Variations in lipid yields and compositions of marine microalgae during cell growth and respiration, and within intracellular structures

To determine variability of marine microalgal lipids during cell growth and respiration, and within intracellular structures, we conducted two series of experiments: (1) batch culturing Isochrysis sp., Gymnodinium sp., Platymonas subcordiforus, Heterosigma akashiwo Hada, and Skeletonema costatum with lipid analysis for cell samples collected in exponential growth and stationary phase respectively; and (2) batch culturing Isochrysis sp. and Gymnodinium sp. from exponential growth to stationary phase and followed by dark incubation with progressive lipid analysis. Moreover, membrane and intracellular storage fatty acids were separated using Si-gel chromatography from representative samples of each species in the second experiment (respectively in exponential growth, stationary phase, and dark respiration). Results from the first experiment showed that no matter how differently microalgae grew, their lipid yields were 4–38× higher in stationary phase than in exponential growth phase. Progressive lipid analysis in the second experiment indicated that total fatty acid (FA) yields in two microalgal cultures continuously increased throughout exponential and stationary phases but total neutral lipid (NL) yields slowed or stopped to increases in stationary phase. During dark incubations, total FA yields decreased steadily in both Isochrysis (with constant cell density) and Gymnodinium (with declining cell density) cultures but total NL yields decreased only in Gymnodinium culture. Lipid compositions and proportions of membrane vs. intracellular fatty acids varied differently with growth phases and respiration in two cultures. This study suggests that the effect of cell growth phase on cellular lipid yield and composition is species-dependent and is likely related to the capacity of cells to synthesize membrane vs. intracellular storage lipids.