However, less attention has been given to the photosynthetic mechanisms of microalgal response to different CO 2 concentrations. In our previous study, the biomass yield (2.78 g L −1) of Chlorella PY-ZU1 cultivated under 15% CO 2 increased by 1.19-fold compared with that of microalgae cultivated under air (1.30 g L −1). Therefore, several studies have focused on CO 2 fixation by microalgae from high concentrations of CO 2 gas, such as flue gas, to increase CO 2 fixation efficiency. However, CCM requires more energy flow in PSI and leaves less energy available for the Calvin–Benson cycle, thus reducing microalgal growth and CO 2 fixation efficiency. Carbonic anhydrase (CAs) and CCM play an important role in the efficient utilization of dissolved inorganic carbon under CO 2-limited conditions. Ī number of studies have been conducted on the mechanisms underlying the acclimation of microalgal cells to limited CO 2 concentrations. Given that CO 2 is the only form of dissolved inorganic carbon that rubisco can fix, the most likely evolutionary goal of the CCM process is to increase dissolved CO 2 concentrations at rubisco locations for fixation. To overcome this challenge, a number of prokaryotic and eukaryotic microalgae have developed a CO 2-concentrating mechanism (CCM) to maximize photosynthesis under limited CO 2 conditions. However, rubisco has a poor apparent affinity with CO 2 when the dissolved CO 2 concentration is less than the K m (CO 2) of rubisco, led to decrease in photosynthetic efficiency. However, the dissolved CO 2 in freshwater is only ~15 μM when in equilibrium with air. The K m for CO 2 of microalgal rubisco, often exceed 25 μM. Moreover, the photosynthetic mechanism ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), the first and stromal enzyme that catalyzes the entry of CO 2 into the Calvin–Benson cycle, is adapted to the considerably higher CO 2 concentrations encountered by C 3 plants. However, the current atmospheric CO 2 concentration of ~0.04% is not enough for microalgae photosynthesis. Utilizing biological conversions by microalgae is a promising approach to reduce CO 2 emissions. Global warming necessitates the reduction of accumulated CO 2 in the atmosphere. Considering the micro and macro growth phenomena of Chlorella PY-ZU1 under different concentrations of CO 2 (0.04–60%), CO 2 transport pathways responses to different CO 2 (0.04–60%) concentrations was reconstructed. These genes related to the two most metabolisms with significantly differential expressions were beneficial for microalgal growth (2.85 g L −1) under 15% CO 2 concentration. ConclusionsĬarbon fixation and nitrogen metabolism are the two most important metabolisms in the photosynthetic cells. Besides, the transcript abundances of most key genes involved in carbon fixation pathways were also enhanced in 15% CO 2-grown cells. The transcript abundance of rubisco (the most important enzyme of CO 2 fixation reaction) was 16.3 times higher in 15% CO 2-grown cells than that under air. More adenosine triphosphates was saved for carbon fixation-related pathways. Extrapolating from the growth conditions and quantitative Real-Time PCR of CCM-related genes, the K m (CO 2) (the minimum intracellular CO 2 concentration that rubisco required) of Chlorella PY-ZU1 might be in the range of 80–192 μM. It was found that carbonic anhydrase (CAs, enzyme for interconversion of bicarbonate to CO 2) dramatically decreased to near 0 in 15% CO 2-grown cells, which indicated that CO 2 molecules directly permeated into cells under high CO 2 stress without CO 2-concentrating mechanism. In this study, the Illumina sequencing and de novo transcriptome assembly of Chlorella PY-ZU1 cells cultivated under 15% CO 2 were performed and compared with those of cells grown under air. The genetic reasons for the higher growth rate, CO 2 fixation rate, and photosynthetic efficiency of microalgal cells under higher CO 2 concentration have not been clearly defined yet.
However, less attention has been given to the microalgae photosynthetic mechanisms response to different CO 2 concentrations. The biomass yield of Chlorella PY-ZU1 drastically increased when cultivated under high CO 2 condition compared with that cultivated under air condition.
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