Abstract
BackgroundThe 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.ResultsIn 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 Km (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.ConclusionsCarbon 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.
Highlights
The biomass yield of Chlorella PY-ZU1 drastically increased when cultivated under high C O2 condition compared with that cultivated under air condition
Gene expression of carbonic anhydrase and rubisco under high CO2 stress Chlorella PY-ZU1 had a higher biomass yield (2.85 g L−1) and shorter growth cycle (7 days) when cultivated in SE medium under continuous aeration with 15% (v/v) C O2 gas
The results indicated that the whole carbon fixation process was driven by 15% C O2 gas
Summary
The biomass yield of Chlorella PY-ZU1 drastically increased when cultivated under high C O2 condition compared with that cultivated under air condition. 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. Global warming necessitates the reduction of accumulated CO2 in the atmosphere. Utilizing biological conversions by microalgae is a promising approach to reduce CO2 emissions [1, 2]. CO2 concentration of ~0.04% is not enough for microalgae photosynthesis [3]. The photosynthetic mechanism ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), the first and stromal enzyme that catalyzes the entry of CO2 into the Calvin–Benson cycle, is adapted to the considerably higher C O2 concentrations encountered by C3 plants [4]. The Km for CO2 of microalgal rubisco, often exceed 25 μM [5].
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