Redistribution of excitation energy between two photosystems during light-shade adaptation in marine diatoms: State conversion of light-harvesting complexes

Marine diatoms effectively photosynthesize by acclimating to the wide range of growth irradiance in the ocean. Using a pennate diatom Phaeodactylum tricornutum and a centric diatom Chaetoceros gracilis, we evaluated differences in the photosynthetic machinery under a wide range of growth irradiances. The chlorophyll a-specific amounts of the major accessory pigments remained relatively constant irrespective of growth irradiance in both diatoms. However, fluorescence spectra at 77K differed drastically depending on the growth irradiance: In P. tricornutum, fluorescence from photosystem II was dominant in high-light-grown cells and negligible in low-light-grown cells, while in C. gracilis, the opposite trend was observed. These drastic changes in fluorescence spectra were slow processes. The amounts of the two reaction centers, as assessed by specific antibodies and absorption changes in P700, remained almost constant under different irradiances. These results indicate that under dim growth irradiance, more excitation energy is diverted to photosystem I in the pennate diatom, and to photosystem II in the centric diatom. Therefore, the light-harvesting antennas balance excitation energy distribution by changing their association between photosystems I and II in different manners between P. tricornutum and C. gracilis, depending on irradiance. This phenomenon is similar to state transitions, but differs in its magnitude and duration. Differences in the preference of energy distribution in the two diatoms suggest that the dynamic state conversion--an antenna rearrangement during the long-term acclimation process in diatoms--is the species-specific strategy to achieve effective photosynthesis under the wide range of growth irradiances in the ocean.