Biomineralisation of Magnetosomes in Bacteria

Microbial Bionanotechnology Chapter 5

2

Magnetotactic • Magnetosome

– Crystalline particles of iron oxide or sulfide– Magnetite Fe3O4– Greigite Fe3S4

• All are either obligate microaerophiles or strict anaerobes

• Motile, aquatic bacteria • Direction of motility is affected by the Earth’s

geomagnetic field • Strains are either north- or south-seeking depending

upon oxic conditions – North-seekers predominate in the northern hemisphere – South-seekers predominate the southern hemisphere– Exist in equal numbers at the equator

• Current hypothesis states that these bacteria use the geomagnetic field to locate lower O2 or anaerobic habitats

Types of magnetotaxis

Types of magneto-aerotaxis

5

There are Two Types of Magneto-aerotaxis...

Axial Magneto-aerotaxis

Polar Magneto-aerotaxis

e.g., Magnetospirillum magnetotacticum

e.g., strain MC-1, a magnetotactic coccus

From http://www.soton.ac.uk/~serg/biotech/mtb-main.htm

Magnetite CrystalsProduced by Magnetotactic

Bacteria

Pc2

Pc1

Sw1

Sw6

Sw3 Sw7

Sw2

Sw5

Sw4

magA,stp5

bfr1,bfr2,nm1,stp1

stp2dnaA

nifl1,rpl6,rpoA

ftsH,nifL2

stp3,stp4

dms,fdx napA,rrn2,

sodB

Pm3 Pm2

Pm1

Pm4

~4.3 mb

The MS-1 GenomeBertani, Weko, Phillips, Gray & Kirschvink, Physical and genetic characterization of theGenome of Magnetosprillum Magnetotacticum,Strain MS-1, Gene 264: 257-263, 2001

Global Swimming

Preferences of Magnetotactic

Bacteria

Equator

N

S

North-Seeking in theNorthern Hemisphere(Blakemore, 1975)

South-Seeking in theSouthern Hemisphere(Kirschvink, 1980)

The equator hassmall numbers of both N- and S-seekers(Frankel et al.,1981; Chang & Kirschvink, 1989)

Pelau

These bugs use thegeomagnetic field asan up/down indicatorat redox gradients.

Other bugs use tumble-run random walks.

The Kalmijn-Blakemore (‘78) Pulse-remagnetization Experiment

Start with North-seeking bacteriaIn a weak biasingField (~50 uT)

North South

SouthNorth

The remagnetized bacteria roll over and start swimming to the South …

Hit with a sharp,antiparallel pulseexceeding Hc

+ =100 mT magnetic pulse!

Unequivocal demonstration of Single-Domain Ferromagnetism!Rock Magnetism and Biology!

(Next video Hc = 1/2 Hmx)

Typical Bacterial Magnetosomes(Courtesy of H. Vali)

3D Imaging of Bacterial MagnetosomesBuseck et al., PNAS 98, 13490-13495, 2001

Higher Animals Also Have Magnetosomes

Salmon Magnetosome Chains (Mann, Sparks, Walker &

Kirschvink, 1988)

Human Brain Magnetite(Kirschvink, Kobayashi &

Woodford, 1992)

MV-1 and Magnetofossils

Biologically Important Features of Magnetosomes:

(Darwinian Selection for Magnetic Properties!!!)

• Size & Shapes with the Single-Domain field• Elongation of the ultrafine crystals• Orienting [111] axis along length• Truncation of ends.• Exclusion of Trace Metal Impurities• Perfect Crystal Lattice• Alignment in chains of similarly-sized particles

Seven Criteria for Identifying Magnetofossils*

*All 7 of these act to maximize the magnetic moment at the cellular level; however, not all magnetofossils will plot in the center

Chemically

Pure

All Inside the

SD Stability

Field

ParticleElongation

Particle

Chains

Intact Crystal

Lattice

[111] Alignment

Truncated Xtl Ends

Magnetosomes show the effect of Natural Selection for their magnetic properties. They are just the right size to be perfect biological bar magnets. Any bacterium that makes crystals outside the SD field will die!

Bacteria MS-1

Bacteria

Protists

Pigeons

Fish, Human

Width/Length

Two-Domain

Superparamagnetic

Single-DomainLe

ngth (um)

Length (A)o

0.0 0.2 0.4 0.8 1.00.610

10

10

10

1

0.1

.001

10

2

3

4

5

Interaction stabilized

Isolated crystals

Magnetosome Elongation by the Magnetotactic Bacteria

(111)

(111)

(111)

(011)

(111)

_

__

_

_

(111)

(111)

(111)

(111)

___

_

_

(111)

(111)

(111)(011)

(111)

___

_

_

(111)

(111)

(111)

(111)

___

_

_

(111)

(100)

(010)

(111)

(111)

(100)

(010)

(111)

(111)

(100)

(010) (111)

(111)

(100)

(010)(111)

Octahedral to Centrosymmetric Hexagonal Prisms (vibroid and coccoid cells)

Cubo-Octahedral to Elongated Cubo-Octahedral

BIOGENIC MAGNETITE GEOMETRIES

Hexaoctahedron

Blue = {100}Red = {110}Green = {111}

{111} {111} @ 109°{111} {100} @ 125°{111} {110} @ 145°{100} {110} @ 135°

TruncatedHexaoctahedronKT-K

Why should the bacteria truncate the ends of their magnetosomes?

