Phaeodactylum tricornutum photorespiration takes part in glycerol metabolism and is important for nitrogen-limited response

Background: Microalgae are potential sources of biofuels and high-value compounds. Mixotrophic conditions usually promote growth of microalgae. The pennate diatom Phaeodactylum tricornutum, with its short life cycle, completely sequenced genome, and ease of transformation, can be used as a model for studying carbon metabolism in microalgae. Results: We compared the growth rate of P. tricornutum (IOCAS-001) under different conditions and labeled the cells using [C-13] glycerol (GL). The results revealed GL promoted the growth of P. tricornutum. Ser and Gly were synthesized via photorespiration. The C-13 enrichment of Ser and Gly under nitrogen-limited conditions was much higher compared to other amino acids, indicating the enhancement of photorespiration. Addition of sodium acetate decreased the growth rate of P. tricornutum under nitrogen-limited conditions. Our results indicated that the GL carbon backbone enters the Calvin cycle in the form of dihydroxyacetone phosphate (DHAP), producing xylulose 5-phosphate (X5P) with a GL2_3-generated carbon backbone distributed at X5P1_2 and ribose 5-phosphate (R5P) with GL1-derived carbon atoms at R5P1 and R5P2. Both R5P and X5P can be converted into ribulose-1,5-bisphosphate (RuBP). By oxygenation of RuBP carboxylase/oxygenase (Rubisco) and metabolism through photorespiration, these RuBPs generate Ser and Gly with GL1 or GL2-derived carbon atoms at position 1 and GL1 or GL3-derived carbon atoms at other positions, resulting in a low level of 13C enrichment of Gly1 and Ser1. Conclusion: Our results indicated different strains of P. tricornutum have different mechanisms for organic carbon metabolism. Photorespiration is involved in GL metabolism and is important for the nitrogen-limited response in P. tricornutum. Classification: Metabolic flux analysis, microalgae