Fluorescence excitation spectra and light utilization in two red tide dinoflagellates

We compared the impact of structural changes in the light‐harvesting machinery of two bloom‐forming dinoflagellates (Prorocentrum minimum and Heterocapsa pygmaea) by quantifying photophysiological changes in order to define the alternative mechanisms of photoacclimation. There are species‐specific and photoacclimative differences in the abundance and composition of functional light‐harvesting complexes (including photosystem 1‐ and 2‐enriched bands) which are reflected in spectral changes in absorption, and photosystem 2 (PS2) fluorescence excitation spectra of whole cells, thylakoid micelles, and discrete photosynthetic pigment‐protein complexes. DCMU‐induced variations in fluorescence excitation spectra are evident in whole cells but not in isolated thylakoid micelles or functional chromoproteins; such variations indicate that intracellular self‐shading which causes variable fluorescence is eliminated in subcellular preparations. We developed a scaling procedure of PS2‐related in vivo fluorescence excitation spectra relative to Chl a‐specific absorption coefficients and used the chromoprotein differences to illustrate how the chromoprotein composition alters cellular fluorescence properties. We also examined the energy transfer efficiency of photosynthetic pigments and the photoprotective role and location of diadinoxanthin.