MEPS 316:85-93 (2006)  -  doi:10.3354/meps316085

Light absorption of Isochrysis galbana (Prymnesiophyceae) under a day–night cycle with ultraviolet radiation

Nobuaki Ohi1,2,*, Akemi Mizobuchi1, Satoru Taguchi1

1Department of Environmental Engineering for Symbiosis, Soka University, 1-236 Tangi-cho, Hachiouji, Tokyo 192-8577, Japan
2Present address: National Aeronautics and Space Administration, Hydrospheric and Biospheric Sciences Laboratory, Goddard Space Flight Center, Wallops Island, Virginia 23337, USA

Abstract: Diel variation in chlorophyll a-specific absorption coefficients, a* ph (λ), m2 (mg chl a)–1, 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 Qa(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 Qa (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.

KEY WORDS: Absorption · Diel cycle · Optical properties · Pigmentation · UV

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