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

Area of Science:

• * Plant and algal physiology
• * Photosynthesis and carbon fixation
• * Molecular biology and transcriptomics

Background:

• * Streptophytic algae, like Chara braunii, inhabit aquatic environments where low CO2 levels can limit photosynthesis.
• * Many algae possess a carbon-concentrating mechanism (CCM) to overcome CO2 limitation, but its presence in Charophyceae is debated.
• * Understanding CCM in Charophyceae is crucial for comprehending algal adaptation to varying carbon availability.

Purpose of the Study:

• * To investigate the presence and regulation of a carbon-concentrating mechanism (CCM) in the streptophytic alga Chara braunii.
• * To analyze the acclimation of photosynthesis and gene expression in response to different inorganic carbon (Ci) conditions.
• * To identify potential molecular players involved in the CCM of Chara braunii.

Main Methods:

• * Cultivation of Chara braunii under varying inorganic carbon (Ci) levels (low vs. high).
• * Measurement of photosynthetic activity and carbon isotope composition (δ13C).
• * Transcriptomic analysis to examine gene expression patterns under different Ci conditions.

Main Results:

• * Photosynthetic activity and Ci compensation points varied significantly between low and high Ci conditions, indicating acclimation.
• * Lower δ13C values in high Ci-grown algae suggest a shift in carbon acquisition strategies.
• * Transcriptomic data revealed differential expression of genes, including carbonic anhydrases and aquaporins, potentially linked to CCM function.

Conclusions:

• * Chara braunii possesses a functional CCM that is active under low Ci conditions and suppressed under high Ci.
• * The CCM enables efficient utilization of inorganic carbon, including CO2 and bicarbonate.
• * Carbonic anhydrases and aquaporins are suggested as potential key components of the CCM in this alga.