Effect of vanadate on photosynthesis and the ATP/ADP ratio in low-CO2-adapted Chlamydomonas reinhardtii cells

We have assessed the effect of vanadate as an inhibitor of plasma-membrane ATPase on photosynthesis and the ATP/ADP ratio in Chlamydomonas reinhardtii CW-92 (a mutant strain lacking a cell wall). This effect was compared in low-CO₂-adapted cells grown in media bubbled with air containing 400 or 70 µL · L^-1 CO₂. Evidence is presented indicating that cells grown at 70 µL · L^-1 CO₂ have a higher rate of photosynthetic O₂ evolution than cells grown at 400 µL · L^-1 CO₂, at limiting carbon concentrations. Extracellular and intracellular carbonic-anhydrase activities were, however, similar in cells grown in both of the low-carbon conditions. Vanadate inhibited, to a different extent, the HCO₃^--dependent O₂ evolution in cells grown at 400 and 70 µL · L^-1 CO₂. At 400 µM vanadate, inhibition reached 70–75% in cells grown at 400 µL · L^-1 but only 50% in those grown at 70 µL · L^-1 CO₂. The ATP/ADP ratios determined with and without vanadate at limiting concentrations of dissolved inorganic carbon indicated that more ATP was hydrolysed in algae grown at 70 µL · L^-1 than in those grown at 400 µL · L^-1 CO₂. We conclude that the maximal capacity to accumulate dissolved inorganic carbon is inversely related to the CO₂ concentration in the medium. Activation and — or synthesis of vanadate-sensitive ATPase may be the major explanation for the higher capacity for HCO₃^--dependent O₂ evolution in cells grown under limited CO₂ concentrations.