A carbonic anhydrase catalyzed CO2 pump in Chlamydomonas reinhardtii: In vivo experiments, computer modelling, and theory

We studied the bicarbonate depletion effects on photosynthesis in high-CO2 grown-cell wall-less mutants of Chlamydomonas reinhardtii with (cw92) and w ithout (cia3/cw15) active carbonic anhydrase (CA) associated with photosyst em II (PSII). The mutants were grown in a high-CO2 medium and then washed w ith a CO2-free buffer. We discuss the shutdown of oxygen evolution in cw92 as a consequence of CO2 depletion of the Calvin cycle. It is suggested that CA acts as part of a PSII-driven pump delivering CO2 from the lumen into t he stroma, where it becomes available to the Calvin cycle. Our model is sup ported by computer simulation data and accommodates the results of in vivo experiments as well as the differences between cw92 and cia3 mutants. We al so present a simple theory of the phenomena discussed, based on a time-scal e hierarchy approach, which describes the CO2 depletion in cw92 using only two rate constants-V-max and K-m of the Calvin cycle.