Two systems for concentrating CO2 and bicarbonate during photosynthesis by Scenedesmus

Abstract

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 K0.5 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 K0.5 (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, 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.