The chl a specific absorption coefficients [ a * ( ), m 2 и mg chl a Ϫ 1 ] were examined in chemostat culture of the Prymnesiophyceae Isochrysis galbana (Parke) under a 12:12-h light:dark cycle at low light (75 mol photons и m Ϫ 2 и s Ϫ 1 ) and high light (500 mol photons и m Ϫ 2 и s Ϫ 1 ) conditions. Other associated measurements such as pigment composition, cell density, and diameter as the measure of cell size were also made at the two light regimes every 2 h for 2 days to confirm the periodicity. A distinct diel variability was observed for the a * ( ) with maxima near dawn and minima near dusk. The magnitude of diel variation in a * (440) was 15% at low light and 22% at high light. Pronounced diel patterns were observed for cell size with minima near dawn and maxima near dusk. The magnitude of diel variation in cell size was 9.3% at low light and 21% at high light. The absorption efficiency factors [ Q a (440)] were determined by reconstruction using intracellular concentrations of pigments and cell size. The Q a (440) also showed a distinct diel variability, with minima near dawn and maxima near dusk. The diel variation in a * ( ) and Q a ( ) was primarily caused by changes in cell size due to growth, although there was some influence from diel variations in the intracellular pigment concentrations. The results presented here indicated that diel variation in a * ( ) was an important component of the optical characterization of phytoplankton.

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Diel variation in chlorophyll a -specific absorption coefficients, a* ph (λ), m 2 (mg chl a), was examined to study the effect of ultraviolet radiation (UVR) on a* ph (λ) of the prymnesiophycean Isochrysis galbana (Parke). A continuous culture was exposed for 2 wk to a 12 h light:12 h dark cycle of photosynthetically available radiation (PAR) supplemented with UVR. Photosynthetic and photoprotective pigments as well as cell density and diameter were also determined every 2 h for 2 d to confirm the periodicity of a* ph (λ). A distinct diel variation was observed, with maxima toward the end of light periods and minima toward the end of dark periods. The magnitude of diel variation in a* ph (440) and a* ph (676) was 14 and 6.6%, respectively. To reconstruct the absorption coefficient, the absorption efficiency factors Q a (λ), were determined using intracellular chl a concentration, cell diameter, and chl a-specific absorption coefficients after solubilization on Triton X, a* TX (λ). The main source of diel variation for Q a (λ) appears to be a* TX (λ). Diel variation in the absorption ratio of photoprotective pigments to chl a was observed to be more significant than that for the absorption ratio of other photosynthetic pigments to chl a. The diel variations in a* ph (λ) were primarily caused by changes in pigmentation of photoprotective pigments due to UVR.