Photosynthetic organisms finely tune their photosynthetic machinery including pigment compositions and antenna systems to adapt to various light environments. However, it is poorly understood how the photosynthetic machinery in the green flagellate Euglena gracilis is modified under high-light conditions. In this study, we examined high-light modification of excitation-energy-relaxation processes in Euglena cells. Oxygen-evolving activity in the cells incubated at 300 µmol photons m⁻² s⁻¹ (HL cells) cannot be detected, reflecting severe photodamage to photosystem II (PSII) in vivo. Pigment compositions in the HL cells showed relative increases in 9′-cis-neoxanthin, diadinoxanthin, and chlorophyll b compared with the cells incubated at 30 µmol photons m⁻² s⁻¹ (LL cells). Absolute fluorescence spectra at 77 K exhibit smaller intensities of the PSII and photosystem I (PSI) fluorescence in the HL cells than in the LL cells. Absolute fluorescence decay-associated spectra at 77 K of the HL cells indicate suppression of excitation-energy transfer from light-harvesting complexes (LHCs) to both PSI and PSII with the time constant of 40 ps. Rapid energy quenching in LHCs and PSII in the HL cells is distinctly observed by averaged Chl-fluorescence lifetimes. These findings suggest that Euglena modifies excitation-energy-relaxation processes in addition to pigment compositions to deal with excess energy. These results provide insights into the photoprotection strategies of this alga under high-light conditions.

光合生物可以微调它们的光合机制，包括色素成分和天线系统，以适应各种光环境。然而，人们对绿色鞭毛虫Euglena gracilis的光合作用机制如何在高光条件下发生变化知之甚少。在这项研究中，我们检查了眼虫细胞中激发-能量-弛豫过程的高光修饰。无法检测到在 300 µmol 光子 m ^-2  s ^-1 (HL 细胞)下孵育的细胞中的氧气释放活性，这反映了体内光系统 II (PSII) 的严重光损伤。HL 细胞中的色素成分显示 9'-顺式新黄质、二十二黄质和叶绿素的相对增加b与在 30 µmol 光子 m ^-2  s ^-1 (LL 细胞)下孵育的细胞进行比较。77 K 的绝对荧光光谱在 HL 细胞中表现出比在 LL 细胞中更小的 PSII 和光系统 I (PSI) 荧光强度。HL 细胞在 77 K 处的绝对荧光衰减相关光谱表明，在时间常数为 40 ps 的情况下，抑制了从光捕获复合物 (LHC) 到 PSI 和 PSII 的激发能量转移。HL 细胞中 LHC 和 PSII 的快速能量淬灭可以通过平均 Chl 荧光寿命明显观察到。这些发现表明眼虫除了颜料成分外，还修改激发-能量-弛豫过程以处理多余的能量。这些结果为了解这种藻类在高光条件下的光保护策略提供了见解。