Chlamydomonas reinhardtii is a unicellular green alga often used as a model organism. It has one large chloroplast and a CO2 concentrating mechanism (CCM) that increases photosynthetic efficiency.
Like most aquatic photosynthetic organisms, the enzyme ribulose 1,5‑bisphosphate carboxylase/oxygenase (Rubisco) is localised in a structure called the pyrenoid within the chloroplast.
Through a series of bicarbonate transporters and carbonic anhydrases, C. reinhardtii increases the CO2 concentration around Rubisco. The locally higher CO2 concentration generated by the CCM increases the rate of carboxylation reactions catalysed by Rubisco and decreases the rate of oxygenation reactions since CO2 and O2 are competitive substrates.
At low CO2 concentrations, the CCM is needed to both deliver Ci to Rubisco and minimise the leak of CO2 from pyrenoid as well as recapture CO2 generated by respiration and photorespiration.
This talk will present evidence of how carbonic anhydrases help C. reinhardtii to recapture CO2 leaking out of the cell.