Chemistry Reference
In-Depth Information
Figure 22
. Model of silicic acid uptake, its intracellular transport and its deposition within the silica
deposition vesicle. With the kind permission of Professor Mark Hildebrand.
of ionophores or other complexing agents including catechol (1,2-dihydroxybenzene) and
its derivatives), the packaging of the silicic acid within small membrane bounded
vesicles, or that the internal chemistry of the diatom has a composition as yet unknown
that is able to “cope with” such high concentrations of silicic acid! The experimental
methods that have been used to obtain estimates of internal silicic acid concentrations
have utilized dilution (helps to depolymerize colloidal and oligomeric species) and
prolonged heating at the boiling point of water (again, aiding depolymerization), thus when
the molybdate method for orthosilicic acid detection is used, perhaps the concentrations of
orthosilicic acid appear higher than they really are in the living organism? A different
approach to the measurement of cellular silicon concentrations has been to use isotopically
labelled silicon 29 and use
29
Si NMR spectroscopy to identify soluble silicon species inside
the diatom (Kinrade et al. 2002), Figure 13. Some published data suggest that hypervalent
silicon may be present within silicifying cells of
Navicula pelliculosa
. Complexation to
give hypervalent silicon containing species is a possible chemical strategy by which silicic
acid may be retained in solution until required for cell wall synthesis. The importance of
“silicon transport vesicles” (Schmid and Schulz 1979) in the preservation of soluble silicon
pools is somewhat doubtful as microscopic evidence suggests that there are an insufficient
number to provide the silicic acid required for cell wall synthesis (Li and Volcani 1985)
and there would still be a requirement for maintenance of readily soluble silicon, perhaps
by complexation of the silicic acid within the vesicles.
The silica deposition environment
For diatoms, the silicon deposition environment is within the silica deposition vesicle
whose structure and composition are unknown largely due to problems with its extraction
