Organization and Elemental Analysis of P-, S-, and Fe-rich Inclusions in a Population of Freshwater Magnetococci

Abstract

We characterized a population of bilophotrichously flagellated freshwater magnetotactic cocci (MC), referred to as ARB-1, morphologically, chemically, and phylogenetically. Cells examined using light microscopy, fluorescence microscopy, environmental scanning electron microscopy (ESEM), and transmission electron microscopy (TEM) contained three types of intracellular inclusions placed in a specific arrangement within the cell. Elemental compositions of the inclusions were determined using energy dispersive X-ray spectroscopy (EDS) from both ESEM and TEM. The spherical to ovoid cells contained two large phosphorus-rich inclusions that occupied most of the cell volume and appeared to be enclosed in a membrane or coating. Several smaller sulfur-rich inclusions were located at the end of the cell opposite the flagellar bundles. The magnetosomes, arranged either as a cluster, a chain, or a combination of both, were located proximal to the two flagellar bundles. Magnetite was identified as the mineral phase of the magnetosomes using selected area electron diffraction (SAED) and by measuring lattice fringe spacings of the crystals. The magnetite crystals were hexagonal prisms that averaged 82 nm in length and thus fit into the single-magnetic-domain size range. Phylogenetic analysis of the 16S rRNA gene sequences suggests that it is a mixed population of MC that form a monophyletic clade distinct from but similar to other uncultured MC.