Introduction
Some organisms in the marine environment produce or aquire natural sunscreens to protect tissues from UV radiation (Hylander and Jephson, 2010). These organisms range from micro-organisms to large vertebrates (Osborn et al., 2015). Microalgae seem to be the largest group producing sunscreen-like chemicals. In order for phytoplankton to survive, grow, and reproduce they must live in the euphotic zone, so some level of resistance to UV radiation is essential. The substances protecting organisms are often mycosporine-like amino acids, but also include scytomenin. By the extraction, purification, and isolation of genes, these products can be exploited by humans to develop natural sunscreens. There is a desire for these products in pharmaceutical markets as the effect of Titanium dioxide (TiO2) and Zinc oxide (ZnO), traditionally used in sunscreens, on human health are not well known (Smijs and Pavel, 2011). The effects of the different regions of the light spectrum are regarded as responsible for the damage caused to cells, and therefore why MAA’s are produced.
Solar Radiation
Solar radiation is essential for primary production. The solar spectrum ranges between 100nm to 1mm, and encompasses Ultraviolet, visible (Photosynthetically active radiation), and Infrared radiation. Photosynthetically active radiation (PAR) can penetrate water to depths of 200m, and allows photosynthesis to occur using just PAR, CO2, and Water. Species living in different temperate regions receive PAR at different intensities, and are therefore adapted to this by their differing morphologies. Ultraviolet (UV) radiation is classified into 3 ranges of wavelength. UV-A and UV-B are known to inhibit photosynthesis (Kureyshevich and Guseynova, 2005). UV-A is believed to reduce the efficacy of chemical reactions in Photosystem II (Shavit and Avron, 1963, Jones and Kok, 1966, Butler and Yamashita, 1968), whilst UV-A and UV-B cause oxidative stress (Muela et al, 2000). Although UV-A can be blocked by cloud cover, if it reaches organisms it has been shown to decrease the rate of photosynthesis by 70% (White, 2002). This reduction was observed in Dunaliella sp. and may vary within other families of marine alga. UV-C is the most damaging to organisms, but is absorbed by gasses in the stratosphere. Therefore, it has very little influence on photosynthetic organisms. Although Infrared radiation contributes almost 50% of solar radiation but can only 1% of this penetrate pure water deeper than 2m (Lembi, 2001). The rate of photosynthesis can be increased by greater temperatures, but this effect can be damaging if enzymes become denatured.
Production of “Sunscreens”
Photosynthetic organisms rely on Solar radiation to produce glucose. They must therefore, remain in the euphotic zone. By doing so, they are exposed to high levels of solar radiation as they are not shaded. DNA is susceptible to photodamage when exposed to UV radiation, as the incorporation of compounds associated with regulation of DNA and proteins (Jeffrey et al., 1996). Furthermore, increased UV-B in the euphotic zone due to stratosphere depletion (Smirnoff, 1998), reduces the rate of photosynthesis, and therefore the rate of growth. Irradiation causes damage to the enzyme, Rubisco and photosynthetic pigments (Bischof, 2000). Some species, however, may be more susceptible to damage by UV than others as shown in co-culture studies of Heterosigma akashiwo and Prorocentrum donghaiense (Xie et al., 2006). The production of “sunscreen” chemicals was first noted in macrophytic red algae (Tsujino, 1961), followed by corals and cyanobacteria (Shibata, 1969). Since this point, the discovery and identification of these products has been noted in both micro and macro algae, and in symbiotic relationships of echinoderms, crustaceans, and ascidians (Shick and Dunlap, 2002).
“Sunscreen Chemicals”
Mycosporine-like amino acids (MAA’s) are the most common sunscreen-like chemicals. They are classed as secondary metabolites as they do not directly control growth and reproduction of organisms (Klisch and Hader, 2008). They do, however, play an important role in the survival of the organisms they are produced by. Although scytonemins also provide UV protection, as of 2008 they had only been found in cyanobacteria and cyanobacterial lichens (Proteau et al., 1993). MAA’s have high UV molar absorptivity (Takano et al., 1979). Upon exposure to UV radiation, organisms have been observed to produce intracellular MAA’s at greater concentrations (Table 1). The concentration and composition of MAA’s in an organism has been shown to vary from species to species, as well as under different environmental stressors, most significantly solar radiation, in macro algal species. (Velasco-Charpentier, Pizarro-Mora and Navarro, 2016)
MAA’s typically contain cyclohexanone or clyclohexenimine ring in the center of the molecule (Figure 1). These ring structures are thought to be responsible for light absorption, and therefore, UV protection (Klisch and Hader, 2008). The energy gained from UV radiation is converted to heat energy and released to the environment (Conde, Churio, and Previtali, 2004).
Potential Uses
Through the identification of the genes causing the production of Mycosporine-like amino acids, there is the potential to create sunscreens for human without potentially harmful chemicals Titanium Dioxide and Zinc Oxide. Following the normal method of DNA extraction, genes responsible to produce MAA’s must be identified. However, the responsible genes have remained elusive, with the biological pathways are yet to be synthesised and sequenced (Spence et al., 2012, Micallef et al., 2015). Until this point, the genetic sequences cannot be inserted into the genomes of gram negative bacteria.
Conclusion
It is essential that organisms living in the euphotic zone can tolerate UV radiation. The reasons and production of natural “sunscreens” in marine primary producers is a well understood process. However, in order to exploit these products, further research must be carried out regarding the genes responsible for these products.