Chemistry Reference
In-Depth Information
Raphe
Raphe
Girdle
bands
Girdle
bands
20
20
µ
µ
m
m
Figure 19
. Field emission scanning electron microscope image of the diatom
Surirella sp
. to
show the two valves, the raphes and girdle bands. Transmission electron microscope image of the
diatom
Navicula pelliculosa
showing examples of the areolae. With the kind permission of
Professor Rick Wetherbee.
mucilage is extruded to assist in movement, Figure 19. The silica from which these
structures are built up is protected from the external environment by an organic matrix.
Although the silica is defined as being “amorphous” at the angstrom level, microfibrillar
and hexagonal columnar arrangements have been observed during the development of
certain diatoms (Li and Volcani 1985). At maturity certain structures are built up from ~5
nm particles (Perry 1989) and small aggregates around 40 nm (Crawford et al. 2001),
Figure 20. Structural studies that have used selective dissolution with sodium hydroxide at
high pH have shown that the silica used to build up the diatom frustules varies within the
frustule and also between species. The silica in diatoms has been proposed to exhibit proton
buffering activity in the oceans, thus enabling the efficient conversion of inorganic
bicarbonate to carbon dioxide (Milligan and Morel 2002).
The diatom reproductive cycle, Figure 21 results in the formation of two daughter
cells, each retaining half of the original frustule with a new valve being formed within the
original cell structure prior to separation of the daughter cells and completion of the
girdle bands. At this point it is important to note that the first stage in the process of
A
A
B
B
150 nm
150 nm
500 nm
500 nm
Figure 20
. Scanning electron microscope image of ca. 40 nm particles visible within the diatom
Pinnularia sp
. cell wall after treatment with mild alkali. With the kind permission of Professor Rick
Wetherbee.
