Networks of Chitin Filaments are Integral Components of Diatom Silica Shells

A whole microcosm of various bizarrely shaped life forms opens up when you look at diatoms, the primary component of ocean plankton, under a microscope. The regularly structured silica shells of these tiny individual life forms have attracted scientists because they are particularly interesting examples of natural hybrid materials and also demonstrate unusual mechanistic and optical properties. The mechanisms of the underlying biomineralization process are not yet fully understood, but the silica shells often provide inspiration for the synthesis of man-made nanostructures. Researchers at TU Dresden and the Max Planck Institute the Chemical Physics of Solids in Dresden have now identified another component of the diatom cell walls. As the team led by Eike Brunner reports in the journal Angewandte Chemie, they found an organic network of crosslinked chitin filaments.

Chitin is a long molecular chain of sugar building blocks, a polysaccharide. It is the second most widespread polysaccharide on Earth after cellulose. In combination with calcium carbonate (lime) and proteins, it forms the shells of insects and crabs. "Chitin plays an important role in the biomineralization of such calcium carbonate based shells and structures," explains Brunner. "We have now been the first to demonstrate that the silica cell walls of the diatom Thalassiosira pseudonana also contain a chitin-based network."

The researchers dissolved the silica components of diatom shells with a fluoride-containing solution. What remained behind appears under a scanning electron microscope as a delicate, net-like scaffolding. This network resembles the cell wall in form and size and consists of crosslinked fibers with an average diameter of about 25 nm. Spectroscopic examinations show that the fibers contain chitin and other, previously unknown biomolecules.

"Our results suggest that the chitin-based network structure serves as a supporting scaffold for silica deposition, while the other biomolecules actively influence it," states Brunner. "This mechanism is thus analogous to calcium carbonate biomineralization. In addition, these networks may also mechanically stabilize the cell walls."