THE ROLE OF ATOMIC FORCE MICROSCOPY IN ADVANCING DIATOM RESEARCH INTO THE NANOTECHNOLOGY ERA - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 406

19.4 DIATOM EPS AND ADHESIVES

SEM approaches have dominated studies on diatom EPS over the last four
decades, and while it has been possible to observe large, conspicuous EPS
macrostructures (e.g. EPS stalks and pads), the ixation, drying and vacuum
operating environment has severely limited the ability to characterize the
vast majority of EPS with conidence that dehydration effects have not
altered the material. With the ability of AFM to probe soft biological systems
in luids, it has been possible to observe the true hydrated morphology of
EPS coatings, 15 adhesives left behind on the substrate (i.e. diatom trails
and bioilms) by motile diatoms 20 as well as adhesives from other algae
species. 21 EPS adhesives that are too sticky, or in the form of an unsupported
3-D structure (e.g. adhesives strand protruding vertically from the cell) are
dificult to image. Applying AFM force measurements in these cases has been
especially important for elucidating the elastic and adhesive properties of
the EPS, in particular at the single cell and molecular level. Localized regions
on the cell surface can be targeted because of the nanometre size (10 20

nm) and lateral positioning of the tip over the desired EPS region. Most of the
work done so far on measuring forces with the AFM has distinguished non-
adhesive and adhesive EPS components and discovered adhesive properties
and designs that give explanation as to why diatoms have the great tenacity
to attach to surfaces.

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19.4.1 Non-Adhesive Components: Cell Coangs and Outer
Frustule Surface

AFM force measurements on the EPS coating have shown a non-linear
increase in the force acting on the cantilever tip as it is indented into the
surface (extending curve), followed by a relaxation in the force (retracting
curve), which is not fully recovered, as the tip is retracted away ( Fig.
19.4a ) . This force proile indicates the properties of a viscoelastic polymer,
which is compressible but does not fully recover its form on the timescale
of the measurement. By analysing the approaching part of the force

measurement (i.e. as the tip is pushed into the surface) with mechanical
models such as Hertz theory, Young's modulus values ranging from 250
to 750 KPa for the EPS coating and outer living cell surface have been
obtained.

13,14

Such measurements have highlighted the diverse polysaccharide
and glycoprotein composition and structure of EPS coatings, as inferred by
signiicant variations in the Young's modulus between species.

15

Similar
measurements have also been used to distinguish the extent of silica
composition in ovoid and triradiate forms of

15

Stiffer ovoid
forms (500 KPa) conirmed a higher silica content compared with fusiform

P. tricornutum. 22