Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan.
Proc Natl Acad Sci U S A. 2012 Oct 23;109(43):17328-35. doi: 10.1073/pnas.1207347109. Epub 2012 Sep 4.
Chlorophylls are essential components of the photosynthetic apparati that sustain all of the life forms that ultimately depend on solar energy. However, a drawback of the extraordinary photosensitizing efficiency of certain chlorophyll species is their ability to generate harmful singlet oxygen. Recent studies have clarified the catabolic processes involved in the detoxification of chlorophylls in land plants, but little is understood about these strategies in aquatic ecosystem. Here, we report that a variety of heterotrophic protists accumulate the chlorophyll a catabolite 13(2),17(3)-cyclopheophorbide a enol (cPPB-aE) after their ingestion of algae. This chlorophyll derivative is nonfluorescent in solution, and its inability to generate singlet oxygen in vitro qualifies it as a detoxified catabolite of chlorophyll a. Using a modified analytical method, we show that cPPB-aE is ubiquitous in aquatic environments, and it is often the major chlorophyll a derivative. Our findings suggest that cPPB-aE metabolism is one of the most important, widely distributed processes in aquatic ecosystems. Therefore, the herbivorous protists that convert chlorophyll a to cPPB-aE are suggested to play more significant roles in the modern oceanic carbon flux than was previously recognized, critically linking microscopic primary producers to the macroscopic food web and carbon sequestration in the ocean.
叶绿素是维持所有最终依赖太阳能的生命形式的光合作用器官的重要组成部分。然而，某些叶绿素物种非凡的光致敏效率的一个缺点是它们能够产生有害的单线态氧。最近的研究阐明了陆地植物中叶绿素解毒涉及的分解代谢过程，但对水生生态系统中这些策略的了解甚少。在这里，我们报告说，各种异养原生动物在摄入藻类后会积累叶绿素 a 分解产物 13(2),17(3)-环脱植基胆绿素 a 烯醇 (cPPB-aE)。这种叶绿素衍生物在溶液中无荧光，并且其在体外不能产生单线态氧，使其成为叶绿素 a 的解毒代谢产物。使用改进的分析方法，我们表明 cPPB-aE 在水生环境中普遍存在，并且它通常是主要的叶绿素 a 衍生物。我们的发现表明，cPPB-aE 代谢是水生生态系统中最重要、分布最广的过程之一。因此，将叶绿素 a 转化为 cPPB-aE 的食草原生动物在现代海洋碳通量中的作用比以前认为的更为重要，将微观初级生产者与宏观食物网和海洋碳固存紧密联系起来。
