We studied the variability of in vivo absorption coefficients and PSII-scaled fluorescence excitation (fl-ex) spectra of high light (HL) and low light (LL) acclimated cultures of 33 phytoplankton species that belonged to 13 different pigment groups (PGs) and 10 different phytoplankton classes. By scaling fl-ex spectra to the corresponding absorption spectra by matching them in the 540-650 nm range, we obtained estimates for the fraction of total chl a that resided in PSII, the absorption of light by PSII, PSI, and photoprotective carotenoids. The in vivo red peak absorption maxima ranged from 673 to 679 nm, reflecting bonding of chl a to different pigment proteins. A simple approach is presented for quantifying intracellular self-shading and evaluating the impact of photoacclimation on biooptical characteristics of the different PGs examined. In view of these results, parameters used in the calculation of oxygenic photosynthesis based on pulse-amplitude-modulated (PAM) and fast-repetition-rate (FRR) fluorometers are discussed, showing that the ratio between light available to PSII and total absorption, essential for the calculation of the oxygen release rate (using the PSII-scaled fluorescence spectrum as a proxy) was dependent on species and photoacclimation state. Three subgroups of chromophytes exhibited 70%-80%, 60%-80%, and 50%-60% chl a in PSII-LHCII; the two subgroups of chlorophytes, 70% or 80%; and cyanobacteria, only 12%. In contrast, the mean fraction for chromo-and chlorophytes of quanta absorbed by PSII was 73% in LL-and 55% in HL-acclimated cells; thus, the corresponding ratios 0.55 and 0.73 might be used as correction factors adjusting for quanta absorbed by PSII for PAM and FRR measurements.Key index words: 13 pigment groups of phytoplankton; absorbed quanta to PSII; chl a AE C )1 ratio; package effect; photoacclimation; PSII-scaled fl-ex spectra; variable fluorescence Abbreviations: ACP, chl a-chl c-peridinin protein complex; chl a AE C )1 ratio, chl a to carbon ratio (w:w); DCMU, dichlorophenyl methyl urea; E, irradiance (lmol photons AE m )2 AE s )1 ); fl-ex, fluorescence excitation; FRR, fast repetition rate;

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) c3, 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 (°ac, 400–700 nm) and chl a‐normalized fluorescence excitation spectra of photosystem II fluorescence with or without addition of DCMU (°F and °FDCMU 400–700 nm) were evident. Gyrodinium aureolum and G. galatheanum exhibited in vivo spectral characteristics similar to chl c3‐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 °ac 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.

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Studies of the marine diatom Skeletonema costatum indicate that for a given irradiance and day length, increases in the rate of supply of a limiting nutrient cause linearly proportional increases in the growth rate and the Chl : C ratio. For a given irradiance, increases in day length cause decreases in the Chl : C ratio, and the growth rate is linearly proportional to the product of day length and the Chl : C ratio. For a given day length, increases in irradiance cause decreases in the Chl : C ratio and quantum yield, and the growth rate and irradiance are curvilinearly related. These observations are incorporated into a formulation of steady state growth, which includes day length, irradiance, and the Chl : C ratio, and two coefficients, which are the product of the maximal quantum yield and the specific absorption coefficient and the product of the average absorption cross section of the photosynthetic unit and the minimal rate of turnover of the unit. The photosynthetic quantum yield is represented by a Poisson distribution and is a function of irradiance alone.