Atomic force microscopy (AFM) characterisation of the porous silica nanostructure of two centric diatoms

The porous silica nanostructure of two marine, centric diatoms, Coscinodiscus sp. and Thalassiosira eccentrica was investigated by atomic force microscopy (AFM). Important morphological features of the silica frustules of diatoms are described, including: the organisation of porous silica layers, their topography, pore size, shape and density. The outer layer of Coscinodiscus sp., commonly called the cribellum, consists of a characteristic hexagonal array of pores with pore sizes of around 45 nm. This thin membrane covers a second structural layer where two different silica surfaces are identified. The outer part, known as the cribrum consists of hexagonally packed pores of about 200 nm diameter. The inner part, known as the foramen layer, consists of larger and radially distributed holes with a diameter of around 1,150 nm. The second diatom species investigated, T. eccentrica produces a frustule with one silica structural layer featuring two different porous surfaces. The outer surface has large (800 nm diameter) holes (foramen) while the inner surface contains a porous wall with pores comparable in size to the Coscinodiscus sp. cribellum. The inner and outer surfaces of the frustule wall of both diatoms are hence in reverse order. However, the size of the small pores is similar for both species. High-resolution AFM also revealed the granular nanostructure of the diatom biosilica with grain sizes from 20 to 70 nm diameters.