In this study, we investigated changes with culture age in growth rate, FPSII e-max (the maximal effective quantum yield of PSII,sF v /F m ), the photosynthetic parameters a, rETR max and I k , fatty acid composition and macromolecular content (via IR spectroscopy) in batch cultures of Phaeodactylum tricornutum and Chaetoceros muelleri. FPSII e-max , and the initial slope of the photosynthesis vs. light curve (a) in the two diatoms exhibited almost the same pattern of variation over the experimental period, first increasing slightly to a maximum value, then decreasing with increasing culture age. There was also a significant decrease, with increasing culture age, in the parameters rETR max and I k in both diatoms. FT-IR spectra of P. tricornutum and C. muelleri all showed changes over the growth period. The bands attributed to lipids (C-H stretching between 3000 cm -1 and 2800 cm -1 along with the ester CsO 1740 cm -1 ) in both diatoms showed the same tendency, increasing with increasing culture age over the entire growth period, consistent with the onset of major nutrient (P) limitation. Changes with culture age were also found in the fatty acid composition. P. tricornutum showed a marked increase in the levels of 16:1 n-7 and 18:1 n-9 and a marked decrease in the levels of 16:3 n-4 and 20:5 n-3 with increasing culture age. There was a slight variation in the fatty acid composition of C. muelleri related to culture age, with the largest increase shown by 16:0 and the largest decrease by 16:3 n-4.