PLANT PHYSIOLOGY , Vol 109, Issue 3 963-972, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Effects of Iron Limitation on Photosystem II Composition and Light Utilization in Dunaliella tertiolecta
I. R. Vassiliev, Z. Kolber, K. D. Wyman, D. Mauzerall, V. K. Shukla and P. G. Falkowski
Oceanographic and Atmospheric Sciences Division, Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973-5000 (I.R.V., Z.K., K.D.W., P.G.F.)
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.
