Enzyme activity highlights the importance of the oxidative pentose phosphate pathway in lipid accumulation and growth of Phaeodactylum tricornutum under CO2 concentration

Background: 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. Results: In 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. Conclusions: The oxidative pentose phosphate pathway may participate in the lipid accumulation in rapid-growth P. tricornutum cells in high CO2 concentration.