34  1999 American Society for Photobiology 0031-8655/99 $5.00+0.00 Photochemistry and Photobiology, 1999, 69(1): 34–40 Attenuation of Biologically Effective UV Radiation in Tropical Atlantic Waters Measured with a Biochemical DNA Dosimeter Peter Boelen* 1,2 , Ingrid Obernosterer 2 , Arie A. Vink 3 and Anita G. J. Buma 2 1 Department of Marine Biology, Centre for Ecological and Evolutionary Studies, University of Groningen, The Netherlands; 2 Department of Biological Oceanography, Netherlands Institute for Sea Research, The Netherlands and 3 TNO Nutrition and Food Research Institute, Zeist, The Netherlands Received 26 March 1998; accepted 10 October 1998 ABSTRACT A biochemical dosimeter was developed to study the at- tenuation of biologically effective UV radiation in marine tropical waters. Small quartz vials were used containing a solution of DNA molecules; the vials were incubated at discrete water depths. Subsequently, DNA damage was determined in these samples, using an antibody directed against thymine dimers followed by chemiluminescent detection. Measurements of DNA damage were com- pared with calculated biologically effective doses, as de- rived from spectroradiometer measurements. The bio- dosimeter was found to be a reliable and easy tool to determine levels of harmful UV radiation in marine wa- ters. The highest attenuation coefficient (1.60 m -1 ) mea- sured with the biochemical dosimeter was found in eu- trophic waters, at a coastal station off Curac ¸ao, Nether- lands Antilles. At the other stations attenuation coeffi- cients ranged from 0.18 m -1 in central Atlantic waters to 0.43 m -1 close to the Curac ¸ao coast line. Latter results indicate that biologically effective UV radiation may eas- ily reach ecologically significant depths, e.g. coral reef communities. INTRODUCTION Tropical regions are characterized by high natural levels of UV radiation (1). Despite the fact that natural UV levels exceed those at higher latitudes, even those influenced by ozone depletion, very little is known about UV stress in trop- ical marine organisms. This is even more surprising, because UV penetration may be very high in these clear oligotrophic waters (2,3). On the other hand, it has been suggested that tropical organisms can overcome high natural levels of UVB (280–315 nm) radiation, for instance by producing UV-ab- sorbing compounds (4–6). Only very recently clear indica- tions of UV stress were described for a variety of marine organisms from the Caribbean and Gulf of Mexico (7–10). Effects of UV radiation on living cells are determined by *To whom correspondence should be addressed at: Department of Marine Biology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands. Fax: +31-503632261; e-mail: P.Boelen@biol.rug.nl a combination of factors. First of all, the penetration char- acteristics of biologically effective radiation in the water col- umn are of major importance. Intensity and spectral com- position of UV at depth depend on chemical characteristics such as yellow substance concentration, pigment concentra- tion and the amount of particles present in the water column. Secondly, the duration of exposure of pelagic organisms de- pends on mixing phenomena, such as wind-induced vertical mixing. Finally, the resultant UV stress will be determined by the vulnerability of the organisms for UVB radiation and their capacity to ameliorate or prevent damage, for instance by repair processes or synthesis of UV-absorbing com- pounds. Because we are interested in UVB radiation impacts on tropical marine communities, our first objective was to describe the penetration of biologically effective UV radia- tion in these waters. One possibility to obtain information about the penetration depth of biologically effective UVB radiation is to perform spectroradiometer measurements at several depths and cal- culate biological effective irradiance using action spectra. Another possibility to determine the potentially hazardous effects of UV radiation at a certain depth is to use a dosi- meter. Several types of dosimeters have been developed for field application, for instance those based on biological ma- terial (11–14), chemical reactions (3,15) and biochemical material, such as bare DNA molecules (16–18). There are several advantages of dosimeter measurements compared to spectral radiometer measurements (16,17). Application of dosimeters allows direct determinations of daily doses at dif- ferent depths by performing underwater incubations during the whole light period. Also, a dosimeter integrates the ab- sorbed energy of all biologically effective wavelengths under investigation. Finally a dosimeter is cheap, small, robust and portable. Structural changes in DNA are considered to be one of the primary consequences of the deleterious effects of UVB on the cellular level (19,20). Cyclobutane pyrimidine dimers, especially thymine dimers (TT)² are the predominant le- sions induced by UVB radiation (21). These photoproducts block transcription and replication of the DNA and cause mutations or even cell death (22,23). ² Abbrevations: BED, biologically effective dose; DCM, deep-chlo- rophyll maximum; PBS, phosphate-buffered saline; TT, thy- mine dimer.