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With respect to the outer frustule surface, a common inding to all these
studies on living diatoms was that a purported tightly bound organic sheath
covering the silica wall and situated beneath the EPS coating was not evident,
suggesting a lack of an additional protective layer, or residual organic
component involved in valve formation. If such a layer were to exist, it would
have to be of a molecular layer thickness covering the silica topography
for it to go undetected by AFM, which has the capability of resolving sub-
nanometre changes in height. It is more likely that the organic sheath
visualized in previous SEM studies
16
results from residual EPS coating after
cell preparation (e.g. chemical ixation and drying). Thus, a clear advantage
of AFM studies is that observations on the structure and properties of the
outer frustule surface can be made under natural physiological conditions.
The integrity of the whole frustule structure is retained, rather than its
disassembly into separate components as is sometimes the case when
frustules undergo chemical treatment and drying. This allows nanoscale
silica structures and frustule components to be observed in relation to one
another and without potential modiication from any prior harsh chemical
treatments, mechanical perturbations or disassembly. The approach will
be of particular use for species such as
whose delicate frustules
collapse and deform under hydrostatic pressure in ambient air conditions.
A clearer representation of the outer living frustule emerging from
AFM imaging of live diatoms is one of a smooth or particulate silica wall,
comprising various nano- and micromorphologies, generally encased within
a structured, visoelastic polymer layer, expect at major openings in the
cell wall. Although parts of this description have always been the “status
quo”, this area of diatom research has possibly redeined our thinking of
the frustule as not just silica with traces of organics but a complete silica-
polymer composite layered
C. australis
structure.
19.3.2 Nanoscale Silica Structures
Preparing acid cleaned frustules provides another method for imaging of
diatom silica structures with AFM. Although the cells are not alive, the EPS
is removed to better expose the silica and provide greater access to different
areas of the frustule, including both distal and proximal surfaces. Early
studies on air-dried
showed the capability of imaging
the whole ellipsoidal frustule structure, including the distal raphe and
pores.
Navicula pelliculosa
frustules, it was found
that the outer surface of the siliceous valve when imaged by SEM or AFM
in contact mode was identical to that of the living cells, whose EPS coating
had been removed by scanning,
17
For chemically cleaned and dried
P. viridis
13,14
as described earlier. This provided a
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