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L. Corbari et al.: Bacteriogenic iron oxides 1299
Corbari et al., Figure 2
a
b
2 µm
c
d
5 µm
e
f
5 µm
Fig. 2. The three levels of the mineral crust. Comparison between the electron back scattering images of the polished-thin sections (vertical
sections) revealing the mineral densities and structures (left column) and the TEM micrographs of horizontal cross-sections exhibiting
mineral associated with bacterial community (right column) at the lower level (a and b), the median level (c and d) and the upper level (e
and f).
The median level of the crust exhibits larger mineral concretions,
globular in shape, (Fig. 2c and d). These globular
concretions often meet to form larger ones that exhibit
a botryoidal structure. In the horizontal cross-sections, the
aggregate density appears rather heterogeneous and consists
of highly mineralised patches interspersed with bacteria rich
areas. The bacterial density in the median layer is always
smaller than in the lower layer; there are fewer rod-shaped
bacteria. Moreover, ghosts of bacteria are also observed
(Fig. 3e and f) in TEM images. These ghosts have bacterial
shapes that are completely enclosed in a heavy mineral
sheath. Sometimes the bacteria are still present but appear
either to be damaged or as membrane remain (Fig. 3e and f).
In other cases, the mineral sheath appears to be empty or to
have been recolonised by other rod-shaped bacteria. These
observations suggest that mineral formation may influence
the survival rate of the bacteria.
www.biogeosciences.net/5/1295/2008/ Biogeosciences, 5, 1295–1310, 2008