Hyperosmotic stress in Chlamydomonas induces metabolomic changes in biosynthesis of complex lipids

To study the early stress response of the green microalga Chlamydomonas reinhardtii CC-503, we exposed cells to hyperosmotic conditions for a period of 5 hours, sampling at eight time points. We performed metabolomic analysis of primary metabolism using gas chromatography/time-of-flight mass spectrometry (GC-TOF MS), and lipidomic analysis by liquid chromatography/quadrupole time-of-flight mass spectrometry (CSH-QTOF MS). Under hyperosmotic stress, many mono- and disaccharides increased along with the concentrations of sugar alcohols similar to osmoprotectants in higher plants. The levels of 13 proteinogenic amino acids increased, suggesting increased biosynthesis over the course of 5 hours, which is clearly a slower response than carbohydrate metabolism. TCA cycle compounds decreased in a tightly co-regulated manner under hyperosmotic stress as a cataplerotic reaction to provide carbon backbones for amino acid biosynthesis. Of note, levels of free palmitoyl-compounds (palmitic acid and monopalmitin) decreased over the stress time course in an opposite pattern to the increase in stearoyl compounds. Triglycerides that increased in concentration under osmotic stress were on average three carbons longer and had four more double bonds compared with triglycerides that remained unchanged. Along with the difference in C18-fatty acids compared with C16-fatty acids, these changes may indicate a specific activation of elongases and desaturases or remodelling from membrane acyl-lipids towards triacylglcyerides.