Diatoms can represent the major component of phytoplankton and contribute massively to global primary production in the oceans. Over tens of millions of years they developed an intricate porous silica shell, the frustule, which ensures mechanical protection, sorting of nutrients from harmful agents, and optimization of light harvesting. Several groups of microalgae evolved different strategies of protection towards ultraviolet radiation (UVR), which is harmful for all living organisms mainly through the formation of dimeric photoproducts between adjacent pyrimidines in DNA. Even in presence of low concentrations of UV-absorbing compounds, several diatoms exhibit significant UVR tolerance. We here investigated the mechanisms involved in UVR screening by diatom silica investments focusing on single frustules of a planktonic centric diatom, Coscinodiscus wailesii, analyzing absorption by the silica matrix …