Diatoms are eukaryotic, unicellular algae that produce intricately nanopatterned silica-based cell walls termed biosilica. Diatoms contain highly modified proteins termed silaffins, that are involved in biosilica formation in a poorly understood way. Silaffin sequences possess no specific domains and are not conserved between diatom species and little is known about their post-translational modifications (PTMs). Some of their lysine residues are modified by compositionally heterogeneous polyamine chains - this modification only occurs in diatoms. However, exact localization of polyamines, their molecular composition and function remain unknown. In this project we pursue two specific aims: first, we would like to characterize the biosilica-associated proteome in three closely related, but morphologically distinct diatom species (Thalassiosira pseudonana, T. oceanica, Cyclotella cryptica). Second, we develop a generic method to discover and quantify the full spectrum of polyamine modifications, including their location at the sequences of biosilica associated proteins and elucidate their role in biosilica morphogenesis.