Tinted windows: The presence of the UV absorbing compounds called mycosporine-like amino acids embedded in the frustules of marine diatoms

Diatom frustule-bound organic compounds presumably play an important role in biomineralization and constitute an important pool of organic matter preserved in diatom frustule-rich sediments. In this study, detailed analysis of diatom frustule-bound organic matter in opal-rich Southern Ocean plankton and sediments revealed for the first time the presence of low molecular weight, UV light absorbing compounds called mycosporine-like amino acids (MAAs). Chemically cleaned diatom frustule-derived biosilica was dissolved in HF, releasing bound or entrapped organic compounds that were subsequently characterized using liquid chromatography with UV–Vis and electrospray ionization mass spectrometry (LC/PDA/ESI-MS). Palythine ([M+H]+ = 245), porphyra-334 ([M+H]+ = 347) and shinorine ([M+H]+ = 333) were the most abundant MAAs detected in HF digests of plankton and sediment. Traces of asterina ([M+H]+ = 289), palythinol ([M+H]+ = 303) and palythinic acid ([M+H]+ = 329) were also detected. MAAs in cleaned HF digested frustules were up to two orders of magnitude more abundant than methanol extractable MAAs. MAAs are substituted with acid hydrolysable amino acid residues. Our results suggest that MAAs, and not proteins, could be responsible for the high proportion of the amino acids glycine and threonine found in hydrolysates of HF digested diatom-rich environmental samples. Total MAAs accounted for 3–27% of the carbon and 2–18% of total nitrogen in the frustules undergoing various chemical cleaning treatments. This is the first report of MAAs in close association with a mineral phase and we hypothesize that the mineral matrix could stabilize these compounds, thereby enhancing photoprotection against the harmful effects of UV light. The presence of frustule-bound MAAs in sediment cores further suggests the possibility that they could be used in compound-specific isotope analysis of diatom-bound organic matter and as indicators of past solar irradiance.