Abstract
SUMMARY The interconversion rate of diadinoxanthin (DD) cycle under high irradiance and subsequent darkness was analyzed using the cultivated centric marine diatom, Chaetoceros gracilis Schutt. A prompt de-epoxidation from diadinoxanthin to diatoxanthin (DT) occurred immediately after the onset of higher irradiance. The fist-order rate constant, k, for this de-epoxidation was 0.1–0.2 min-1 irrespective of the irradiance. The difference in photon fluence rate lead to the difference of the final amount of DT, leaving the rate constant at almost the same value. After turning off the light, epoxidation from DT to DD occurred. The first-order constant of epoxidation was much slower than that of de-epoxidation: 0.005 – 0.009 min-1. Independent of this epoxidation process, de novo synthesis of DD-cycle pigments was also observed under the subsequent darkness. Based on these findings, a common nature of the DD-cycle as a protection mechanism for photo-systems was demonstrated for C. gracilis.
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