Oecologia (2002) 133:90–93 DOI 10.1007/s00442-002-0963-z Abstract Eﬀectsof enhanced UV-B (representing a 15% ozone depletion) on cyanobacterial nitrogen ﬁxa- tion were measured at a high arctic site (Adventdalen, 79°N, Svalbard) and a subarctic site (Abisko, 68°N, Sweden). Nitrogen ﬁxation potential (acetylene reduc- tion) by cyanobacteria associated with the moss Sanionia uncinata in vegetation exposed to experimentally en- hanced levels of UV-B for 3 and 4 years in the high arc- tic in Adventdalen was reduced by 50% compared to controls after 3 years. No reduction in nitrogen ﬁxation potential was observed in cyanobacteria associated with the moss Hylocomium splendens when previously ex- posed to enhanced UV-B in Abisko for a 7-year period. However, in the same experiment a 50% increase in summer precipitation stimulated nitrogen ﬁxation poten- tial by up to 6-fold above the natural precipitation treat- ments both in cyanobacteria associated with vegetation exposed to natural and enhanced UV-B radiation. In con- trast to the lack of UV eﬀect on moss-associated nitro- gen ﬁxation at the subarctic site, nitrogen ﬁxation pote tial by the dominant lichen species Peltigera aphth was reduced by 50% when measured after 8 years expo sure to elevated UV-B treatment. Evidence from th studies highlights the importance of UV-B radiation for cyanobacterial nitrogen ﬁxation in the Arctic and future impact on nitrogen availability in such plant comm ties. Keywords Arctic cryptograms · Cyanobacteria · Nitrogen ﬁxation · UV-B Introduction Enhancement of UV-B radiation as a consequence o ozone depletion has wide-ranging impacts on natur plant communities, particularly via impacts on decompo sition and nutrient cycling (Zepp et al. 1998). This coul be very important to arctic communities where ozone d pletion is pronounced (Anonymous 2000) and nitrogen limiting factorfor plantgrowth (Shaver and Chapin 1980). A major source of nitrogen in arctic environmen arises from biological nitrogen ﬁxation, mainly by cya- nobacteria, contributing up to 82% of the annual ecosy tem nitrogen input and 20% of total annual nitrogen up take by plants (Chapin and Bledsoe 1992). In vegetated areas of the Svalbard archipelago, cyanobacteria in ass ciation with bryophytes were by far the most important source of biologically ﬁxed nitrogen (Solheim et al. 1996). Lack of available water is often a major constraint to cyanobacterial nitrogen ﬁxation in the Arctic (Dicks 2000). There is also evidence that UV-B radiation can a fect cyanobacterial nitrogen ﬁxation (Rai et al. 199 Cyanobacteria are especially prone to DNA damage UV radiation because of the very short distance betwee the cell surface and genome and also because of limited ﬁltration of enhanced UV-B by chemical protection (e.g mycosporine-like amino acids) (Rozema et al. 1999). In- B. Solheim Department of Biology, University of Tromsø, 9037 Tromsø, Norway U. Johanson Department of Plant Physiology, Box 117, 221 00 Lund, Sweden T.V. Callaghan Sheﬀield Centre for Arctic Ecology, 26 Taptonville Road, Department of Animal and Plant Sciences, The University of Sheﬀield, Sheﬀield S10 5BR, UK T.V. Callaghan Abisko Naturvetenskapliga Station, Abisko, 981 07, Sweden J.A. Lee Department of Animal and Plant Sciences, University of Sheﬀield, Sheﬀield, S10 2TN, UK D. Gwynn-Jones ( ✉ ) Institute of Biological Sciences, University of Wales, Aberystwyth, Ceredigion, SY23 3DA, Wales e-mail: DYJ@aber.ac.uk Fax: +44-1970-622350 L.O. Björn Department of Cell and Organism Biology, Lund University, 223 62, Lund, Sweden G L O B A L C H A N G E E C O L O G Y Bjørn Solheim · Ulf Johanson · Terry V. Callaghan John A. Lee · Dylan Gwynn-Jones · Lars O. Björn The nitrogen ﬁxation potential of arctic cryptogram species is inﬂuenced by enhanced UV-B radiation Received: 9 June 2001 / Accepted: 25 April 2002 / Published online: 11 July 2002 © Springer-Verlag 2002