Ultrastructure of Nannochloropsis spp. cells during nitrate depletion. Electron micrographs show a representative cell grown in replete medium on day 4 (A) and representative cells grown in nitrogen-deficient medium on day 13 (B) and day 28 (C and D). LD, Lipid droplets; M, mitochondrion; N, nucleus; ST, stacks of thylakoids; V, vacuoles. Bars = 100 nm for A to C and 500 nm for D.