Ecotoxicological studies usually focus on single microbial species under controlled conditions. As a result, little is known about the responses of different microbial functional groups or individual species to stresses. In an aim to assess the response of complex microbial communities to pollution in their natural habitat, we studied the effect of a simulated lead pollution on the microbial community (bacteria, cyanobacteria, protists, fungi and micro-metazoa) living on Sphagnum fallax. Mosses were grown in the laboratory with 0 (control), 625 and 2500 µg.L-1 of Pb2+ diluted in a standard nutrient solution and were sampled after 0, 6, 12 and 20 weeks. The biomass of heterotrophic bacteria, micro-algae, testate amoebae and ciliates were dramatically and significantly decreased in both Pb addition treatments after 6, 12 and 20 weeks in comparison with the control. The biomass of cyanobacteria declined after 6 and 12 weeks in the highest Pb treatment. The biomasses of fungi, rotifers and nematodes decreased along the duration of the experiment but were not significantly affected by lead addition. Consequently, the total microbial biomass was lower for both Pb addition treatments after 12 and 20 weeks than in the controls. The community structure was strongly modified due to changes in the densities of testate amoebae and ciliates, while the relative contribution of heterotrophic bacteria to the microbial biomass was stable. Differences in responses among the microbial groups suggest changes in the trophic links among them. The correlation between the biomass of heterotrophic bacteria and that of ciliates or testate amoebae increased with increasing Pb loading. We interpret this result as an effect on the grazing pathways of these predators and by the Pb effect on other potential prey (i.e. smaller protists). The community approach used here complements classical ecotoxicological studies by providing clues to the complex effect of pollutant affecting organisms both directly and indirectly through trophic effects and could potentially find applications for pollution monitoring. Introduction