Evidence for a CO2-concentrating mechanism in the model streptophyte green alga Chara braunii

Abstract

Oxygenic photosynthesis in streptophytic algae, such as Charophyceae, is often impeded by low CO₂ levels in aquatic habitats. Consequently, many algal groups evolved a CO₂-concentrating mechanism (CCM). However, its presence in Charophyceae remains controversial. To explore this, we analyzed the acclimation of photosynthesis, carbon isotope composition, and gene expression in Chara braunii under varying inorganic carbon (Ci) conditions. The photosynthetic activity changed complementarily under low- or high-Ci levels. Notably, the Ci compensation point of photosynthesis was significantly lower in thalli grown at ambient Ci than in elevated Ci. Correspondingly, the delta ¹³C levels were lower in thalli from high than low Ci. These results indicate that C. braunii performs a CCM under low Ci, which is suppressed under high Ci. Transcriptomic analyses of algae from different Ci cultivations provided insight into Ci-regulated genes and pointed to the possible association between carbonic anhydrases and aquaporins with the CCM. Collectively, our results indicate that C. braunii expresses a CCM allowing efficient use of CO₂ and bicarbonate under limiting Ci conditions. A tentative scenario is provided summarizing the role of potential players in the CCM.

Keywords: CO2 limitation; Charophyceae; bicarbonate; carbonic anhydrase; gene expression; transport.