BIO‐OPTICAL CHARACTERISTICS AND PHOTOADAPTIVE RESPONSES IN THE TOXIC AND BLOOM‐FORMING DINOFLAGELLATES GYRODINIUM AUREOLUM, GYMNODINIUM GALATHEANUM , AND TWO STRAINS OF PROROCENTRUM MINIMUM 1

Photoadaptive responses in the toxic and bloom‐forming dinoflagellates Gyrodinium aureolum Hulbert, Gymnodinium galatheanum Braarud, and two strains of Prorocentrum minimum (Pavillard)Schiller were evaluated with respect to pigment composition, light‐harvesting characteristics, carbon and nitrogen contents, and growth rates in shade‐ and light‐adapted cells. The two former species were grown at scalar irradiances of 30 and 170 μmol · m −2 at a 12‐h daylength at 20° C. The two strains of P. minimum were grown at 35 and 500 μmol. m −2 · s −1 at a 2‐h daylength at 20° C. For the first time, chlorophyll (chl) c 3 , characteristic of several bloom‐forming prymnesiophytes, was detected in G. aureolum and G. galatheanum. Photoadaptional status affected the pigment composition strongly, and the interpretation of the variation depended on whether the pigment composition was normalized per cell, carbon, or chl a. Species‐specific and photoadaptional differences in chl a ‐specific absorption (°a c , 400–700 nm) and chl a ‐normalized fluorescence excitation spectra of photosystem II fluorescence with or without addition of DCMU (°F and °F DCMU 400–700 nm) were evident. Gyrodinium aureolum and G. galatheanum exhibited in vivo spectral characteristics similar to chl c 3 ‐containing prymnesiophytes in accordance with their similar pigmentation. Prorocentrum minimum had in vivo absorption and fluorescence characteristics typical for peridinin‐containing dinoflagellates. Species‐specific differences in in vivo absorption were also observed as a function of package effect vs. growth irradiance. This effect could be explained by differences in intracellular pigment content, cell size/shape, and chloroplast morphology/numbers. Light‐ and shade‐adapted cells of P. minimum contained 43 and 17% of photoprotective carotenoids (diadino + diatoxanthin) relative to chl a , respectively. The photoprotective function of these carotenoids was clearly observed as a reduction in °F and °F DCMU at 400–540 nm compared to °a c in light‐adapted cells of P. minimum. Spectrally weighted light absorption (normalized to chl a and carbon, 400–700 nm) varied with species and growth conditions. The use of quantum‐corrected and normalized fluorescence excitation spectra with or without DCMU‐treated cells to estimate photosynthetically usable light is discussed. The usefulness of in vitro absorption and fluorescence excitation spectra for estimation of the degradation status of chl a and the ratio of chl a to total pigments is also discussed.