The influence of inorganic mercury (Hg(II)) exposure on photosynthetic microorganisms and their pigments remains understudied. Here, we employed resonance Raman (RR) spectroscopy to investigate the responses of two freshwater phytoplankton species, the green alga Chlamydomonas reinhardtii and the diatom Cyclotella meneghiniana to Hg(II) exposure. We selectively recorded the spectral RR signature of carotenoids in intact cells exposed to concentrations of 10 nM and 100 nM of Hg(II), representative for contaminated environment and unexposed control cells. A two-hour exposure of C. reinhardtii resulted in a slight decrease in lutein and β-carotene levels, while total carotenoids RR band broadening, as revealed by the FWHM of the υ1(C=C) stretching mode from averaged RR spectra, suggested conformational changes in pigments. Higher Hg(II) concentration induced more pronounced conformational changes. Similarly, a two-hour exposure of C. meneghiniana resulted in slight decreased level of the fucoxanthin, while diadinoxanthin showed an opposite trend compared to control: when fucoxanthin decreased, diadinoxanthin increased under 10 nM Hg (II) exposure. At higher concentrations, the decrease in fucoxanthin was less pronounced, accompanied by a broadening of the band area, (with FHHM increased), indicating possible conformer occurrence in response to Hg-induced stress. The changes in the main carotenoid species of the two algae are species-specific, Hg(II) concentration-specific, and dependent on exposure time. The calculated spectral differences in absorbances from UV–VIS spectra of methanol extracts from each group supported the main findings obtained by RR, though with caution due to the selective extraction efficiency of the respective carotenoids. This study highlighted for a first time the capability of single-cell RR spectroscopy as a valuable tool for toxicity assessment and for comprehending early-stage alterations in carotenoid metabolism due to toxic metal exposure in vivo.

中文翻译：

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无机汞对淡水藻类中类胡萝卜素的影响：来自单细胞共振拉曼光谱的见解


无机汞 （Hg（II）） 暴露对光合微生物及其色素的影响仍未得到充分研究。在这里，我们采用共振拉曼 （RR） 光谱来研究两种淡水浮游植物物种，即绿藻莱茵衣藻和硅藻 Cyclotella meneghiniana 对 Hg（II） 暴露的响应。我们选择性地记录了暴露于 10 nM 和 100 nM Hg（II） 浓度的完整细胞中类胡萝卜素的光谱 RR 特征，代表受污染的环境和未暴露的对照细胞。莱茵梭菌暴露 2 小时导致叶黄素和 β-胡萝卜素水平略有降低，而总类胡萝卜素 RR 带展宽，如平均 RR 光谱中 υ1（C=C） 拉伸模式的 FWHM 所揭示的那样，表明色素的构象变化。较高的 Hg（II） 浓度诱导更明显的构象变化。同样，C. meneghiniana 暴露 2 小时导致岩藻黄质水平略有降低，而与对照组相比，二甲氧黄质表现出相反的趋势，当岩藻黄质降低时，二甲氧黄质在 10 nM Hg （II） 暴露下增加。在较高浓度下，岩藻黄质的减少不太明显，伴随着条带面积的扩大（FHHM 增加），表明在 Hg 诱导的应激下可能发生构象异构体。两种藻类的主要类胡萝卜素种类的变化是物种特异性的，Hg（II） 浓度特异性的，并且取决于暴露时间。从每组甲醇提取物的紫外-可见光谱光谱中计算出的吸光度光谱差异支持 RR 获得的主要发现，但由于相应类胡萝卜素的选择性提取效率，因此需要谨慎。 这项研究首次强调了单细胞 RR 光谱法作为毒性评估和理解由于体内有毒金属暴露而导致的类胡萝卜素代谢早期改变的宝贵工具的能力。