Calcium plays an important role in the physiology of bacterial cells and as a free soluble form inside the cells, it is tightly regulated at a low concentration. However, much higher amounts of stored calcium exist within cells and can be remobilized at diverse occasions. As a consequence, there has been a variety of techniques developed to quantitatively map the different forms of Ca in cells. Here we show how scanning transmission x-ray microscopy (STXM), a synchrotron-based microscopy technique, conducted in the soft x-ray range at the Ca L_2,3 edges (340–360 eV) offers an original and informative quantitative view of different Ca reservoirs in bacterial cells at a spatial resolution better than 100 nm. To illustrate this, we analyzed mutants of the cyanobacterium Synechococcus elongatus PCC 7942, overexpressing different versions of a gene called ccyA from two phylogenetically distant cyanobacteria: Gloeomargarita lithophora and Synechococcus sp. PCC 6312. This gene is diagnostic of the capability of some cyanobacteria to form intracellular amorphous calcium carbonate (iACC) but its function remains unknown. Here we show that the overexpression of the ccyA gene in the iACC non-forming cyanobacterium S. elongatus PCC 7942 results in an increased Ca content, especially in some Ca-rich cells. Moreover, we show that STXM can discriminate four different reservoirs of Ca in cyanobacteria and provides quantitative assessment of their relative importance.

钙在细菌细胞的生理学中起着重要作用，并且作为细胞内的游离可溶形式，它在低浓度下受到严格调节。然而，细胞内存在大量储存的钙，并且可以在不同的场合重新动员。因此，人们开发了多种技术来定量绘制细胞中不同形式的 Ca。_{在这里，我们展示了扫描透射 X 射线显微镜 (STXM)（一种基于同步加速器的显微镜技术）如何在 Ca L 2,3边缘 (340–360 eV)}的软 X 射线范围内进行，提供原始且信息丰富的定量视图细菌细胞中不同 Ca 储库的空间分辨率优于 100 nm。为了说明这一点，我们分析了蓝藻细长聚球藻的突变体PCC 7942，过表达来自两种系统发育较远的蓝藻的ccyA基因的不同版本：Gloeomargarita lithophora和Synechococcus sp。PCC 6312。该基因可诊断某些蓝细菌形成细胞内无定形碳酸钙 (iACC) 的能力，但其功能仍然未知。在这里，我们表明ccyA基因在 iACC 非形成蓝细菌S. elongatus PCC 7942 中的过表达导致 Ca 含量增加，特别是在一些富含 Ca 的细胞中。此外，我们表明 STXM 可以区分蓝藻中四种不同的 Ca 储库，并对其相对重要性进行定量评估。