Biophysical Characterization of Scytonemin: A Sunscreening Pigment in Cyanobacterial Biofilms

Abstract

Scytonemin is a natural photoprotectant against high intensity ultraviolet radiation (UVR) which has been reported in more than 300 cyanobacteria. Cyanobacteria are Gram-negative prokaryotic microorganisms that constitute the greatest biodiversity in terrestrial, freshwater and marine environments. Cyanobacteria are exposed to potentially lethal levels of solar ultraviolet radiation and accumulate photoprotective compound (PPC) scytonemin in their extracellular polysaccharide sheath to counteract the damaging effects. UV-B radiation has deleterious effects on physiological and biochemical processes of living organisms. In this study, cyanobacterial epilithic biofilms collected from diverse habitat were screened to detect the presence of scytonemin. Scytonemin was partially separated and characterized by using high performance liquid chromatography [Retention time: 4.8 min]. Thereafter, scytonemin was further characterized by using various spectroscopic techniques such as UV-Vis [UVλ_max: 386 nm]; electrospray ionization-mass spectrometry [ESI-MS) ([M+H] ⁺ m/z: 545.2]; and Fourier Transform Infrared (FTIR) [3431 cm⁻¹, 2940 cm^-1, 2920 cm^-1 1585, 1385 1173]. The photoprotective capability of scytonemin against UV-B radiation was assessed on survivability of Escherichia coli. Our results suggest that scytonemin plays a crucial role in the better survival of cyanobacteria in stressed habitats facing harsh environmental conditions. Scytonemin is an eco-friendly natural sunscreen with potential for dermatological or biomedical applications and a safe alternative to synthetic UV sunscreens.