RESPONSE OF THE PHOTOSYNTHETIC APPARATUS OF PHAEODACTYLUM-TRICORNUTUM(BACILLARIOPHYCEAE) TO NITRATE, PHOSPHATE, OR IRON STARVATION

The effects of nitrate, Phosphate, and iron starvation and resupply on photosynthetic pigments, selected photosynthetic proteins, and photos ystem II (PSII) photochemistry were examined in the diatom Phaeodactyl um tricornutum Bohlin (CCMP1327). Although cell chlorophyll a (chl a) content decreased in nutrient-starved cells, the ratios of light-harve sting accessory pigments (chl c and fucoxanthin) to chl a were unaffec ted by nutrient starvation. The chl a-specific light absorption coeffi cient (aBAR) and the functional absorption cross-section of PSII (sig ma) increased during nutrient starvation, consistent with reduction of intracellular self-shading (i. e. a reduction of the ''package effect '') as cells became chlorotic. The light-harvesting complex proteins r emained a constant proportion of total cell protein during nutrient st arvation, indicating that chlorosis mirrored a general reduction in ce ll protein content. The ratio of the xanthophyll cycle pigments diatox anthin and diadinoxanthin to chl a increased during nutrient starvatio n. These pigments are thought to play a photoprotective role by increa sing dissipation of excitation energy in the pigment bed upstream from the reaction centers. Despite the increase in diatoxanthin and diadin oxanthin, the efficiency of PSII photochemistry, as measured by the ra tio Of variable to maximum fluorescence (F(nu)/F(m)) of dark adapted c ells, declined markedly under nitrate and iron starvation and moderate ly under phosphate starvation. Parallel to changes in F(nu)/F(m) were decreases in abundance of the reaction center protein D1 consistent wi th damage of PSII reaction centers in nutrient-starved cells. The rela tive abundance of the carboxylating enzyme, ribulose bisphosphate carb oxylase/oxygenase (RUBISCO), decreased in response to nitrate and iron starvation but not phosphate starvation. Most marked was the decline in the abundance of the small subunit of RUBISCO in nitrate-starved ce lls. The changes in pigment content and fluorescence characteristics w ere typically reversed within 24 h of resupply of the limiting nutrien t,