Philippine Journal of Science 139 (1): 5-16, June 2010 ISSN 0031 - 7683 Aquatic Microalgae As Potential Sources Of Uv-Screening Compounds Maribel L. Dionisio-Sese Institute of Biological Sciences, College of Arts and Sciences, University of the Philippines Los Baños, College, Laguna 4031, Philippines Microalgae are a polyphyletic and biochemically diverse assemblage of chlorophyll a-containing microorganisms capable of oxygenic photosynthesis that are predominantly found in aquatic environments with observed high levels of ultraviolet (UV) radiation. Certain microalgae produce organic metabolites, such as sporopollenin, scytonemin and mycosporine-like amino acids, to protect themselves from UV radiation while allowing visible radiation involved in photosynthesis to pass through. Sporopollenin, an acetolysis-resistant inert biopolymer usually observed in plant pollens and spores, was detected in the cell wall of some UV-tolerant chlorophytes. Scytonemin, a yellow-brown lipid-soluble dimeric pigment, was found in the extracellular polysaccharide sheath of some cyanobacteria. Mycosporine- like amino acids, which belong to a family of water-soluble compounds, were reported in several free-living cyanobacteria, chlorophytes, haptophytes, diatoms, and dinoflagellates, as well as in several marine invertebrate-microalgal symbiotic associations. Their capacity to intercept UV radiation and dissipate its energy as heat without the formation of radical intermediates makes these microalgal compounds potential sources of protection from UV and photo-oxidative stress. Key Words: microalgae, mycosporine-like amino acids, scytonemin, sporopollenin, UV-absorbing/ screening compounds, UV photoprotection INTRODUCTION the high UV transparency of clear tropical ocean water. In high latitudes, enhanced UV radiation due to stratospheric Microalgae refers to a polyphyletic and biochemically ozone depletion is also a major stress factor for many diverse assemblage of chlorophyll-a containing organisms. Ozone depletion has been observed in both microorganisms whose most distinct common the Antarctic and Arctic regions, where ozone levels have physiological attribute is their capability to undertake been reported to decline by as much as 50% during late oxygenic photosynthesis. They also share the characteristic winter and early spring in the polar vortex (Smith et al. ability to thrive in aquatic environments wherein they are 1992; Von der Gathen et al. 1995). regarded as the most important biomass producers on a global scale. UV radiation is customarily divided into three spectral regions: UV-C (200-280 nm), UV-B (280-320 nm), and High levels of visible and UV radiation in the tropical UV-A (320-400 nm). UV-C, the most damaging spectral region, particularly in the low-latitude tropics, occur due to region, is eliminated from the earth’s atmosphere through the shorter light path for sunlight in the region, the thinness absorption by ozone and other atmospheric gases. The of the earth’s protective ozone layer near the equator, and decreasing ozone concentration in the stratosphere, due *Corresponding author: [email protected] to the increasing concentrations of chlorofluocarbons, 5 Philippine Journal of Science Dionisio-Sese ML: UV-Screemomg Compounds in Microalgae Vol. 139 No. 1, June 2010 nitrogen dioxide and other pollutants in the atmosphere, Sporopollenin has led to increased penetration of UV-B, but not of UV- Sporopollenin is an acetolysis-resistant inert biopolymer A, on the surface of the earth (Kerr and McElroy 1993; possessing a complicated structure with aliphatic (mainly Madronich et al. 1998). Thus UV-B, which is deemed isoprenoid) and aromatic components variably present. deleterious, can penetrate biologically significant depths It is found in the cell wall of some algae (Atkinson et (up to several meters) into the water column (Karentz al. 1972; Pickett-Heaps and Staehelin 1975) and in plant and Lutze 1990; Kuwahara et al. 2000), thereby affecting pollens and spores (Shaw and Yeadon 1964; Osthoff aquatic ecosystems. and Wiermann 1987; Guilford et al. 1988). Xiong et al. (1997) observed that UV-B tolerant chlorophyte species The detrimental effects of UV-B on primary producers are of Characium terrestre, Coelastrum microporum, Enallax varied, mediated primarily by damaging molecular targets coelastroides, Scenedesmus sp., Scotiella chlorelloidea, such as nucleic acids, proteins and pigments, and indirectly, Scotiellopsis rubescens, and Spongiochloris spongiosa by producing reactive oxygen species. These can lead to contain large amounts of sporopollenin. The biopolymer the inhibition of cell division and growth, affecting various was reported to be present also in Dunaliella salina physiological and biochemical processes, such