Nanostructure of the diatom frustule as revealed by atomic force and scanning electron microscopy

The cell wall (frustule) of the freshwater diatom Pinnularia viridis (Nitzs ch) Ehrenberg is composed of an assembly of highly silicified components an d associated organic layers. We used atomic force microscopy (AFM) to inves tigate the nanostructure and relationship between the outermost surface org anics and the siliceous frustule components of live diatoms under natural h ydrated conditions. Contact mode AFM imaging revealed that the walls were c oated in a thick mucilaginous material that was interrupted only in the vic inity of the raphe fissure. Analysis of this mucilage by force mode AFM dem onstrated it to be a nonadhesive, soft, and compressible material. Applicat ion of greater force to the sample during repeated scanning enabled the muc ilage to be swept from the hard underlying siliceous components and piled i nto columns on either side of the scan area by the scanning action of the t ip. The mucilage columns remained intact for several hours without dissolvi ng or settling back onto the cleaned valve surface, thereby revealing a coh esiveness that suggested a degree of crosslinking. The hard silicified surf aces of the diatom frustule appeared to be relatively smooth when living ce lls were imaged by AFM or when field-emission SEM was used to image chemica lly cleaned walls. AFM analysis of P. viridis frustules cleaved in cross-se ction revealed the nanostructure of the valve silica to be composed of a co nglomerate of packed silica spheres that were 44.8 +/- 0.7 nm in diameter. The silica spheres that comprised the girdle band biosilica were 40.3 +/- 0 .8 nm in diameter. Analysis of another heavily silicified diatom, Hantzschi a amphioxys (Ehrenberg) Grunow, showed that the valve biosilica was compose d of packed silica spheres that were 37.1 +/- 1.4 mn and that silica partic les from the girdle bands were 38.1 +/- 0.5 mn. These results showed little variation in the size range of the silica particles within a particular fr ustule component (valve or girdle band), but there may be differences in pa rticle size between these components within a diatom frustule and significa nt differences are found between species.