Figure 5.

¹³C labeling of fatty acids and starch during N deprivation in cells cultured in TAP media at 160 μmol photons m⁻² s⁻¹. Cells grown to log phase in N-replete media were moved at time 0 to TAP media lacking N with 100% uniformly labeled ¹³C acetate. After 40 h, the media was replaced with TAP media lacking N and containing unlabeled acetate for the remainder of the time course. The proportions of different mass isomers are plotted as a function of time. Panel A shows mass isomers of the m/z = 319 fragment ion containing carbons 2-6 of the Glc monomers of starch. Filled squares represent M+0, filled diamonds M+1, empty triangles m+2, X’s M+3, and empty circles M+4. Panel B is a histogram of the mass isomer distributions for a representative 16:0 FAME mass spectrum showing the abundance of highly labeled molecules together with unlabeled molecules (M+0 and naturally occurring M+1 masses). C to E represent mass isomer distributions for ions of intact C16:0 FAME molecules from TAG, MGDG, and polar lipid fractions, respectively. Filled squares represent unlabeled molecules (M+0); filled diamonds, M+1 (one ¹³C atom); empty triangles, M+2; empty circles, the sum of M+3 through M+16 (fully labeled). Error bars indicate range (n = 2).