as motility, zygotes (Komaristaya and Gorbulin 2006). orientation in motile organisms and photosynthesis, which can consequently alter community structure and function. Rozema et al. (2001) has demonstrated the UV-B absorbing property of extracts containing sporopollenin. UV-A can have a net damaging influence on photosynthesis The UV-B screening role of sporopollenin is evidenced (Cullen et al. 1992). However, it is generally considered that by the high UV-B optical density of the sporopollenins a more positive effect is exerted on physiology by inducing isolated from UV-B tolerant chlorophytes and its photoreactivation or repair activities. increase upon UV-B exposure. In Xiong et al. (1997), Sinha et al. (1998) identified the following four adaptation the sporopollenin content (mean ±SE) increased from strategies through which some organisms attempt to cope 0.73±0.12% to 0.85±0.14% (w/w) in Scenedesmus sp. and with UV radiation: from 0.58 ±0.10% to 0.73±0.09 % in Enallax coelastroides upon UV-B exposure. Aside from its UV-B screening (1) Availability of a number of DNA repair mechanisms, role, the presence of sporopollenin in the cell wall of such as photoreactivation and light-independent Pediastrum duplex has been associated with the resistance nucleotide excision repair of DNA, and UV-A/blue of this chlorophyte to microbial decomposition in natural light-guided repair of the photosynthetic apparatus; waters (Gunnison and Alexander 1975). The presence of (2) Production of enzyme systems (e.g., superoxide sporopollenin in the cell wall of Chlorella fusca also has dismutase which reacts with and neutralizes the been associated to its resistance to extreme extraction highly toxic reactive oxygen species produced procedure (Atkinson et al. 1972). by UV radiation) and induced formation of Since sporopollenin is a non-toxic, safe natural material quenching agents (e.g., carotenoids, especially that has the ability to absorb UV-B radiation and to bind those in the xanthophyll cycle, which, in addition heavy metal (Arslan et al. 2004), its use in the cosmetic to removing reactive oxygen species, can dissipate treatment of skin against age-related/sun-related wrinkle excess excitation energy as heat or deactivate formation and as a chelating agent of ion-exchangers excited chlorophyll, reducing photoinhibition of in wastewater purification has been patented in the photosynthesis); United States in 2007 (http://www.patentgenius.com/ (3) Behavioral modification to avoid exposure by patent/7182965.html). migrating to habitats with reduced UV exposure; and (4) Production of UV-absorbing substances that screen Scytonemin their cells from harmful UV radiation. Scytonemin is a yellow-brown lipid-soluble dimeric In terrestrial environments, where higher plants are the pigment with a molecular mass of 544 kDa and a foremost primary producers, several studies have shown structure based on indolic and phenolic subunits (Proteau that harmful UV radiation in higher plants is absorbed by et al. 1993). It has an in vivo absorption maximum of epidermally located phenylpropanoids, mainly flavonoid 370 nm (in vitro at 386 nm) but purified scytonemin also derivatives (Kootstra 1994). In aquatic environments, significantly absorbs at 252, 278, and 300 nm (Proteau where microalgae figure prominently, the presence and et al. 1993). Scytonemin can be readily extracted with role of UV-absorbing compounds like sporopollenin, non-polar solvents such as acetone or ethyl acetate scytonemin, and mycosporine-like amino acids (MAAs) and authenticated through spectrophotometry or have been established. chromatography. 6 Philippine Journal of Science Dionisio-Sese ML: UV-Screemomg Compounds in Microalgae Vol. 139 No. 1, June 2010 Microalgal scytonemin appears restricted to cyanobacteria, Aside from its UV-absorbing property, scytonemin was specifically in the extracellular polysaccharide sheath found to have anti-inflammatory and anti-proliferative of Chlorogloeopsis sp., Calothrix sp., Scytonema sp., capabilities through its inhibition of human polo-like Rivularia sp., and Nostoc commune (Sinha et al. 1998). It kinase (Stevenson et al. 2002a, 2002b). In 2002, the is reportedly the most important UV-absorbing compound United States has patented methods of using scytonemin in Lyngbya cf. aestuarii, where its area content seems to in treating disorders associated with cell cycle progression, follow the seasonal fluctuation of solar intensity (Karsten et cell proliferation, kinase activity, tissue hyperplasia or al. 1998). It was found to be the common pigment identified angiogenesis, as in cancer or inflammatory diseases in the extracellular sheath of more than 30 species of (http://www.patentgenius.com/patent/6495586.html). cyanobacteria cultures