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

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 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. © Inter-Research 2006.