268  2002 American Society for Photobiology 0031-8655/02 $5.00+0.00 Photochemistry and Photobiology, 2002, 76(3): 268–273 Estimation of Biologically Damaging UV Levels in Marine Surface Waters with DNA and Viral Dosimeters ¶ Steven W. Wilhelm* 1 , Wade H. Jeffrey 2 , Curtis A. Suttle 3 and David L. Mitchell 4 1 Department of Microbiology, The University of Tennessee, Knoxville, TN; 2 Center for Environmental Diagnostics and Bioremediation, University of West Florida, Pensacola, FL; 3 Departments of Earth and Ocean Sciences, and Microbiology and Botany, The University of British Columbia, Vancouver, BC, Canada and 4 Department of Carcinogenesis, The University of Texas MD Anderson Cancer Center, Smithville, TX Received 17 April 2002; accepted 30 May 2002 ABSTRACT We have surveyed the biologically harmful radiation penetrating the water column along a transect in the western Gulf of Mexico using dosimeters consisting of intact viruses or naked calf-thymus DNA (ctDNA). The indigenous marine bacteriophage PWH3a-P1, which ly- tically infects the heterotrophic bacterium Vibrio natrie- gens (strain PWH3a), displayed decay rates for infectiv- ity approaching 1.0 h -1 in surface waters when deployed in a seawater-based dosimeter. The accumulation of py- rimidine dimers in ctDNA dosimeters provided a strong correlation to these results, with pyrimidine dimers rep- resenting more than 0.3% (up to ca 3800 dimers Mb -1 DNA) of the total DNA in dosimeters exposed to sea sur- face levels of solar radiation. The results demonstrate a strong correlation between the dimer formation in the DNA dosimeters, the decay rates of viral infectivity and the penetration of UVB radiation into the water column. The decay of viral infectivity attenuated with depth in a manner similar to the decay of solar radiation and was still significant at 10 m in offshore oligotrophic water and at dimer frequencies less than 0.1% (ca 200–300 dimers Mb -1 DNA). INTRODUCTION Increasing evidence suggests that stratospheric ozone (O 3 ) levels are decreasing because of the presence of anthropo- genically derived chlorofluorocarbons in the upper atmo- sphere (1,2). Stratospheric ozone functions as the main at- mospheric attenuator of ultraviolet (UV) radiation, and thus ozone decay is predicted to lead to increased levels of in- cident UV radiation at the earth’s surface (3). Because of ¶Posted on the web site on 20 June 2002. *To whom correspondence should be addressed at: The Department of Microbiology, The University of Tennessee, Knoxville, TN 37993, USA. Fax: 865-974-4007; e-mail: wilhelm@utk.edu Abbreviations: CPD, cyclobutane pyrimidine dimers; ctDNA, calf- thymus DNA; PAR, photosynthetically active radiation; PFU, plaque-forming units; PVC, polyvinyl chloride; RIA, radioim- munoassay; UV, ultraviolet. the direct deleterious effects of UV on living organisms, it is important to develop indicators of UV exposure that would measure biologically relevant UV damage (4,5). Quantifying the exposure to UV radiation is difficult and expensive. Spectroradiometers are commonly used to mea- sure the energy of discreet wavelengths but are costly and limited in their ability to integrate over long periods of ex- posure and to account for sporadic anomalies such as cloud- iness (6,7). Chemical indicators of UV exposure are more cost-effective but are limited by the wavelengths of their specific chemical reactions (8,9). Given these considerations, it is important to develop UV detection systems that inte- grate dosage over time, respond to all wavelengths of solar radiation and provide biologically relevant results in a cost- effective manner. Recently, DNA dosimeters, consisting of naked DNA within quartz enclosures, have been used to estimate the damaging UV radiation on the basis of the formation of cis– syn cyclobutane pyrimidine dimers (CPD) in the DNA (6,10). Although the dimer formation within DNA is most efficient at 254 nm (11), wavelengths less than 280 nm do not reach the earth’s surface, leaving wavelengths of ca 305 nm to be the most biologically effective (12). UVB radiation induces dimer formation that is ca 90 times less effective than at 254 nm but remains four orders of magnitude more damaging than light at 360 nm (13). The UVB (290–320 nm, as defined by the American Society for Photobiology) component of the solar spectrum can directly damage DNA through the formation of CPD as well as induce the forma- tion of pyrimidine [6-4] pyrimidone photoproducts (14). Both types of damage are inhibitory to cellular replication processes (15) and have been examined in viruses in marine systems (16,17). But pyrimidine [6-4] pyrimidone photo- products can be converted to the Dewar isomer by longer- wavelength irradiance (18), making them more difficult to be accurately estimated in situ (16). Viruses are pervasive within aquatic environments at titers of 10 5 –10 9 infective units per milliliter (19,20). Viral parti- cles demonstrate host-specific relationships with many aquatic organisms, including bacteria, cyanobacteria and eu- karyotic plankton. The cosmopolitan distribution of viruses and their sensitivity to solar radiation suggest that they may