Scytonemin-imine, a mahogany-colored UV/Vis sunscreen of cyanobacteria exposed to intense solar radiation

Abstract

We report the structure of a novel sunscreen based on the scytoneman skeleton. This pigment, scytonemin-3a-imine, was isolated from cultures of the cyanobacterium, Scytonema hoffmani, only when grown under high to intense (300–1500+ μmolquantam−2s−1) light conditions, with or without added UVR (ultraviolet radiation). It was also isolated from samples of natural cyanobacterial mats growing in shallow/short hydroperiod fresh water (Florida Everglades), soils (Loxahatchee, Florida) and saline cyanobacterial mats (Eleuthera, The Bahamas). These natural samples were all growing under intense (e.g. > 1500μmolquantam−2s−1) light conditions. Scytonemin-3a-imine may eventually become a biomarker for such biota.Characterization included UV/Vis spectrophotometry, Fourier transform infrared (FTIR) spectroscopy, matrix assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry (MS and MSn), and both 1- and 2-D 1H nuclear magnetic resonance (NMR) spectrometry. Derivatization (NaBH4 reduction, acetylation, deuterium exchange) were utilized to confirm structural features. The UV/Vis spectrum had maxima at 237, 366, 437 and 564nm. The absorption of UVR is in line with other scytoneman-based pigments. The strong visible absorption bands (437 and 564nm) indicate an alternate or additional physiological role for the pigment. We propose that it may protect both the chlorophyll reaction centers and the cytochromes from excess radiation. We speculate that excitation of the cytochromes, potentially leading to excess electron transport, or photo-oxidative destruction of the cytochromes, may enhance reactive oxygen species (ROS) generation.