Figure 1.
Transmission electron micrographs of three different morphotypes of magnetotactic bacteria extracted from lake sediments.
(A) Candidatus Magnetobacterium bavaricum (MBav), (B) magnetotactic vibrio, and (C) magnetotactic coccus. (scale-bars are 1 µm).
Figure 2.
Raman-based compositional mapping of bacteria samples.
(532 nm, laser power 0.17 mW) A) Scanning-electron micrograph (SEM) showing 6–10 µm long rod-shaped cells of MBav and less elongated (1–3 µm) vibrios. B) Confocal reflectance, simulated from scattering intensity at Rayleigh peak. C) map of magnetite (303, 535, 665 ), which forms linear structures (magnetosome chains). D) map of orthophosphate (1080
, P-O stretching mode), E) map of
(filter: 151, 219. 467
), which occurs in the form of intracellular globules, F) map of 800 to 950
band, see also Figure 3Bi and 3E upper spectrum. G) map of 747
(cytochrome), closely associated with the plasma membrane, H) composite map of B, C, D, E, G. I) SEM of another sample, with corresponding composite Raman map shown in K (532 nm laser, 0.25 mW, same filters and coloring scheme as in H).
Figure 3.
Typical Raman spectra of magnetic bacteria.
A-B) recorded at a single-pixel (400 nm size), as marked in Figure 2 (white open boxes). A) vibrio: (i) intracellular phosphorous reservoir (white box in Figure 2D) with a sharp line at 1080
, characteristic of orthophosphate. (ii) Magnetosome chain (box
in Figure 2C), with the three characteristic lines of magnetite at 303, 535, and 665
(white bars). B) MBav (i) intracellular sulfur globules (box
in Figure 2E), dominated by
rings (151, 219, 467
); arrow shows broad band (
800
) assigned to two-phonon peak of
. The line at 1440
is typical of fatty acids. (ii) Magnetite (white bars) in cells containing also sulfur globules (box
in Figure 2E). (iii) Magnetite in cells without sulfur globules (box
in Figure 2C). C) EDX-spectrum of magnetic vibrio, dominated by phosphorous and oxygen. The Si-line is due to the microscope slide, and the Au-line due to the sputtered gold at the surface. D) EDX-spectrum of MBav. Sulfur is clearly present, while oxygen is less abundant than in C. E, F) Average spectra obtained by averaging over those regions that exclusively produce Raman lines of sulfur in MBav (E, upper graph), of orthophosphate in vibrio (E, lower graph), and of magnetite in MBav (F, upper graph) and in vibrio (F, lower graph).
Figure 4.
Raman spectrum of a single crystal of greigite (10 µm grain size).
The band at 351 has a satellite at 327
and a shoulder at 367
. Another line is located at 253
. Note the absence of lines at wavenumbers> 500
, particularly in the 660–670
range, where magnetite and hematite are active. The 190
line is just not detectable. The spectrum is consistent with observations from literature (c.f. Table 2 and [96]).
Figure 5.
Single Raman spectra of a magnetotactic coccus, taken at two different spots with otherwise identical settings.
A) Well expressed magnetite lines at 190, 303, 535, 665 (white bars). B) lines of heme group (most likely of reduced cytochrome c) at 747, 1126, 1314, 1586
. The line 1177
is present in A) and B) similarly strong, and therefore is not assigned to the heme group, but to polyphosphate (Table 2).
Table 1.
Characteristic Raman lines of iron-oxides and iron-sulfides, compiled from literature.
Table 2.
Raman lines identified in our bacteria samples, and in synthetic reference samples.
Table 3.
Candidate phosphorous compounds in bacterial inclusions (according to DoCampo [53]) and characteristic Raman lines for inorganic equivalents compiled from literature.
Table 4.
Characteristic Raman lines of sulfur allotropes, compiled from literature.
Table 5.
Characteristic Raman lines of relevant organic compounds or functional groups (compiled from literature).
Figure 6.
Raman spectrum of a sulfur inclusion in MBav, recorded at three different excitation wavelengths.
The characteristic lines of (indicated by the white bars) appear consistently.