Solar UVB-Induced DNA Damage and Photoenzymatic DNA Repair in Antarctic Zooplankton (Ozone Depletion͞dna Damage and Repair͞photolyase͞marine Ecosystems)
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Proc. Natl. Acad. Sci. USA Vol. 94, pp. 1258–1263, February 1997 Ecology Solar UVB-induced DNA damage and photoenzymatic DNA repair in antarctic zooplankton (ozone depletionyDNA damage and repairyphotolyaseymarine ecosystems) KIRK D. MALLOY*, MOLLY A. HOLMAN*, DAVID MITCHELL†, AND H. WILLIAM DETRICH III*‡ *Department of Biology, Northeastern University, Boston, MA 02115; and †Department of Carcinogenesis, The University of Texas M. D. Anderson Cancer Research Center, Smithville, TX 78957 Communicated by George Somero, Stanford University, Pacific Grove, CA, December 13, 1996 (received for review August 18, 1996) ABSTRACT The detrimental effects of elevated intensi- of marine organisms in coastal regions and the upper photic ties of mid-UV radiation (UVB), a result of stratospheric zone of open oceans may be affected deleteriously by the ozone depletion during the austral spring, on the primary projected long-term increase in UVB flux (16–20). producers of the Antarctic marine ecosystem have been well Although UVB damages most biological macromolecules, documented. Here we report that natural populations of including lipids, proteins, and nucleic acids (21, 22), the Antarctic zooplankton also sustain significant DNA damage principal cause of UV-induced mutation, degeneration, [measured as cyclobutane pyrimidine dimers (CPDs)] during andyor death in animal cells is modification of DNA (23). The periods of increased UVB flux. This is the first direct evidence most significant DNA lesions generated by UVB are cyclobu- that increased solar UVB may result in damage to marine tane pyrimidine dimers (CPDs), which constitute '70–90% of organisms other than primary producers in Antarctica. The the aberrant DNA photoproducts (23). CPDs increase linearly extent of DNA damage in pelagic icefish eggs correlated with with UVB exposure (23, 24), but the dose–response relation- daily incident UVB irradiance, reflecting the difference be- ship varies significantly between different species and taxa tween acquisition and repair of CPDs. Patterns of DNA (25–27). Nevertheless, the CPD burden resulting from suble- damage in fish larvae did not correlate with daily UVB flux, thal doses of UVB may inhibit embryonic and larval devel- possibly due to different depth distributions andyor different opment and ultimately decrease survival, by slowing transcrip- capacities for DNA repair. Clearance of CPDs by Antarctic tion and mitosis and by imposing energetic costs associated fish and krill was mediated primarily by the photoenzymatic with DNA repair (28). repair system. Although repair rates were large for all species The impact of elevated UVB on marine ecosystems has been evaluated, they were apparently inadequate to prevent the documented most extensively for the primary producers of transient accumulation of substantial CPD burdens. The polar latitudes (refs. 13 and 29; reviewed in ref. 30). Primary capacity for DNA repair in Antarctic organisms was highest productivity in the Southern Ocean declines by as much as 15% in those species whose early life history stages occupy the in areas affected by the ozone hole (13). Survival of and water column during periods of ozone depletion (austral photosynthesis by Antarctic diatoms are reduced by UVB, spring) and lowest in fish species whose eggs and larvae are although the susceptibility of these single-celled algae to abundant during winter. Although the potential reduction in UVB-induced DNA damage varies significantly, depending fitness of Antarctic zooplankton resulting from DNA damage upon cell surface:volume ratios, pigmentation, and DNA is unknown, we suggest that increased solar UV may reduce repair rates (25). UVB has also been shown to inhibit nitrogen recruitment and adversely affect trophic transfer of produc- uptake by phytoplankton from the North Atlantic (31). Al- tivity by affecting heterotrophic species as well as primary though previous investigators have strongly suggested that producers. elevated UVB flux may perturb marine ecosystems as a whole (32, 33), its effects on marine organisms other than primary The concentration of stratospheric ozone has decreased sig- producers have not yet been systematically investigated. Both- nificantly during the past two decades, the result of catalytic well et al. (34), after demonstrating that herbivorous grazers destruction of ozone mediated by the photodegradation prod- were the keystone species affected by UVB in shallow tem- ucts of anthropogenic chlorofluorocarbons (1, 2). Ozone de- perate streams, concluded that ‘‘predictions of the response of pletion has been most dramatic over Antarctica, where ozone entire ecosystems to elevated UVB cannot be made on single levels typically decline .50% during the austral spring ‘‘ozone trophic-level assessments.’’ hole’’ (3–5). Elsewhere, ozone concentrations have fallen Most organisms possess defense mechanisms that act either gradually at temperate latitudes (6, 7), and further depletion to prevent UVB-induced DNA damage (via behavioral, phys- over a broader geographical range is anticipated during the ical, andyor chemical strategies) or to repair it after it has next 25–100 years (7, 8). Atmospheric ozone strongly and occurred. The two primary repair systems are photoenzymatic selectively absorbs solar UVB (290–320 nm), thus reducing the repair (PER; ‘‘light repair’’) and nucleotide excision repair intensity of the most biologically damaging solar wavelengths (NER; ‘‘dark repair’’) (25). PER repairs primarily CPDs via that penetrate the atmosphere (9), and decreased stratospheric the enzyme photolyase, which uses near-UV light (320–400 ozone has been linked directly to increased UVB flux at the nm) as its source of energy. NER, by contrast, involves multiple earth’s surface (10). UVB penetrates to ecologically significant proteins (e.g., UVR-A, -B, and -C in Escherichia coli), does not depths (20–30 m) in the ocean at intensities that cause require light for catalysis and repairs preferentially the 6–4 measurable biological damage (11–16). Therefore, the fitness pyrimidine–pyrimidinone photoproduct and the Dewar pyri- midinone (35). The publication costs of this article were defrayed in part by page charge To assess the potential impact of ozone depletion and payment. This article must therefore be hereby marked ‘‘advertisement’’ in increased UVB intensities on populations of marine hetero- accordance with 18 U.S.C. §1734 solely to indicate this fact. Copyright q 1997 by THE NATIONAL ACADEMY OF SCIENCES OF THE USA Abbreviations: CPD, cyclobutane pyrimidine dimer; PER, photoen- 0027-8424y97y941258-6$2.00y0 zymatic repair; NER, nucleotide excision repair. PNAS is available online at http:yywww.pnas.org. ‡To whom reprint requests should be addressed. 1258 Downloaded by guest on October 5, 2021 Ecology: Malloy et al. Proc. Natl. Acad. Sci. USA 94 (1997) 1259 trophs in Antarctica, we analyzed field-collected zooplankton complexes were collected by precipitation overnight at 48C for UVB-induced DNA damage (CPDs) during the ozone hole with goat anti-rabbit IgG and carrier IgG followed by centrif- (October–November) of 1994. Because DNA damage and ugation. After washing, pellets were dissolved in NCS II repair occur contemporaneously in the field, we also evaluated solubilizer (Amersham), and radioactivity was quantified by the ability of three Antarctic heterotrophs [two teleosts, liquid scintillation counting in Scintiverse (Fisher). Specimen Notothenia coriiceps (Nototheniidae; rockcods) and Chaeno- DNA damage (CPDs per Mb) was evaluated by use of a cephalus aceratus (Channichthyidae; icefishes) and one species standard curve generated with unlabeled DNA containing of krill, Euphausia superba] to repair UV-induced DNA dam- known quantities of CPDs. age at ambient temperatures (21to118C). Finally, to evaluate DNA Repair Measurements. To determine the relative the relative efficiency of DNA repair in these cold-adapted importance of PER and NER repair systems in Antarctic Antarctic species, we measured DNA repair rates of a tem- marine heterotrophs, we measured light and dark DNA repair perate, eurythermal teleost (Fundulus heteroclitus) at three rates of juvenile specimens of two abundant fish species (the acclimation temperatures. Our results indicate that Antarctic rockcod N. coriiceps and the icefish C. aceratus) and of adult zooplankters accumulate significant CPD levels during periods specimens of krill (E. superba). The fishes, whose embryonic of increased UVB flux, that repair of this damage is mediated and larval stages differ in seasonal exposure to solar illumi- largely by the photoenzymatic repair system, and that DNA nation (see Results), were collected during RyV Polar Duke repair capacity is highest for species whose eggs and larvae cruises 94-10, 95-2, and 95-3 (October 1994–November 1994, occupy the water column during the austral spring. Although March 1995, and May 1995, respectively). Krill were collected the reduction in zooplankton fitness resulting from this DNA with a plankton net from surface waters near Palmer Station damage is unknown, we suggest that increased solar UVB flux in February 1995. As a benchmark for comparison of DNA may reduce recruitment and trophic transfer of productivity by repair efficiency in these cold-living poikilotherms, we also affecting both the heterotrophic species and the primary evaluated repair rates of the eurythermal killifish, F. hetero- producers of the Antarctic marine ecosystem. clitus, at three acclimation temperatures (6, 10, and 258C).