Magnetic Colloids from Magnetotactic Bacteria:  Chain Formation and Colloidal Stability

Abstract

Single-domain magnetite (Fe3O4) crystals, harvested from magnetotactic bacteria, display on transmission electron micrographs the cluster morphologies (folded chains, flux-closure rings) predicted for magnetic colloids with dominant dipolar attractions. These strong attractions are responsible for the linear magnetite chains inside bacteria but do not affect the colloidal stability of the bacteria, as confirmed by analytic sedimentation experiments. Calculations of the interaction energy between dipole chains show that the magnetic component of the interbacteria interaction is negligible due to screening of dipolar forces: the bacteria sense only the geomagnetic field but not each other's compass.

Experimental observation of magnetosome chain collapse in magnetotactic bacteria: Sedimentological, paleomagnetic, and evolutionary implications

“…Despite the evidence for linear chains in vivo, chains do collapse during typical preparation for TEM; this has been independently predicted and observed by Philipse & Maas [20]. In particular, even apparently linear images of such chains, when viewed via a single 2D electron shadow image in a TEM, cannot detect oscillations in the vertical plane.…”

“…3 Without some form of rigid support, linear chains of single-domain magnetite crystals will collapse spontaneously because they are physically unstable. For elongate crystals, magnetostatic energy calculations [10,20] show that the collapse of chains into clumps of crystals is favored strongly, as, by reducing the center-to-center spacing of the dipoles, it can reduce the magnetostatic potential energy of the cell by ~ 10 4 kT (where kT, the product of the Boltzmann constant and the absolute temperature, is the one-dimensional thermal background energy). Paired anti-parallel magnetosome crystals have been seen in TEM images of cells with elongate crystal morphologies (e.g., [19]).…”

The Acidic Repetitive Domain of the Magnetospirillum gryphiswaldense MamJ Protein Displays Hypervariability but Is Not Required for Magnetosome Chain Assembly

“…However, the assembly and stabilization of magnetosome chains have remained a puzzle, because magnetic dipoles tend to arrange into more energetically favored assemblages such as rings and aggregates, unless stabilized by a biological structure (11,12,21). Recently, several studies addressed the mechanism of magnetosome chain assembly at the molecular level.…”

Magnetic tests for magnetosome chains in Martian meteorite ALH84001

“…This is because a chain of equant magnetite crystals has higher magnetostatic energy than a ring of the crystals and is therefore not commonly observed for abiogenic magnetite in nature. Magnetosome chains are thought to be stabilized in the bacteria by a rigid biomechanical structure, because when removed from the cell they often collapse into the lower-energy ring or clumped configuration (10)(11)(12). However, the proposed chains in the images of Friedmann et al (5) do not appear to be isolated from surrounding magnetite crystals, which calls into question the appropriateness of their being labeled ''chains'' at all rather than members of a three-dimensional clumped assemblage of crystals.…”