459 J. Phycol. 37, 459–467 (2001) DNA DAMAGE AND PHOTOSYNTHETIC PERFORMANCE IN THE ANTARCTIC TERRESTRIAL ALGA PRASIOLA CRISPA SSP. ANTARCTICA (CHLOROPHYTA) UNDER MANIPULATED UV-B RADIATION 1 Daniela Lud 2 Netherlands Institute of Ecology, Centre for Estuarine and Coastal Ecology, P.O. Box 140, 4400 AC Yerseke, The Netherlands Anita G.J. Buma, Willem van de Poll Department of Marine Biology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands Tanja C.W. Moerdijk and Ad H.L. Huiskes Netherlands Institute of Ecology, Centre for Estuarine and Coastal Ecology, P.O. Box 140, 4400 AC Yerseke, The Netherlands The effect of reduced, natural ambient, and en- hanced UV-B radiation (UVBR) on photosynthesis and DNA damage in the Antarctic terrestrial alga Prasiola crispa ssp. antarctica (Kützing) Knebel was investigated in two field experiments. Samples of P. crispa were collected underneath snow cover and ex- posed outside to reduced and natural UVBR in the austral spring. In a second experiment at the end of the austral summer, samples were exposed to ambi- ent and enhanced UVBR. PSII efficiency, net photo- synthetic rate (NP), dark respiration rate (DR), UV- absorbing pigments, and cyclobutyl pyrimidine dimer (CPD) formation were measured during the experi- ments. In October 1998, a spring midday maximum of 2.0 Wm -2 of UVBR did not significantly affect ef- fective quantum yield (F/F m '), and a reduction in the ratio of variable to maximal fluorescence (F v /F m ) in the late afternoon was transient. Exposure to natu- ral ambient UVBR in October increased CPD values significantly. Midday maxima of UVBR during the experiments in October and January were compara- ble, but Setlow-DNA-weighted UVBR was more than 50% lower in January than in October. In January, 0.5 Wm -2 additional UVBR during 10 h did not have a negative effect on F/F m '. The reduction in F v /F m was not significant. NP and DR were not af- fected by supplementation of UVBR. Although pho- tosynthetic activity remained largely unaffected by UVBR treatment, DNA damage was shown to be a sensitive parameter to monitor UVBR effects. Sup- plementation of additional UVBR did significantly enhance the amounts of CPD in exposed samples and repair took place overnight. It is concluded that PSII and whole-chain photosynthesis of P. crispa is well adapted to ambient and enhanced levels of UVBR but that CPD formation is more sensitive to UVBR than to photosynthesis. Key index words: Antarctica; chlorophyll; chlorophyll fluorescence; cyclobutyl pyrimidine dimers; DNA damage; photosynthesis; Prasiola crispa; terrestrial alga; thymine dimers; UV-B radiation; UV-B supple- mentation Abbreviations: BED, biologically effective dose; CPD, cyclobutyl pyrimidine dimer; DR net rate of dark respiration; F/F m ', effective quantum yield in the light; F m , F v , maximum and variable chl fluores- cence after dark incubation respectively; F v /F m , opti- mal quantum yield; MAA, mycosporine-like amino acid; NP, net rate of photosynthesis; UVAR, UVBR UV-A radiation, UV-B radiation, respectively Terrestrial phototrophic organisms are exposed to PAR (400–700 nm), UV-A radiation (UVAR, 320–400 nm), and UV-B radiation (UVBR, 280–400 nm). The absolute fluxes of UVBR are low in polar regions, but the relative increase of UVBR due to stratospheric ozone depletion is more pronounced in polar regions than elsewhere (Madronich et al. 1995). In October, ozone column depths over the Antarctic Peninsula can be reduced by over 60%, leading to increased UVBR on the earth’s surface. The highest values of UVBR in the Antarctic Peninsula region can be ex- pected in late November. Later in the austral summer after thawing of the snow cover above the vegetation incident, UVBR is lower than in the austral spring but still higher compared with preozone hole conditions (Jones and Shanklin 1995). UVBR has been reported to have negative effects on primary production (Behrenfeld et al. 1993, Post and Larkum 1993, Franklin and Forster 1997, Karsten et al. 1999) and growth (Grobe and Murphy 1994, Buma et al. 1997) of algae. chl fluorescence parame- ters, especially the ratio of variable to maximum fluo- rescence (F v /F m ), were found to be useful to monitor UVBR effects on algal photosynthesis (Clendennen et al. 1996, Dring et al. 1996b). The decrease of F v /F m due to UVBR is attributed to a decline in F v , indicat- ing damage to PSII (Heraud and Beardall 2000). Bis- chof et al. (1998) found a reduction of F v /F m in nine species of Antarctic inter- and subtidal macroalgae. The authors observed a more severe reduction in sub- tidal than in intertidal species. Larkum and Wood 1 Received 17 November 2000. Accepted 29 April 2001. 2 Author for correspondence: e-mail lud@cemo.nioo.knaw.nl.