| The pigment composition, absorption, and fluorescence excitation spectra of phytoplankton were analysed in two different environments of the North Atlantic: the Central North Atlantic, and the Labrador Sea. The distribution of different groups of algae could be explained, in great part, by photoadaptation (i.e., evolutionary adaptation of different species to have different pigment composition). A background of small cells was more-or-less uniformly distributed in the whole study area. Large cells contributed, on average, most of the total biomass. The coefficients of variation for absorption and fluorescence excitation spectra were of similar magnitude for the small and for the large size-fractions of phytoplankton. Both photoadaptation and photoacclimation (i.e., temporary changes in pigment concentrations in a given species) contributed to variations in the optical characteristics of phytoplankton. This was especially evident for the fluorescence excitation spectra, which varied not only with the concentration of photoprotective pigments (photoacclimation to high light), but also with the arrangement of the photosynthetic apparatus, which differed with the species present. Laboratory experiments were carried out with three species of phytoplankton grown at different light levels. In the diatom Thalassiosira weissflogii all types of pigments serve both photosystems, and as a consequence the shapes of the absorption and fluorescence excitation spectra were similar, except for the effect of photoprotective pigments at high light In the cyanophyte Synechococcus sp., where different pigments served each individual photosystem, the shapes of the absorption and fluorescence spectra showed marked differences even at low light. The diatom Chaetoceros sp. represented an intermediate case where part of the difference between absorption and fluorescence excitation could be attributed to pigments associated exclusively to photosystem I. These results are presented in an ecological and physiological context. |
