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Basking in the sun on an Australian beach, I began to feel a burning sensation from excessive UV irradiation and decided not to wait for the ozone layer to return. As first aid I scraped some algae and cyanobacteria from the blistering rocks and smeared them on my exposed skin. Why? Because these organisms contain sunscreen compounds to protect themselves from harmful doses of UV‐B (280–315 nm) and UV‐A (315–400 nm) radiation. Cyanobacteria are prominent in many superficial habitats exposed to high solar irradiance, including deserts, polar regions and intertidal marine flats. On rocky marine substrates, many cyanobacteria form crusts or small cushions in the high intertidal or supratidal zone. In response to intense solar radiation, cyanobacteria and some other microorganisms have evolved a variety of defence mechanisms including the biosynthesis of UV‐absorbing/screening compounds such as mycosporine‐like amino acids (MAAs) and scytonemin. So far, scytonemin has been found to be produced mainly by cyanobacteria (Fig. 1), while mycosporine and MAAs are widespread and are accumulated by a range of microorganisms, prokaryotic (cyanobacteria) as well as eukaryotic (microalgae, yeasts and fungi), and a variety of marine macroalgae, corals and other marine life forms. Excellent reviews on this topic can be found in Klisch and Hader (2008), Sinha and Hader (2008), Rastogi and Sinha (2009), Rastogi et al. (2010) and Singh et al. (2010a).
The very recent elucidation of biosynthetic pathways and identification of associated genes now allows data mining of microbial genomes to assess their potential for producing sunscreen compounds. A brief overview is given here to wet your appetite.
Mycosporines and MAAs
Mycosporines and MAAs are colourless compounds found intracellularly in many marine and freshwater organisms (Sinha et al., 2007; Klisch and Hader, 2008; Llewellyn and Airs, 2010). These natural products are characterized by a cyclohexenone or cyclohexenimine chromophore core conjugated with amino acids or imino alcohol substituents (Fig. 2). These are attached to the core through imine linkages, leading to a combination of resonance tautomers which facilitates absorption of UV light. Differences in the absorption spectra of MAAs, with maxima ranging from 310 to 360 nm (Fig. 3C), are due to variations in the attached side groups and nitrogen substituent. Figure 2A shows chemical structures...