| Abstract: | The fluorescence excitation spectrum is sometimes used as aproxy for the action spectrum of photosynthesis in phytoplankton.The main assumption behind this approximation is that the shapesof absorption and fluorescence excitation spectra are similarexcept for the absorption by photoprotective pigments, whichdo not contribute to the fluorescence spectrum. In this study,we compare the shapes of the absorption and fluorescence spectrain three species of phytoplankton grown at differentirradiances:two diatoms (Thalassiosira weissflogii and Chaetoceros sp.)and a cyanophyte (Synechococcus sp.). The contribution to absorptionby photoprotective pigments was estimated for each experiment.Results showed that the differences between the shapes of absorptionand fluorescence spectra were similar to the estimated absorptionby photoprotective pigments only in the case ofT. weissflogii.In Synechococcus sp., and to a lesser degree in Chaetocerossp., the differences between the two types of spectra were largerthan the absorption by photoprotective pigments. In the caseof Synechococcus sp., the difference between these spectra wasapparently due mainly to the extreme imbalance of chlorophylla distribution between the two photosystems. Chaetoceros sp.seemed to be an intermediate case: a small part of the chlorophylla of the cell appeared to be exclusively associated with photosystemI and therefore did not contribute to fluorescence. Fluorescenceand absorption values were normalized to their values at 545nm, and the ratio of normalized absorption to normalized fluorescencewas computed for the blue (439 nm) and red (676 nm) peaks inthe spectra. The results showed that these peak ratios can beused to distinguish between the effects of photoprotective pigmentsand the arrangement of the photosynthetic apparatus on differencesbetweenfluorescence and absorption spectra. |
