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Metabolism and Enzymology

Two Systems for Concentrating CO₂ and Bicarbonate during Photosynthesis by Scenedesmus¹

Fachbereich Biologie der Philipps Universität, D-3550 Marburg, West Germany, Department of Biochemistry, Michigan State University, East Lansing, Michigan 48824

Scenedesmus cells grown on high CO₂, when adapted to air levels of CO₂ for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO₂ from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O₂ evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO₂ pump with K_0.5 for O₂ evolution of less than 5 micromolar CO₂ was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO₃^– concentrating process between pH 8 to 10 lowered the K_0.5 (DIC) from 7300 micromolar HCO₃^– in high CO₂ grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO₃^– pump was inhibited by vanadate (K_i of 150 micromolar), as if it involved an ATPase linked HCO₃^– transporter. The CO₂ pump was formed on low CO₂ by high-CO₂ grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO₃^– pump was partially activated on low CO₂ within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO₃^– pump at pH 9 had requirements similar to the activation of the CO₂ pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO₂-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO₂ pump and in some algae an additional vanadate-sensitive plasmalemma HCO₃^– transporter for a HCO₃^– pump.

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