Abstract Increasing CO 2 availability is a common practice at the industrial level to trigger biomass productivity in microalgae cultures. Still, the consequences of high CO 2 availability in microalgal cells exposed to relatively high light require further investigation. Here, the photosynthetic, physiologic, and metabolic responses of the green microalga model Chlamydomonas reinhardtii were investigated in high or low CO 2 availability conditions: high CO 2 enabled higher biomass yields only if sufficient light energy was provided. Moreover, cells grown in high light and high CO 2 availability were characterized, compared to cells grown in high light and low CO 2 , by a relative increase of the energy‐dense triacylglycerols and decreased starch accumulation per dry weight. The photosynthetic machinery adapted to the increased carbon availability, modulating Photosystem II light‐harvesting efficiency and increasing Photosystem I photochemical activity, which shifted from being acceptor side to donor side limited: cells grown at high CO 2 availability were characterized by increased photosynthetic linear electron flow and by the onset of a balance between NAD(P)H oxidation and NAD(P) + reduction. Mitochondrial respiration was also influenced by the conditions herein applied, with reduced respiration through the cytochrome pathway compensated by increased respiration through alternative pathways, demonstrating a different use of the cellular reducing power based on carbon availability. The results suggest that at high CO 2 availability and high irradiance, the reducing power generated by the oxidative metabolism of photosynthates is either dissipated through alternative oxidative pathways in the mitochondria or translocated back to the chloroplasts to support carbon assimilation and energy‐rich lipids accumulation.