In vivo fluorescence excitation and absorption spectra of marine phytoplankton: I. Taxonomic characteristics and responses to photoadaptation

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Abstract
Absorption and fluorescence excitation spectra were measured for batch cultures of five species of marine phytoplankton grown under high and low light. These spectra were examined for properties characteristic of taxonomic position and of photoadaptive response. While regions of absorption and excitation of chlorophyll a fluorescence diagnostic of pigment composition were identifiable, photoadaptive response had greater influence on spectral variability. Although reduced growth irradiance caused changes in both the absorption and fluorescence excitation spectra, the fluorescence excitation spectrum appears to be more sensitive to alterations in the ambient light field for growth than does the absorption spectrum. For a single species. the fluorescence excitation spectrum for a sample grown at low irradiance showed greater structure than that for the sample grown at a high irradiance. Under low light conditions, the excitation of chlorophyll a fluorescence by accessory pigments increased relative to the excitation by chlorophyll a itself The highest fluorescence yields occur in the blue-green region of the spectrum, corresponding to bands of peak absorption by the accessory pigments. Changes in absorption spectra are less marked, but two features recur. First. in the blue-green region of the spectrum from -500–560 nm. absorption is enhanced in the low-light cells relative to that of the high-light cells. Second, the ratio of absorption at 435 nm to that at 676 nm was greater for the high-light cells. Correlating changes in pigment concentrations were observed. The influence of photoadaptation on the properties of fluorescence excitation spectra is as great or greater than the influence of pigment complements characteristic of specific algal taxa.

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