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Seasonal ozone depletion over Antarctica leads to enhanced UVB (280 to 320 nm) radiation throughout the period of greatest biological production. The effect of UV radiation on bacterioplankton has received little attention, and its effects on marine microheterotrophs and viruses, which mediate bacterial biomass, are poorly understood. This study examined the impact of ambient solar W radiation on bacterioplankton in natural Antarctic microbial communities. Following a lag of 2 d, bacterial concentrations increased all light treatments. Inhibition of bacterial growth increased with increasing UV irradiance and duration of exposure, reaching 27 % inhibition in hlgh UV treatments (c2.0 m equivalent depth) compared to controls after 7 d exposure. Bacterioplankton growth rates declined in all treatments during post-UV incubation, particularly at lower UV irradiances (23.0 m equivalent depth), indicating UV-induced inhibition of bacterial mortality during irradiation. Positive bacterial growth coincided with both phytoplankton mortality and increased microheterotroph concentrations following exposure to high UV irradiances. Exposure of Antarctic microbial communities to ambient UV is likely to increase microbial respiration of carbon in surface waters and reduce vertical carbon flux.

Ozone depletion over Antarctica has enhanced ultraviolet-B radiation (UVBR, 280 to 320 nm wavelength). We measured the effect of ambient solar UV radiation on the biomass and species composition of phytoplankton, protozoa, bacteria and dissolved organic carbon (DOC) in natural microbial assemblages from Antarctic coastal waters. Results were modelled to determine the features of the irradiance responsible for changes in the biomass of these microbial components and responses of individual phytoplankton taxa. Model results showed that changes in phytoplankton biomass were primarily due to dose rate, indicating that their UV-induced mortality resulted from the equilibrium between damage and repair. However, there was considerable variability between individual species in their response to dose and dose rate. Changes in protozoan biomass were mainly due to dose and were likely due to community-level, trophodynamic interactions. UV radiation did not measurably affect bacterial biomass, but resulted in increasing concentrations of DOC. We found a threshold of erythemal irradiance of 28 mW m -2 , approximating peak noon-time irradiance at 3.6 m depth near the summer solstice in Antarctic coastal waters, below which no change in the community structure was observed, but above which phytoplankton mortality and protozoan biomass increased. Our results indicate that enhanced UVB radiation in Antarctic waters increases phytoplankton mortality and causes changes in the structure, function and composition of the microbial community that are likely to return more photoassimilated carbon to the atmosphere. KEY WORDS: Antarctic · Model · UV · Ozone · Marine microbes Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 42: [75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90] 2006 pervasive, with molecular and cellular impacts on autotrophs flowing, via the food web, to cause changes on the ecosystem level (Vincent & Roy 1993). Most research on the effect of UV on marine microbes has focussed on its impact on phytoplankton (Davidson 1998). However, Bothwell et al. (1994) first showed that studies of UV-induced effects on single species or trophic levels could not be used to predict the impacts in natural communities, as they did not incorporate the effect of UV on trophic interactions. Recent studies show that UVB also directly impacts the production, growth, survival and species composition of bacteria, viruses and protozoa , Mostajir et al. 2000, Vernet 2000, increasing the likelihood that ecosystem-level changes will result from Antarctic ozone depletion. However, few studies have addressed UV-induced changes in the dynamics of natural marine microbial assemblages (Keller et al. 1997a,b, Wickham & Carstens 1998, Mostajir et al. 1999) and findings vary greatly. To our knowledge, no studies have been conducted in Antarctic marine waters other than those we have performed (Davidson & van der Heijden 2000, Davidson & Belbin 2002.Models can be used to better unde...