Department of Integrated Biosciences, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba, 277-8562, Japan.
Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies, Tsukuba, Ibaraki, 305-8506, Japan.
Sci Rep. 2018 Apr 4;8(1):5617. doi: 10.1038/s41598-018-23854-w.
The unicellular green alga Haematococcus pluvialis accumulates large amounts of the red ketocarotenoid astaxanthin to protect against environmental stresses. Haematococcus cells that accumulate astaxanthin in the central part (green-red cyst cells) respond rapidly to intense light by distributing astaxanthin diffusively to the peripheral part of the cell within 10 min after irradiation. This response is reversible: when astaxanthin-diffused cells were placed in the dark, astaxanthin was redistributed to the center of the cell. Although Haematococcus possesses several pigments other that astaxanthin, the subcellular distribution and content of each pigment remain unknown. Here, we analyzed the subcellular dynamics and localization of major pigments such as astaxanthin, β-carotene, lutein, and chlorophylls under light irradiation using time-lapse and label-free hyperspectral imaging analysis. Fluorescence microscopy and freeze-fracture transmission electron microscopy showed that, preceding/following exposure to light, astaxanthin colocalized with lipid droplets, which moved from the center to the periphery through pathways in a chloroplast. This study revealed that photoresponse dynamics differed between astaxanthin and other pigments (chlorophylls, lutein, and β-carotene), and that only astaxanthin freely migrates from the center to the periphery of the cell through a large, spherical, cytoplasm-encapsulating chloroplast as a lipid droplet. We consider this to be the Haematococcus light-protection mechanism.
单细胞绿藻雨生红球藻积累大量的红色酮类类胡萝卜素虾青素，以抵御环境胁迫。积累虾青素的雨生红球藻中央部分（绿-红囊泡细胞）在受到强光照射后 10 分钟内，通过将虾青素扩散到细胞的外周部分，迅速对强光做出反应。这种反应是可逆的：当虾青素扩散的细胞被放置在黑暗中时，虾青素会重新分布到细胞的中心。尽管雨生红球藻还含有几种除虾青素以外的色素，但每种色素的亚细胞分布和含量仍然未知。在这里，我们使用延时和无标记高光谱成像分析，分析了在光照下主要色素（如虾青素、β-胡萝卜素、叶黄素和叶绿素）的亚细胞动态和定位。荧光显微镜和冷冻断裂透射电子显微镜显示，在暴露于光之前/之后，虾青素与脂滴共定位，脂滴通过叶绿体中的途径从中心移动到外周。这项研究表明，虾青素和其他色素（叶绿素、叶黄素和β-胡萝卜素）的光响应动力学不同，只有虾青素可以作为脂滴自由地从细胞中心迁移到外周，通过一个大的、球形的、包裹细胞质的叶绿体。我们认为这就是雨生红球藻的光保护机制。
