Bacteria synthesize a wide range of intracellular submicrometer-sized inorganic precipitates of diverse chemical compositions and structures, called biominerals. Their occurrences, functions and ultrastructures are not yet fully described despite great advances in our knowledge of microbial diversity. Here, we report bacteria inhabiting the sediments and water column of the permanently stratified ferruginous Lake Pavin, that have the peculiarity to biomineralize both intracellular magnetic particles and calcium carbonate granules. Based on an ultrastructural characterization using transmission electron microscopy (TEM) and synchrotron-based scanning transmission X-ray microscopy (STXM), we showed that the calcium carbonate granules are amorphous and contained within membrane-delimited vesicles. Single-cell sorting, correlative fluorescent in situ hybridization (FISH), scanning electron microscopy (SEM) and molecular typing of populations inhabiting sediments affiliated these bacteria to a new genus of the Alphaproteobacteria. The partially assembled genome sequence of a representative isolate revealed an atypical structure of the magnetosome gene cluster while geochemical analyses indicate that calcium carbonate production is an active process that costs energy to the cell to maintain an environment suitable for their formation. This discovery further expands the diversity of organisms capable of intracellular Ca-carbonate biomineralization. If the role of such biomineralization is still unclear, cell behaviour suggests that it may participate to cell motility in aquatic habitats as magnetite biomineralization does.

细菌合成各种化学成分和结构的细胞内亚微米大小的无机沉淀物，称为生物矿物。尽管我们对微生物多样性的认识取得了很大进展，但它们的发生、功能和超微结构尚未完全描述。在这里，我们报告了居住在永久分层的铁质湖 Pavin 的沉积物和水柱中的细菌，这些细菌具有生物矿化细胞内磁性颗粒和碳酸钙颗粒的特性。基于使用透射电子显微镜 (TEM) 和基于同步加速器的扫描透射 X 射线显微镜 (STXM) 的超微结构表征，我们发现碳酸钙颗粒是无定形的并且包含在膜分隔的囊泡中。单细胞分选，相关荧光原位杂交 (FISH)、扫描电子显微镜 (SEM) 和居住在沉积物中的种群的分子分型将这些细菌与 Alphaproteobacteria 的新属相关联。代表性分离物的部分组装基因组序列揭示了磁小体基因簇的非典型结构，而地球化学分析表明碳酸钙的产生是一个活跃的过程，它消耗细胞能量以维持适合其形成的环境。这一发现进一步扩大了能够进行细胞内碳酸钙生物矿化的生物的多样性。如果这种生物矿化的作用仍不清楚，细胞行为表明它可能像磁铁矿生物矿化一样参与水生栖息地的细胞运动。扫描电子显微镜 (SEM) 和居住在沉积物中的种群的分子分型将这些细菌与 Alphaproteobacteria 的新属相关联。代表性分离物的部分组装基因组序列揭示了磁小体基因簇的非典型结构，而地球化学分析表明碳酸钙的产生是一个活跃的过程，它消耗细胞能量以维持适合其形成的环境。这一发现进一步扩大了能够进行细胞内碳酸钙生物矿化的生物的多样性。如果这种生物矿化的作用仍不清楚，细胞行为表明它可能像磁铁矿生物矿化一样参与水生栖息地的细胞运动。扫描电子显微镜 (SEM) 和居住在沉积物中的种群的分子分型将这些细菌与 Alphaproteobacteria 的新属相关联。代表性分离物的部分组装基因组序列揭示了磁小体基因簇的非典型结构，而地球化学分析表明碳酸钙的产生是一个活跃的过程，它消耗细胞能量以维持适合其形成的环境。这一发现进一步扩大了能够进行细胞内碳酸钙生物矿化的生物的多样性。如果这种生物矿化的作用仍不清楚，细胞行为表明它可能像磁铁矿生物矿化一样参与水生栖息地的细胞运动。