differences found in O₂ uptake capacity between the two physiological types under conditions in which CO₂ was saturating or CO₂ uptake was inhibited. It was only under CO₂-limited conditions that pronounced differences between the two physiological types were observed. This fact suggests that differences in O₂ metabolism and sensitivity between the two types really reflect differences in their capacity to assimilate inorganic carbon; in this respect they are analogous to C₃ and C₄ plants.

The hypothesis that air-grown Scenedesmus can assimilate HCO₃^– by directly monitoring the time course of dissolved CO₂, O₂ uptake, and O₂ evolution in illuminated algal suspensions at alkaline pH was tested. Inasmuch as the measuring technique employed was fast compared to the nonenzymic equilibration of the inorganic carbon species, it was possible to determine the degree to which the CO₂ concentration deviated from equilibrium (with the other inorganic carbon species) during the course of illumination. The observed kinetics in air-grown and CO₂-grown algae in the presence and absence of carbonic anhydrase, and a comparison of these kinetics with theoretical (computer-generated) time courses, support the idea that air-adapted algae are able to assimilate HCO₃^– actively at a high rate. The data suggest that these algae preferentially assimilate CO₂ and supply the balance of their needs by taking up HCO₃^–. Since (unlike C₄ plants) these algae have no special CO₂ pump, and thus have a relatively low affinity for CO₂, HCO₃^– assimilation is the major carbon uptake process at alkaline pH even when the total CO₂ is present in millimolar concentrations.

² Dedicated to the late Dr. Bessel Kok, beloved friend, colleague and mentor.

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