A Novel Magnetotactic Alphaproteobacterium Producing Intracellular Magnetite and Calcium-Bearing Minerals

Abstract

Magnetotactic bacteria (MTB) are prokaryotes that form intracellular magnetite (Fe 3 O 4 ) or greigite (Fe 3 S 4 ) nanocrystals with tailored sizes, often in chain configurations. Such magnetic particles are each surrounded by a lipid bilayer membrane, called a magnetosome, and provide a model system for studying the formation and function of specialized internal structures in prokaryotes. Using fluorescence-coupled scanning electron microscopy, we identified a novel magnetotactic spirillum, XQGS-1, from freshwater Xingqinggong Lake, Xi’an City, Shaanxi Province, China. Phylogenetic analyses based on 16S rRNA gene sequences indicate that strain XQGS-1 represents a novel genus of the Alphaproteobacteria class in the Proteobacteria phylum. Transmission electron microscope analyses reveal that strain XQGS-1 forms on average 17 ± 3 magnetite magnetosome particles with ideal truncated octahedral morphology with average length and width of 88.3 ± 11.7 nm and 83.3 ± 11.0 nm, respectively. They are tightly organized into a single chain along the cell long axis close to the concave side of the cell. Intra-chain magnetic interactions likely result in these large equidimensional magnetite crystals behaving as magnetically stable single domain particles that enable bacterial magnetotaxis. Combined structural and chemical analyses demonstrate that XQGS-1 cells also biomineralize intracellular amorphous calcium phosphate (2-3 granules per cell; 90.5 ± 19.3 nm average size) and weakly crystalline calcium carbonate (2-3 granules per cell; 100.4 ± 21.4 nm average size) in addition to magnetite. Our results expand the taxonomic diversity of MTB and provide evidence for intracellular calcium phosphate biomineralization in MTB. IMPORTANCE Biomineralization is a widespread process in eukaryotes that form shells, teeth, or bones. It also occurs commonly in prokaryotes, resulting in more than sixty known minerals formed by different bacteria under wide-ranging conditions. Among them, magnetotactic bacteria (MTB) are remarkable because they might represent the earliest organisms that biomineralize intracellular magnetic iron minerals (i.e., magnetite (Fe 3 O 4 ) or greigite (Fe 3 S 4 )). Here we report a novel magnetotactic spirillum (XQGS-1) that is phylogenetically affiliated with the Alphaproteobacteria class. In addition to magnetite crystals, XQGS-1 cells form intracellular sub-micron calcium carbonate and calcium phosphate granules. This finding supports the view that MTB are also an important microbial group for intracellular calcium carbonate and calcium phosphate biomineralization.

Identification of sulfate‐reducing magnetotactic bacteria via a group‐specific 16S rDNA primer and correlative fluorescence and electron microscopy: Strategy for culture‐independent study

“…It is relatively straightforward to directly link each 16S rRNA genotype with an MTB phenotype through routine sample preparation procedures and use of conventional FM, SEM and TEM instruments. Together with analysing full‐length 16S rRNA gene sequences, the correlative FISH‐SEM/TEM approach is being used increasingly to identify novel MTB strains in China (Li et al, 2019; Li et al, 2020b; Li et al, 2022b; Liu et al, 2021a; Liu et al, 2021b; Liu et al, 2022; Zhang et al, 2017), France (Monteil et al, 2021; Qian et al, 2019; Qian et al, 2020) and Russia (Koziaeva et al, 2019; Koziaeva et al, 2020). Our results confirm that combining the group‐specific primer pair 390F/1492R and the correlative FISH‐SEM/TEM approach is efficient for recognizing MTB from the Desulfobacterota phylum even in low abundances.…”

Intracellular silicification by early-branching magnetotactic bacteria

“…About 5 μl of the cell suspension was used to amplify the 16 S rRNA gene of WYHC-5 cells by polymerase chain reaction with the universal bacterial primers 27F (5-′AGAGTTTGATCCTGGCTCAG-3′) and 1492R (5′-GGTTACCTTGTTACGACTT-3′) ( 24 ), as described in detail by Li et al . ( 32 ).…”

“…About 5 l of the cell suspension was used to amplify the 16S rRNA gene of WYHC-5 cells by polymerase chain reaction with the universal bacterial primers 27F (5-′AGAGTTTGATCCTGGCTCAG-3′) and 1492R (5′-GGTTACCTTGTTACGACTT-3′) (24), as described in detail by Li et al (32). The amplification products were purified using a QIAquick Gel Extraction Kit (QIAGEN, Germany), ligated with the pMD19-T vector (TaKaRa, Japan), and then cloned in Escherichia coli (strain DH5)-competent cells (Huada, Beijing, China) according to the manufacturer's instructions.…”

“…Despite both their phylogenetic and morphological diversity, MTB share the ability to produce intracellular nanocrystals of magnetite (Fe 3 O 4 ) or greigite (Fe 3 S 4 ) under strict genetic control in a subcellular compartment called the magnetosome (19). In the past decade, it has been increasingly shown that MTB also have the capability to form diverse types of intracellular inclusions such as sulfur (S 0 ) globules, Ca/Mg-rich polyphosphate granules, and calcium carbonate or phosphate particles (20)(21)(22)(23)(24). This suggests that this group of ecologically important prokaryotes may drive biogeochemical elemental cycling of C, N, P, S, and Fe within aquatic OATZ by coupling geomagnetic-guided up-down shuttling with redox-related metabolic dynamics (20)(21)(22).…”

Biogeochemical Niche of Magnetotactic Cocci Capable of Sequestering Large Polyphosphate Inclusions in the Anoxic Layer of the Lake Pavin Water Column

“…MTB forming 2-4 large granules of intracellular amorphous CaCO 3 (iACC), occupying up to 65% of their cell volume, were affiliated to two undescribed genera within the Rhodospirillaceae family of the Alphaproteobacteria (Monteil et al, 2021). Intracellular carbonatogenesis coupled with magnetosome formation in magnetotactic Alphaproteobacterium was also recently evidenced in sediment of Xingqinggong Lake (Liu et al, 2021). Before these observations, only Cyanobacteria and the non-magnetotactic Gammaproteobacterium Achromatium were known to form carbonate phases in undersaturated solutions (Head et al, 1996;Couradeau et al, 2012;Salman et al, 2015;Cam et al, 2016;Benzerara et al, 2020).…”