Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), UMR CNRS 7590, Muséum National d'Histoire Naturelle, Sorbonne Université, Paris, France.
CEA Cadarache, UMR7265 Institute of Biosciences and Biotechnologies of Aix-Marseille, CEA, CNRS, Aix-Marseille University, Saint-Paul-lez-Durance, France.
Geobiology. 2021 Mar;19(2):199-213. doi: 10.1111/gbi.12424. Epub 2020 Dec 21.
Achromatium is a long known uncultured giant gammaproteobacterium forming intracellular CaCO that impacts C and S geochemical cycles functioning in some anoxic sediments and at oxic-anoxic boundaries. While intracellular CaCO granules have first been described as Ca oxalate then colloidal CaCO more than one century ago, they have often been referred to as crystalline solids and more specifically calcite over the last 25 years. Such a crystallographic distinction is important since the respective chemical reactivities of amorphous calcium carbonate (ACC) and calcite, hence their potential physiological role and conditions of formation, are significantly different. Here, we analyzed the intracellular CaCO granules of Achromatium cells from Lake Pavin using a combination of Raman microspectroscopy and scanning electron microscopy. Granules in intact Achromatium cells were unequivocally composed of ACC. Moreover, ACC spontaneously transformed into calcite when irradiated at high laser irradiance during Raman analyses. Few ACC granules also transformed spontaneously into calcite in lysed cells upon cell death and/or sample preparation. Overall, the present study supports the original claims that intracellular Ca-carbonates in Achromatium are amorphous and not crystalline. In that sense, Achromatium is similar to a diverse group of Cyanobacteria and a recently discovered magnetotactic alphaproteobacterium, which all form intracellular ACC. The implications for the physiology and ecology of Achromatium are discussed. Whether the mechanisms responsible for the preservation of such unstable compounds in these bacteria are similar to those involved in numerous ACC-forming eukaryotes remains to be discovered. Last, we recommend to future studies addressing the crystallinity of CaCO granules in Achromatium cells recovered from diverse environments all over the world to take care of the potential pitfalls evidenced by the present study.
血杆菌是一种已知的长期未培养的巨型γ变形菌，能够在一些缺氧沉积物中和氧化-缺氧边界形成细胞内 CaCO3，从而影响 C 和 S 的地球化学循环。虽然细胞内 CaCO3 颗粒在一个多世纪前首次被描述为草酸钙，然后是胶体 CaCO3，但在过去 25 年中，它们通常被称为结晶固体，更具体地说是方解石。这种晶体学上的区别很重要，因为无定形碳酸钙 (ACC) 和方解石的各自化学反应性，因此它们的潜在生理作用和形成条件有很大的不同。在这里，我们使用拉曼微光谱和扫描电子显微镜对来自 Pavin 湖的 Achromatium 细胞的细胞内 CaCO3 颗粒进行了分析。完整的 Achromatium 细胞中的颗粒明确由 ACC 组成。此外，当在拉曼分析中用高激光辐照度照射时，ACC 会自发转化为方解石。在细胞死亡和/或样品制备时，少数 ACC 颗粒也会在裂解细胞中自发转化为方解石。总的来说，本研究支持了细胞内 Ca-碳酸盐在 Achromatium 中是无定形而不是结晶的原始主张。从这个意义上说，Achromatium 与一大群蓝细菌和最近发现的磁生α变形菌相似，它们都形成细胞内的 ACC。本文讨论了 Achromatium 的生理学和生态学意义。负责在这些细菌中保存这些不稳定化合物的机制是否与参与众多形成 ACC 的真核生物的机制相似，还有待发现。最后，我们建议未来的研究在全世界不同环境中研究从 Achromatium 细胞中回收的 CaCO3 颗粒的结晶度时，要注意本研究中发现的潜在陷阱。
