Different physiological and molecular responses of the green algae Chlorella variabilis to long-term and short-term elevated CO2

Chlorella variabilis is a high-efficiency photosynthetic green plant, and its photosynthesis capacity is considerably higher than other plants. With increasing CO2 concentration in the atmosphere C. variabilis may change the metabolism to respond to elevated CO2. To investigate the response of C. variabilis when exposed to increased CO2 concentrations, physiological and transcriptome analyses were carried out on C. variabilis that had been cultivated at 1000 ppm CO2 for long (LT) and short (ST) periods of time and these were compared with samples from the control group (CT). The common responses of the ST and LT C. variabilis included increased pigment levels, and higher Fv/Fm and carbon fixation rates. These results indicated that current CO2 levels could limit C. variabilis growth and photosynthesis. Transcriptome changes in the ST samples were coincident with the observed physiological responses, which suggested that carbon flow to the acetyl-CoA pool from photosynthesis enhanced the production of lipids and fatty acids. This was confirmed by the depressed β-oxidation levels, upregulation of fatty acids and triglyceride synthesis pathways, lower soluble carbohydrate contents, and lipid accumulation. However, we found no change or downregulation of genes involved in cellular metabolism in the LT samples, such as the genes participating in protein, fatty acid, and triglyceride synthesis. The results demonstrated that C. variabilis was generally sensitive to the ST condition, but hardly responded to the LT treatment and failed to evolve any specific adaptations.