Because sharp edges produce magnetic ‘Flower Structures’, which warp the internal magnetization directions. Putting those iron atomsin the next crystal increases the magnetic moment of the cells!See Fabian et al., GJI 124:89-104, 1996; Newell & Merrill JGR 105: 19377-19391)

Martian Magnetofossil(enlarged ~1,600,000 times)

Modeled after K. Thomas-Keprta et al. PNAS 98: 2165, 2001 by Koseki J. Kobayashi-Kirschvink

A PDF file for this model is available on the Magnetofossil home page At Caltechwww.gps.caltech.edu/users/jkirschvink/magnetofossil.html 111

110

100

Iron Uptake & Purification in the Magnetotactic Bacteria

Magnetosome Membranes

Cell membrane

Iron (Fe and Fe )Other Cations

SiderophoresTransferrin

SourceRock

Trans-membraneiron channel (mag-A?)

3+ 2+

Biomineralisation of Fe

Magnetotactic Bacteria

• Electron cryotomography of Magnetospirillum magneticum sp. AMB-1 reveals that magnetosomes are invaginations of the inner membrane. (A) General features of AMB-1 cells highlighted in a 12-nm-thick section of an ECT reconstruction. Outer membrane, OM; inner membrane, IM; peptidoglycan layer, PG; ribosomes, R; outer membrane bleb, B; chemoreceptor bundle, CR; poly-ß-hydroxybutyrate granule, PHB; gold fiduciary marker, G; magnetosome chain, MG. Scale bar, 500 nm. (B to E) Representative magnetosomes containing no magnetite (B), small (C), mediumsized (D), and fully-grown (E) crystals are invaginations of the inner membrane. Scale bar, 50 nm.

• Magnetosome chains are flanked by long cytoskeletal filaments. (A) Larger view of the magnetosome chain in Fig. 1A. (B) Similar view of a magnetosome chain grown in the absence of iron, which prevents the formation of magnetite crystals. Arrows point to the long filaments. (C) Three-dimensional organization of magnetosomes (yellow) and their associated filaments (green) shown in (B) with respect to the whole cell (blue). Scale bars, 100 nm.

Bakterielle Magnetosomer

TEM image of two Itaipu-1 cocci. Each bacterium has two chains of magnetosomes (arrows) and two phosphorus-rich globules (P). Scale bar, 1 μm.

High-resolution TEM images of Itaipu-1 magnetosomes with indexed bars parallel to lattice planes. Obvious symmetries between even the very small facets on opposite sides of the crystal diagonals can be seen. Comparison with other crystals of the chain in Fig. 2 also indicates that this symmetry regularly alternates between crystals.

(A) Electrostatic contribution to the holographic phase shift from the Itaipu magnetosomes shown in Fig. 2, oriented to a [110] projection. The contours represent the projected thickness and show a flat-topped morphology and steep sides. (B) Projected thickness contours for the same crystals after tilting by 30° about the chain axis to a [211] orientation. The contours show that the crystal is much thicker along its center than along its edges, having a central ridge formed by intersecting faces. (C) Line profiles (solid line for panel A and dashed line for panel B) across the magnetosomes from the indicated positions (arrows), converted to values of one-half their thickness, reveal a 120° angle between the facets for the [211] projection, which is consistent with the intersection of [110] faces. Scale bar, 150 nm (panels A and B).

• TEM images of Itaipu-1 and Itaipu-3 magnetosomes. (A) Chain of large magnetosomes from magnetotactic bacterial strain Itaipu-1 surrounded by smaller, elongated magnetosomes from strain Itaipu-3. The inset is a [211] diffraction pattern from the second large Itaipu-1 crystal (arrow). (B) Same chain as in panel A tilted 30o about the [111] axis. The inset [110] diffraction pattern from the second large Itaipu-1 crystal shows (111) fringes from the magnetically easy axis. Corner faces {111} and {200} are mirrored about the vertical (or horizontal) axis for alternating crystals (double arrows); see detailed image in Fig. 3. Scale bar, 200 nm.

Tomographic reconstruction of a magnetite nanocrystal from an undescribed coccus collected from Sweet Springs Nature Reserve, Morro Bay, CA, reconstructed from a tilt series of STEM HAADF images obtained at 300 kV on a Philips CM300 FEG TEM over a range of ± 56°. The tableau shows the three-dimensional morphology of the crystal viewed from a range of directions.

Magnetosome Element Analysis

Cu-Fe Analysis

Magnetosomes

Magnetosome crystal morphology

• MamK, a homolog of the bacterial actin-like protein MreB, forms filaments in vivo. (A) Phylogenetic relationship between MamK and other bacterial actin-like proteins demonstrated by an unrooted tree. These proteins separate into three distinct groups: MamK (green), ParM/StbA (red) and MreB (blue). (B) MamK fused to GFP (green) appears to form filaments in vivo localized to the inner curvature of the cell (cell membrane stained red with FM4-64).

• MamK is required for the proper organization of the magnetosome chain. (A) Three-dimensional reconstruction of a wild-type AMB-1 cell. The cell membrane (gray), magnetosome membrane (yellow), magnetite (orange), and magnetosome-associated filaments (green) are rendered. (B) mamK mutant, where magnetosomes appear disordered and no filaments are found in their vicinity. (C) mamK cell expressing mamK-GFP on a plasmid showing full reversal of the mutant phenotype.

Magnetosome membrane proteins

Comparative genome analysis

Magnetotaxis genes

Mam gene regulation in Cells

Magnetosome membrane

Magnetosome membrane protein

Biotechnological applications

• Delivery systems• Separation systems• DNA arrays• RNA arrays• Thermo treatment• Sensor systems

Applications of Magnetosome particles