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 photosystem II (PSII) photochemistry were examined in the diatom Phaeodactylum tricornutum Bohlin (CCMP1327). 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 chl a-specific light absorption coefficient (a*BAR) and the functional absorption cross-section of PSII (sigma) 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 remained a constant proportion of total cell protein during nutrient starvation, indicating that chlorosis mirrored a general reduction in cell protein content. 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. Despite the increase in diatoxanthin and diadinoxanthin, the efficiency of PSII photochemistry, as measured by the ratio Of variable to maximum fluorescence (F(nu)/F(m)) of dark adapted cells, declined markedly under nitrate and iron starvation and moderately under phosphate starvation. Parallel to changes in F(nu)/F(m) were decreases in abundance of the reaction center protein D1 consistent with damage of PSII reaction centers in nutrient-starved cells. The relative abundance of the carboxylating enzyme, ribulose bisphosphate carboxylase/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 cells. The changes in pigment content and fluorescence characteristics were typically reversed within 24 h of resupply of the limiting nutrient,