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.

Phaeodactylum tricornutum Bohlin is an ideal model diatom; its complete genome is known, and it is an important economic microalgae. Although silicon is not required in laboratory and factory culture of this species, previous studies have shown that silicon starvation can lead to differential expression of miRNAs. The role that silicon plays in P. tricornutum growth in nature is poorly understood. In this study, we compared the growth rate of silicon starved P. tricornutum with that of normal cultured cells under different culture conditions. Pigment analysis, photosynthesis measurement, lipid analysis, and proteomic analysis showed that silicon plays an important role in P. tricornutum growth and that its presence allows the organism to grow well under green light and low temperature.

SummaryThe development of microalgae on an industrial scale largely depends on the economic feasibility of mass production. High light induces productive suspensions during cultivation in a tubular photobioreactor. Herein, we report that high light, which inhibited the growth of Chlorella sorokiniana under autotrophic conditions, enhanced the growth of this alga in the presence of acetate.We compared pigments, proteomics and the metabolic flux ratio in C. sorokiniana cultivated under high light (HL) and under low light (LL) in the presence of acetate.Our results showed that high light induced the synthesis of xanthophyll and suppressed the synthesis of chlorophylls. Acetate in the medium was exhausted much more rapidly in HL than in LL. The data obtained from LC-MS/MS indicated that high light enhanced photorespiration, the Calvin cycle and the glyoxylate cycle of mixotrophic C. sorokiniana. The results of metabolic flux ratio analysis showed that the majority of the assimilated carbon derived from supplemented acetate, and photorespiratory glyoxylate could enter the glyoxylate cycle.Based on these data, we conclude that photorespiration provides glyoxylate to speed up the glyoxylate cycle, and releases acetate-derived CO 2 for the Calvin cycle. Thus, photorespiration connects the glyoxylate cycle and the Calvin cycle, and participates in the assimilation of supplemented acetate in C. sorokiniana under high light.