Biology Reference
In-Depth Information
19.3 THE DIATOM CELL WALL
19.3.1 The Living Outer Frustule
The irst AFM images of the frustule were taken on the surface of living
diatoms.
13,14
To enable imaging of the motile, pennate diatoms,
Pinnularia
viridis
, cells in
artiicial media were settled onto an adhesive polymer surface (poly-L-
lysine or polyethylenimine) for immobilization and the AFM cantilever tip
brought directly into contact with living cells positioned either on their
girdle or valve face. The original intention of this approach was to probe
the outer EPS layers; however it was established that contact mode imaging
at higher forces easily removed the EPS coating to reveal the underlying
frustule structures.
13,14
After “sweeping” away the EPS, the large, lat valve
face of
,
Craspedostauros australis
and
Nitzchia navis-varingica
was amenable for observing common microstructures such
as the raphe opening and endings
(
Fig. 19.2a
)
, while the nanostructure of
other valve components exhibiting small changes in their surface height,
including foramen chambers, raised circular nodules and the surrounding
silica wall, were more clearly resolved in AFM images than in scanning
electron microscopy (SEM) images of chemically cleaned frustules.
13
Live
C. australis
P. viridis
cells positioned on their valve could not be imaged because of
their instability, though imaging of the latter girdle region to observe their
silica bands and 30
girdle region subsequently allowed the direct visualization of EPS secretion
emanating from the pores. The girdle regions of live
-
cells
in logarithmic growth phase were void of an EPS coating and were shown to
consist of numerous 50
N. navis-varingica
previous SEM reports of “silica warts” for this species. The silica particles
were only weakly connected to the frustule, as they could be removed by
nanonewton lateral forces imposed by the cantilever tip, suggesting that
particle formation occurred through the ine, nanoscale deposition, or
bottom-up assembly, of silica at the distal girdle surface. When in their
stationary growth phase,
-
produced an EPS coating on the
girdle region and silica particles, but not the valve mantle openings, which
instead had branching polymer strands adhering to the surface. Studies on
live
N. navis-varingic
revealed that the triradiate form had a clean,
smooth surface morphology, in contrast to the rougher, streaky appearance
of the ovoid form indicating the presence of EPS. Further studies on
Phaeodactylum tricornutum
P. viridis
and
aimed at preserving the EPS coatings using low amplitudes
to reduce the tapping force on the cells revealed the EPS coatings had
distinct nanostructure speciic to each species.
15
The EPS coating for
C. australis
C.
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