Abstract
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, diffraction by frustule ultrastructure and also UV conversion into photosynthetically active radiation exerted by nanostructured silica photoluminescence. We identified the defects and organic residuals incorporated in frustule silica matrix which mainly contribute to absorption; simulated and measured the spatial distribution of UVR transmitted by a single valve, finding that it is confined far away from the diatom valve itself; furthermore, we showed how UV-to-blue radiation conversion (which is particularly significant for photosynthetic productivity) is more efficient than other emission transitions in the visible spectral range.
Highlights
Detrimental effects of ultraviolet radiation (UVR) exposure on living organisms, especially UVC (100–280 nm) and UVB (280–315 nm), are well known
Radiation characterized by wavelengths
Light microscope observations of monoalgal culture of the strain CCAP 1013/9 of the centric diatom species C. wailesii allowed for observation of cells that are circular in valve face (Fig. 1a) and barrel shaped in a side view, with a pervalvar axis smaller than a mean diameter of about 150 μm, containing many small discoid chloroplasts
Summary
Detrimental effects of ultraviolet radiation (UVR) exposure on living organisms, especially UVC (100–280 nm) and UVB (280–315 nm), are well known. Several molecular mechanisms allow protection of phytoplankton cells from UVR They comprise photoenzymatic repair (PER) photoreactivation, which consists of direct monomerization of cyclobutane dimers by photolyase in the presence of visible and UVA light[2,5]; synthesis of photoprotective pigments such as carotenoids (e.g. diadinoxanthin and beta-carotene)[3], accumulated in the cytoplasm and acting as passive www.nature.com/scientificreports/. MAAs accumulate in response to high light exposure[3,4,6] and osmotic stress Despite these photoprotection mechanisms, a great variability in UV susceptibility and consequent induced damages has been observed across microalgal genera and even species[2,3,9], probably due to different cell morphologies, placement of organelles (possible chloroplast shielding or nuclear hiding), different concentrations of UV-absorbing pigments, and DNA content (e.g. genomes with high thymine content will lead to a considerable proportion of lesions due to thymine-thymine cyclobutane dimers)[2]. In some cases the concentration of UV-absorbing compounds in diatom populations has been estimated to be 2 to 5 orders of magnitude lower per cell unit than in Phaeocystis[10], a major component of Antartic phytoplankton communities
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