||Brookhaven National Lab., Upton, NY. Oceanographic and Atmospheric Sciences Div.;Environmental Research Lab., Narragansett, RI.;Department of Energy, Washington, DC.;National Science Foundation, Washington, DC.;National Aeronautics and Space Administration, Washington, DC.
The role of iron in regulating light harvesting and photochemical energy conversion processes was examined in the marine unicellular chlorophyte Dunaliella tertiolecta and the marine diatom Phaeodactylum tricornutum. In both species, iron limitation led to a reduction in cellular chlorophyll concentrations, but an increase in the in vivo, chlorophyll-specific, optical absorption cross-sections. Moreover, the absorption cross-section of photosystem II, a measure of the photon target area of the traps, was higher in iron-limited cells and decreased rapidly following iron addition. Iron-limited cells exhibited reduced variable/maximum fluorescence ratios and a reduced fluorescence per unit absorption at all wave-lengths between 400 and 575 nm. Following iron addition, variable/maximum fluorescence ratios increased rapidly, reaching 90 percent of the maximum within 18 to 25 h. Thus, although more light was absorbed per unit of chlorophyll, iron limitation reduced the transfer efficiency of excitation energy in photosystem II.