PHOTOSYSTEM II HETEROGENEITY IN THE MARINE DIATOM Phaeodactylum tricornutum

Abstract— The kinetics of photosystem II photochemistry are analyzed in the marine diatom Phaeodacfylum tricornutum by measurement of fluorescence induction in cell suspensions treated with 3–(3,4-dichlorophenyl)-1,1-dimethylurea. Photosystem II kinetics are found to be biphasic, the sum of two exponential components, suggesting that biphasic energy conversion in photosystem II may be a general consequence of thylakoid membrane appression. The emission wavelength-dependence of fluorescence induction suggests that the two photosystem II components have different variable fluorescence emission spectra. The slower component exhibits characteristic emission of the diatom light-harvesting complexes while emission from the faster component resembles that of the photosystem II reaction center. Variable fluorescence emission (293 K) at wavelengths > 700 nm is assigned to photosystem II. Application of model equations indicates that the two photosystem II unit types differ primarily in antenna size. A new analytical procedure is presented which eliminates ambiguities in the kinetic analysis associated with the incorrect assignment of the maximal fluorescence yield.