Time-resolved transcriptome analysis of Scenedesmus obliquus HTB1 under 10% CO₂ condition

Certain microalgal species can grow under high CO₂ concentrations providing potential for mitigating CO₂ pollution in flue gas produced by power plants. Microalga Scenedesmus obliquus strain HTB1 was isolated from the Chesapeake Bay and grow rapidly in a high level of CO₂ . However, little is known about the molecular responses of HTB1 to high CO₂ levels. Here, we investigated how HTB1 responds to 10% CO₂ using the time-resolved transcriptome analysis. Gene expression profiles indicated that HTB1 responds quickly (in 2 h) and becomes adaptive within 12 h when exposed to 10% CO₂ . Interestingly, certain genes of light-harvesting, chlorophyll synthesis and carbon fixation (i.e. rbcS) were up-regulated at 10% CO₂ , and these functional responses are consistent with the increased photosynthesis efficiency and algal biomass under 10% CO₂ . Nitrate assimilation was strongly enhanced, with amino acid biosynthesis and aminoacyl tRNA biosynthesis genes being markedly up-regulated, indicating that HTB1 actively takes up nitrogen and accelerates protein synthesis at 10% CO₂ . Carbon metabolism including fatty acid biosynthesis and TCA cycle was enhanced at 10% CO₂ , supporting the earlier observation of increased lipid content of Scenedesmus sp. under high CO₂ levels. Interestingly, key genes like RuBisCO (rbcL) and carbonic anhydrase in carboxysomes did not respond actively to 10% CO₂ , implying that exposure to 10% CO₂ has little impact on the carbon concentrating mechanisms and CO₂ fixation of the Calvin cycle. It appears that HTB1 can grow rapidly at 10% CO₂ without significant metabolic changes in carbon fixation and ATP synthesis.