The effects of nitrate, phosphate, and iron starvation and resupply on photosynthetic pigments, selected photosynthetic proteins, and photosystem II (PSII) photochemistry were examined in the diatom Phaeodactylum tricornutum Bohlin (CCMP 1327). Although cell chlorophyll a (chl a) content decreased in nutrient-starved cells, the ratios of light-harvesting accessory pigments (chl c and fucoxanthin) to chl a were unaffected by nutrient starvation. The ratio of the xanthophyll cycle pigments diatoxanthin and diadinoxanthin to chl a increased during nutrient starvation. These pigments are thought to play a photoprotective role by increasing dissipation of excitation energy in the pigment bed upstream from the reaction centers. The relative abundance of the carboxylating enzyme, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), decreased in response to nitrate and iron starvation but not phosphate starvation. The changes in pigment content and fluorescence characteristics were typically reversed within 24 h of resupply of the limiting nutrient.