Ashwani Rai, Timothy Chen, Jim Moroney

Chlamydomonas reinhardtii can grow photosynthetically using CO₂ or in the dark using acetate as the carbon source. In the light in air, the CO₂ concentrating mechanism (CCM) of C. reinhardtii accumulates CO₂, enhancing photosynthesis. A combination of carbonic anhydrases (CAs) and bicarbonate transporters in the CCM of C. reinhardtii increases the CO₂ concentration at Ribulose 1,5-bisphosphate carboxylase oxygenase (Rubisco) in the chloroplast pyrenoid. Previously, CAs important to the CCM have been found in the periplasmic space, surrounding the pyrenoid and inside the thylakoid lumen. Two almost identical mitochondrial CAs, CAH4 and CAH5, are also highly expressed when the CCM is made, but their role in the CCM is not understood. Here, we adopted an RNAi approach to reduce the expression of CAH4 and CAH5 to study their possible physiological functions. RNAi mutants with low expression of CAH4 and CAH5 had impaired rates of photosynthesis under ambient levels of CO₂ (0.04% CO₂ [v/v] in air). These strains were not able to grow at very low CO₂ (<0.02% CO₂ [v/v] in air), and their ability to accumulate inorganic carbon (C_i = CO₂ + HCO3−⁠) was reduced. At low CO₂ concentrations, the CCM is needed to both deliver C_i to Rubisco and to minimize the leak of CO₂ generated by respiration and photorespiration. We hypothesize that CAH4 and CAH5 in the mitochondria convert the CO₂ released from respiration and photorespiration as well as the CO₂ leaked from the chloroplast to HCO3- thus “recapturing” this potentially lost CO₂.