Mitochondrial carbonic anhydrases are needed for optimal photosynthesis at low CO2levels in Chlamydomonas

Abstract

Chlamydomonas reinhardtii can grow photosynthetically using CO2or in the dark using acetate as the carbon source. In the light in air, the CO2concentrating 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 CO2concentration 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 (Ci = CO2 HCO?3-) was reduced. At low CO2concentrations, the CCM is needed to both deliver Ci to Rubisco and to minimize the leak of CO2generated by respiration and photorespiration. We hypothesize that CAH4 and CAH5 in the mitochondria convert the CO2released from respiration and photorespiration as well as the CO2leaked from the chloroplast to HCO3 thus "recapturing" this potentially lost CO2