Changes in the in vivo absorption and fluorescence excitation spectra with growth irradiance in three species of phytoplankton

The fluorescence excitation spectrum is sometimes used as a proxy for the action spectrum of photosynthesis in phytoplankton. The main assumption behind this approximation is that the shapes of absorption and fluorescence excitation spectra are similar except for the absorption by photoprotective pigments, which do not contribute to the fluorescence spectrum. In this study; we compare the shapes of the absorption and fluorescence spectra in three species of phytoplankton grown at different irradiances: two diatoms (Thalassiosira weissflogii and Chaetoceros sp.) and a cyanophyte (Synechococcus sp.). The contribution to absorption by photoprotective pigments was estimated for each experiment. Results showed that the differences between the shapes of absorption and fluorescence spectra were similar to the estimated absorption by photoprotective pigments only in the case of T. weissflogii. In Synechococcus sp., and to a lesser degree in Chaetoceros sp., the differences between the two types of spectra were larger than the absorption by photoprotective pigments. In the case of Synechococcus sp., the difference between these spectra was apparently due mainly to the extreme imbalance of chlorophyll a distribution between the two photosystems. Chaetoceros sp. seemed to be an intermediate case: a small part of the chlorophyll a of the cell appeared to be exclusively associated with photosystem I and therefore did not contribute to fluorescence. Fluorescence and absorption values were normalized to their values at 545 nm, and the ratio of normalized absorption to normalized fluorescence was computed for the blue (439 nm) and red (676 nm) peaks in the spectra. The results showed that these peak ratios can be used to distinguish between the effects of photoprotective pigments and the arrangement of the photosynthetic apparatus on differences between fluorescence and absorption spectra.