Abstract
We investigated the photoresponses of diadinoxanthin (DD) and diatoxanthin (DT) using Isochrysis galbana cultures that had been acclimated to four different levels of photon flux density (PFD). DD can play a role as a photosynthetic light-harvesting pigment, whereas DT can play a role as a photoprotective pigment. The PFD ranged from limited light conditions (LL) to saturated light conditions (SL) for growth. The photoresponses of these pigments during light–dark transitions at each acclimation PFD were consistent. The photoresponses of these xanthophyll pigments, as normalized to either cell density or chlorophyll a (Chl a) concentration, to light–dark transitions varied with acclimation PFD. The stoichiometric variations between the DD or DT : Chl a ratio observed at the two higher acclimation PFDs suggested that these xanthophyll dynamics are controlled by xanthophyll cycling processes in addition to de novo processes and interconversion between DD and fucoxnathin under SL conditions. Significant linear relationships were observed between the levels of DD and/or DT normalized to either the cell density or Chl a concentration. Based on the cellular pigment content and the xanthophyll : Chl a ratio, the slopes of the linear relationships normalized to either the cell density or Chl a concentration were significantly related to the acclimation PFD linearly and sigmoidally, respectively. Similarly, the acclimation PFD at the interaction of the sigmoidal change and the transition point from the LL to SL condition suggests that these xanthophyll pigment dynamics can be characterized by acclimation to the light conditions. An examination of the slope may provide information leading to a better understanding of photoacclimation in the algal photosynthetic apparatus.
