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 C_i 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.