RESUMO
The effects of iron limitation on photosystem II (PSII) composition and photochemical energy conversion efficiency were studied in the unicellular chlorophyte alga Dunaliella tertiolecta. The quantum yield of photochemistry in PSII, inferred from changes in variable fluorescence normalized to the maximum fluorescence yield, was markedly lower in iron-limited cells and increased 3-fold within 20 h following the addition of iron. The decrease in the quantum yield of photochemistry was correlated with increased fluorescence emission from the antenna. In iron-limited cells, flash intensity saturation profiles of variable fluorescence closely followed a cumulative one-hit Poisson model, suggesting that PSII reaction centers are energetically isolated, whereas in iron-replete cells, the slope of the profile was steeper and the calculated probability of energy transfer between reaction centers increased to >0.6. Immunoassays revealed that in iron-limited cells the reaction center proteins, D1, CP43, and CP47, were markedly reduced relative to the peripheral light-harvesting Chl-protein complex of PSII, whereas the [alpha] subunit of cytochrome b559 was about 10-fold higher. Spectroscopic analysis established that the cytochrome b559 peptide did not contain an associated functional heme. We conclude that the photochemical conversion of absorbed excitation energy in iron-limited cells is limited by the number of photochemical traps per unit antenna.
RESUMO
The effects of PAR and UV radiation on PS II photochemistry were examined in natural phytoplankton communities from coastal waters off Rhode Island (USA) and the subtropical Pacific. The photochemical energy conversion efficiency, the functional absorption cross section and the kinetics of electron transfer on the acceptor side of PS II were derived from variable fluorescence parameters using both pump and probe and fast repetition rate techniques. In both environments, the natural phytoplankton communities displayed marked decreases in PS II photochemical energy conversion efficiency that were correlated with increased PAR. In the coastal waters, the changes in photochemical energy conversion efficiency were not statistically different for samples treated with supplementary UV-B radiation or screened to exclude ambient UV-B. Moreover, no significant light-dependent changes in the functional absorption cross section of PS II were observed. The rate of electron transfer between QA (-) and QB was, however, slightly reduced in photodamaged cells, indicative of damage on the acceptor side. In the subtropical Pacific, the decrease in photochemical energy conversion efficiency was significantly greater for samples exposed to natural levels of UV-A and/or UV-B compared with those exposed to PAR alone. The cells displayed large diurnal changes in the functional absorption cross section of PS II, indicative of non-photochemical quenching in the antenna. The changes in the functional absorption cross section were highly correlated with PAR but independent of UV radiation. The time course of changes in photochemical efficiency reveals that the photoinhibited reaction centers rapidly recover (within an hour or two) to their preillumination values. Thus, while we found definitive evidence for photoinhibition of PS II photochemistry in both coastal and open ocean phytoplankton communities, we did not find any effect of UV-B on the former, but a clear effect on the latter. The results of this study indicate that the effects of UV-B radiation on phytoplankton photosynthesis are as dependent on the radiative transfer properties of the water body and the mixing rate, as on the wavelength and energy distribution of the radiation and the absorption cross sections of the biophysical targets.
