Ultraviolet-B (UVB, 280-320 nm) radiation is a natural component of sunlight that harms organisms and disturbs natural communities in surface waters. A natural planktonic assemblage of organisms (Ͻ240 m) was studied in a mesocosm experiment for 7 d under varying conditions of UVB radiation: UVB excluded, natural radiation, and UVB enhanced at two different levels. The dynamics of several populations at different trophic levels comprising heterotrophic bacteria (Ͻ1 m), heterotrophic flagellates (2-10 m), small phytoplankton (Ͻ5 m), large phytoplankton (5-20 m), and ciliates (15-35 m) were monitored during the experiment. Enhanced UVB provoked a significant decrease in the number of ciliates (66%) and large phytoplankton (63%) relative to natural UVB conditions. The severe effects of UVB radiation on ciliates and large phytoplankton communities shown here would strongly limit upward transfer of mass and energy. The decline of predator abundance (ciliates) under UVB stress relative to natural conditions resulted in a positive feedback between enhanced UVB radiation and prey abundances, shown by increased abundances of bacteria (49%), heterotrophic flagellates (up to 300%), and small phytoplankton (41%). Similarly, with respect to carbon partitioning, the decrease in ciliate and diatom carbon biomass (64 and 56%, respectively) under enhanced UVB exposure was balanced by an increase in the carbon biomass of heterotrophic bacteria (48%), heterotrophic flagellates (126%), and autotrophic flagellates (162%). As a manifestation of enhanced UVB at the community level, the ecosystem develops toward a microbial food web in preference to an herbivorous food web. Thus, enhanced UVB radiation can change the structure and dynamics of the pelagic food web.The pelagic planktonic community functions through a web of energy and nutrient exchanges mediated by a diverse array of producers and consumers, which ultimately depend on the energy supplied by sunlight. Following the discovery of stratospheric ozone depletion (Farman et al. 1985) and the resulting increase in intensity of biologically harmful UVB radiation (280-320 nm) reaching Antarctic waters, the majority of UVB studies have focused on phytoplankton be- AcknowledgmentsWe thank F. Rassoulzadegan, F. Azam, and T. Sime-Ngando for comments; C. Lovejoy and L. Bérard for help with the identification of some planktonic species; and D. Bourget and N. Lafontaine for nutrient analyses.This work was supported by NSERC of Canada, Fonds FCAR of Québec, and FODAR (University of Québec). International collaboration was made possible by NATO collaborative research grant (CRG 95139) to S.D. and P.M. This investigation is a contribution to the research programs of the Groupe de Recherche en Environnement Côtier.

Whereas vertebrates possess only two thioredoxin genes, higher plants present a much greater diversity of thioredoxins. For example, Arabidopsis thaliana has five cytoplasmic thioredoxins (type h) and at least as many chloroplastic thioredoxins. The abundance of plant thioredoxins leads to the question whether the various plant thioredoxins play a similar role or have specific functions. Because most of these proteins display very similar activities on artificial or biological substrates in vitro, we developed an in vivo approach to answer this question. The disruption of both of the two Saccharomyces cerevisiae thioredoxin genes leads to pleiotropic effects including methionine auxotrophy, H 2 O 2 hypersensitivity, altered cell cycle characteristics, and a limited ability to use methionine sulfoxide as source of methionine. We expressed eight plant thioredoxins (six cytoplasmic and two chloroplastic) in yeast trx1, trx2 double mutant cells and analyzed the different phenotypes. Arabidopsis type h thioredoxin 2 efficiently restored sulfate assimilation whereas Arabidopsis type h thioredoxin 3 conferred H 2 O 2 tolerance. All thioredoxins tested could complement for reduction of methionine sulfoxide, whereas only type h thioredoxins were able to complement the cell cycle defect. These findings clearly indicate that specific interactions between plant thioredoxins and their targets occur in vivo.Thioredoxins are small oxidoreductases (molecular mass Ϸ12 kDa) that all contain two redox-active half-cystine residues in an exposed active center, having the amino acid sequence WCXPC (1). In its reduced form, thioredoxin can function as hydrogen donor for a variety of target proteins. The oxidized form of thioredoxin, having an intramolecular disulfide bridge between the two cysteine residues from the catalytic center, is generally reduced by a thioredoxin reductase. Thioredoxins are ubiquitous and have been described in prokaryotes as well as in eukaryotes, including, fungi, plants, invertebrates, and vertebrates (2-4). Higher plants are known to possess at least three types of thioredoxins: thioredoxin f and m, which are both encoded by the nuclear genome but are located within the chloroplast, and thioredoxin h, which is a cytosolic protein.The chloroplastic thioredoxins f and m are thought to provide a functional link between the light-absorbing pigments and several key metabolic enzymes such as fructose-1,6-bisphosphatase, phosphoribulose kinase, and malate dehydrogenase (5-7). Both oxidized thioredoxin f and thioredoxin m are reduced by a ferredoxin-dependent thioredoxin reductase (8-10). In contrast, the cytosolic thioredoxins h are reduced by an NADPH-dependent thioredoxin reductase (11,12). Recent progress in the systematic sequencing of plant genomes provides evidence that the diversity of thioredoxins is much greater than previously expected, each class of thioredoxin being encoded by multigene families. For instance, Arabidopsis thaliana has been shown to express at least two thioredoxins f (L.V. and...