Abstract
BackgroundRising CO2 concentration was reported to increase phytoplankton growth rate as well as lipid productivity. This has raised questions regarding the NADPH supply for high lipid synthesis as well as rapid growth of algal cells.ResultsIn this study, growth, lipid content, photosynthetic performance, the activity, and expression of key enzymes in Calvin cycle and oxidative pentose phosphate pathway (OPPP) were analyzed in the marine diatom Phaeodactylum tricornutum under three different CO2 concentrations (low CO2 (0.015 %), mid CO2 (atmospheric, 0.035 %) and high CO2 (0.15 %)). Both the growth rate and lipid content of P. tricornutum increased significantly under the high CO2 concentration. Enzyme activity and mRNA expression of three Calvin cycle-related enzymes (Rubisco, 3-phosphoglyceric phosphokinase (PGK), phosphoribulokinase (PRK)) were also increased under high CO2 cultivation, which suggested the enhancement of Calvin cycle activity. This may account for the observed rapid growth rate. In addition, high activity and mRNA expression of G6PDH and 6PGDH, which produce NADPH through OPPP, were observed in high CO2 cultured cells. These results indicate OPPP was enhanced and might play an important role in lipid synthesis under high CO2 concentration.ConclusionsThe oxidative pentose phosphate pathway may participate in the lipid accumulation in rapid-growth P. tricornutum cells in high CO2 concentration.
Highlights
Rising CO2 concentration was reported to increase phytoplankton growth rate as well as lipid productivity
The consumption of NO3− during algal rapid growth may cause the increase of culture pH from day 2 to 7 under high CO2 concentration, and this increase of pH might correlate with algal growth rate and carbon fixation efficiency
Rubisco acts as a CO2 receptor and takes part in the carbon fixation stage of the Calvin cycle, and 3-phosphoglyceric phosphokinase (PGK) participates in the reduction stage of the Calvin cycle, whereas PRK is incorporated in the regeneration phase in the Calvin cycle; high activities of Rubisco, PGK, and PRK in high CO2 cultured cells indicated that the Calvin cycle activity was enhanced and was operating effectively in algal cells under high CO2 cultivation
Summary
Rising CO2 concentration was reported to increase phytoplankton growth rate as well as lipid productivity. This has raised questions regarding the NADPH supply for high lipid synthesis as well as rapid growth of algal cells. Various studies have shown that rising CO2 concentration increases lipid productivity as well as phytoplankton growth rate, such as in Phaeodactylum tricornutum [12], Nannochloropsis oculata [13], and Chlorella vulgaris [14], high levels of CO2 concentration enhanced both biomass production and lipid content, shedding light on the potential for biodiesel production from microalgae. Most previous studies have only focused on microalgal growth rate, lipid content, and tolerance to high levels of CO2 [17,18,19,20]. Little effort has been directed toward the analysis of the mechanism involved in lipid accumulation in microalgae and their simultaneous rapid growth rate
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