Microalgae represent a potential solution to reduce CO 2 emission exploiting their photosynthetic activity. Here, the physiologic and metabolic responses at the base of CO 2 assimilation were investigated in conditions of high or low CO 2 availability in two of the most promising algae species for industrial cultivation, Chlorella sorokiniana and Chlorella vulgaris. In both species, high CO 2 availability increased biomass accumulation with specific increase of triacylglycerols in C. vulgaris and polar lipids and proteins in C. sorokiniana. Moreover, high CO 2 availability caused only in C. vulgaris a reduced NAD(P)H/NADP + ratio and reduced mitochondrial respiration, suggesting a CO 2 dependent increase of reducing power consumption in the chloroplast, which in turn influences the redox state of the mitochondria. Several rearrangements of the

Microalgae represent potential solutions to reduce the atmospheric CO2 level through photosynthesis. To boost CO2 fixation by microalgae it is essential to understand physiologic and metabolic responses at the base of CO2 assimilation and carbon flow. In this work two Trebouxiophyceae species, Chlorella sorokiniana and Chlorella vulgaris, were investigated for their metabolic responses to high and low CO2 (air level) availability. High CO2 availability resulted in an increase in biomass accumulation in both species but with a different chloroplast and mitochondrial responses. In C. sorokiniana we observed increased polar lipids and protein amount and a balanced NADPH redox state and a similar total respiration in the two conditions analysed. In contrast, in C. vulgaris high CO2 level caused an increase in TAG accumulation and a higher NADPH consumption suggesting a CO2 dependent increase of reducing power consumption in the chloroplast, which in turn influences the redox state of the mitochondria by lowering total dark respiration. Several rearrangements of the photosynthetic machinery were observed in both species, which differ from those described for the model organism Chlamydomonas reinhardtii. In the case of C. reinhardtii, adaptation of the photosynthetic apparatus to different CO2 availability relies on the translational repressor NAB1. NAB1 homologous protein could be identified only in C. vulgaris but lacked the regulation mechanisms previously described in C. reinhardtii. These findings highlight that the acclimation strategies to cope with a fluctuating inorganic carbon supply are diverse among green microalgae and point to new biotechnological strategies to boost CO2 fixation.