Transcriptome and key genes expression related to carbon fixation pathways in Chlorella PY-ZU1 cells and their growth...

The biomass yield of Chlorella PY-ZU1 drastically increased when cultivated under high CO₂ condition compared with that cultivated under air condition. However, less attention has been given to the microalgae photosynthetic mechanisms response to different CO₂ concentrations. The genetic reasons for the higher growth rate, CO₂ fixation rate, and photosynthetic efficiency of microalgal cells under higher CO₂ concentration have not been clearly defined yet.

In this study, the Illumina sequencing and de novo transcriptome assembly of Chlorella PY-ZU1 cells cultivated under 15% CO₂ were performed and compared with those of cells grown under air. It was found that carbonic anhydrase (CAs, enzyme for interconversion of bicarbonate to CO₂) dramatically decreased to near 0 in 15% CO₂-grown cells, which indicated that CO₂ molecules directly permeated into cells under high CO₂ stress without CO₂-concentrating mechanism. Extrapolating from the growth conditions and quantitative Real-Time PCR of CCM-related genes, the K_m (CO₂) (the minimum intracellular CO₂ concentration that rubisco required) of Chlorella PY-ZU1 might be in the range of 80-192 μM. More adenosine triphosphates was saved for carbon fixation-related pathways. The transcript abundance of rubisco (the most important enzyme of CO₂ fixation reaction) was 16.3 times higher in 15% CO₂-grown cells than that under air. Besides, the transcript abundances of most key genes involved in carbon fixation pathways were also enhanced in 15% CO₂-grown cells.

Carbon fixation and nitrogen metabolism are the two most important metabolisms in the photosynthetic cells. These genes related to the two most metabolisms with significantly differential expressions were beneficial for microalgal growth (2.85 g L^-1) under 15% CO₂ concentration. Considering the micro and macro growth phenomena of Chlorella PY-ZU1 under different concentrations of CO₂ (0.04-60%), CO₂ transport pathways responses to different CO₂ (0.04-60%) concentrations was reconstructed.