EFFECTS OF IRON LIMITATION ON PHOTOSYSTEM-II COMPOSITION AND LIGHT UTILIZATION IN DUNALIELLA-TERTIOLECTA

The effects of iron limitation on photosystem II (PSII) composition an d photochemical energy conversion efficiency were studied in the unice llular chlorophyte alga Dunaliella tertiolecta. The quantum yield of p hotochemistry in PSII, inferred from changes in variable fluorescence normalized to the maximum fluorescence yield, was markedly lower in ir on-limited cells and increased 3-fold within 20 h following the additi on of iron. The decrease in the quantum yield of photochemistry was co rrelated with increased fluorescence emission from the antenna. In iro n-limited cells, flash intensity saturation profiles of variable fluor escence closely followed a cumulative one-hit Poisson model, suggestin g that PSII reaction centers are energetically isolated, whereas in ir on-replete cells, the slope of the profile was steeper and the calcula ted probability of energy transfer between reaction centers increased to > 0.6. Immunoassays revealed that in iron-limited cells the reactio n center proteins, D1, CP43, and CP47, were markedly reduced relative to the peripheral light-harvesting Chi-protein complex of PSII, wherea s the alpha subunit of cytochrome b(559) was about 10-fold higher. Spe ctroscopic analysis established that the cytochrome b(559) peptide did not contain an associated functional heme. We conclude that the photo chemical conversion of absorbed excitation energy in iron-limited cell s is limited by the number of photochemical traps per unit antenna.