Mixotrophic microorganisms are able to use organic carbon as well as inorganic carbon sources and thus, play an essential role in the biogeochemical carbon cycle. In aquatic ecosystems, the alteration of carbon dioxide (CO2 ) fixation by toxic metals such as cadmium - classified as a priority pollutant - could contribute to the unbalance of the carbon cycle. In consequence, the investigation of cadmium impact on carbon assimilation in mixotrophic microorganisms is of high interest. We exposed the mixotrophic microalga Chlamydomonas reinhardtii to cadmium in a growth medium containing both CO2 and labelled 13 C-[1,2] acetate as carbon sources. We showed that the accumulation of cadmium in the pyrenoid, where it was predominantly bound to sulphur ligands, impaired CO2 fixation to the benefit of acetate assimilation. Transmission electron microscopy (TEM)/X-ray energy dispersive spectroscopy (X-EDS) and micro X-ray fluorescence (μXRF)/micro X-ray absorption near-edge structure (μXANES) at Cd LIII- edge indicated the localization and the speciation of cadmium in the cellular structure. In addition, nanoscale secondary ion mass spectrometry (NanoSIMS) analysis of the 13 C/12 C ratio in pyrenoid and starch granules revealed the origin of carbon sources. The fraction of carbon in starch originating from CO2 decreased from 73 to 39% during cadmium stress. For the first time, the complementary use of high-resolution elemental and isotopic imaging techniques allowed relating the impact of cadmium at the subcellular level with carbon assimilation in a mixotrophic microalga.

中文翻译：

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镉的类固醇螯合会损害微藻中的二氧化碳固定。

混合营养微生物能够利用有机碳以及无机碳源，因此在生物地球化学碳循环中起着至关重要的作用。在水生生态系统中，被归类为优先污染物的镉等有毒金属对二氧化碳（CO2）固定的改变可能会导致碳循环的不平衡。因此，研究镉对混合营养微生物中碳同化的影响是非常重要的。我们在含有CO 2和标记为13 C- [1,2]乙酸盐作为碳源的生长培养基中，将混养微藻莱茵衣藻暴露于镉。我们表明，主要在与硫配体结合的类胡萝卜素中积累的镉会损害CO2固定，从而有利于乙酸同化。透射电子显微镜（TEM）/ X射线能量色散谱（X-EDS）和Cd LIII边缘的微X射线荧光（μXRF）/微X射线吸收近边缘结构（μXANES）表示定位和细胞结构中镉的形态。此外，对类胡萝卜素和淀粉颗粒中13 C / 12 C比的纳米级二次离子质谱（NanoSIMS）分析揭示了碳源的来源。在镉胁迫期间，源自CO2的淀粉中的碳含量从73％降低至39％。高分辨率元素和同位素成像技术的互补使用首次允许将镉在亚细胞水平的影响与混合营养微藻中的碳同化联系起来。