Radioprotection, vol. 46,n ◦ 6 (2011) S331–S337 C  EDP Sciences, 2011 DOI: 10.1051 / radiopro / 20116985s Microbial communities in an oligotrophic reservoir inﬂuenced by high levels of ionizing radiation L.B. Ronqui 1 , H. Azevedo 1 , M.H.R. Seleghim 2 , C.R. Ferrari 1 , S. Rodgher 2 , J.F. Macacini 1 , C.V. Roque 1 , G.B. Mourão 3 and M.R.L. Nascimento 1 1 Poços de Caldas Laboratory, Brazilian Nuclear Energy Commission, Poços de Caldas – Andradas Road, Km 13, 37701-970 Poços de Caldas, MG - Brazil 2 Department of Ecology and Evolutionary Biology, Federal University of São Carlos, Washington Luis Road, (SP 310) Km 235, 13565-905 São Carlos, SP - Brazil 3 Department of Exact Sciences, “Luiz de Queiroz” College of Agriculture, São Paulo University, Pádua Dias Avenue, 11, 13418-900 Piracicaba, SP - Brazil Abstract. In this study, we investigated the microbial community (bacteria and protozooplankton) of a Brazilian reservoir that receives neutralized acid mine drainage ﬂowing from piles of earth produced during uranium extraction. Thus, this research represents an attempt to ﬁll a gap in the information on the ecology of freshwater microorganisms in tropical oligotrophic habitats, in particular in systems affected by high levels of the uranium. During the study, we observed very low chlorophyll a contents and protozooplankton cell densities and biomass. These cell densities were between 0 and 89 cells L -1 and were lower than those frequently reported for oligotrophic freshwater lakes. In contrast, bacterial densities were normal or even slightly high, between 1.85 to 6.0 × 10 9 cells L -1 . The present study has demonstrated very low protozooplankton cell densities and biomass in oligotrophic reservoir under the inﬂuence of acidic mining efﬂuents and of high levels of ionizing radiation. It is likely that the ciliate cell densities and biomasses recorded in this study, which fall signiﬁcantly below the previously published values, can be explained by the chronic fractionated exposure of these protozoa to the high levels of uranium to this system. 1. INTRODUCTION Bacteria and protozoa are components of the microbial food web of the plankton and benthos in fresh- water habitats. The bacteria are recognized as important decomposers and remineralizers of inorganic nutrients in aquatic ecosystems, as well as channeling carbon and energy, acquired from the dissolved organic matter excreted by algae, to organisms at higher trophic levels in the food chain [16, 17]. Few experimental or descriptive studies have been published on microbial food webs in oligotrophic and ultraoligotrophic temperate lakes, compared to their eutrophic and mesotrophic counterparts [18, 19]. According to Laybourn-Parry [19], the oligotrophic systems are harder to study, since they contain highly-stressed organisms at low densities. The Antas Reservoir (Minas Gerais state, Brazil), olig- otrophic system, is located in a tropical region of raised background radioactivity (due to local uranium and thorium anomalies) and receives the neutralized acid leach solution draining from waste heaps gen- erated by uranium mining. This study was a season-by-season analysis of the abundance and biomass of ciliated protozoa among the plankton of Antas Reservoir and their interactions with other components of the microbial food web (density and biomass of the dinoﬂagellate Peridinium sp., density of bacterial plankton and biomass of phytoplankton) and with the physical and chemical characteristics of the water. Thus, this research represents an attempt to ﬁll a gap in the information on the ecology of freshwater