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The mercury rejected in the water system, from mining operations and lixiviation of soils after deforestation, is considered to be the main contributors to the contamination of the ecosystem in the Amazon Basin. The objectives of the present study were to examine cytogenetic functions in peripheral lymphocytes within a population living on the banks of the Tapajós River with respect to methylmercury (MeHg) contamination, using hair mercury as a biological indicator of exposure. Our investigation shows a clear relation between methylmercury contamination and cytogenetic damage in lymphocytes at levels well below 50 micrograms/gram, the level at which initial clinical signs and symptoms of mercury poisoning occur. The first apparent biological effect with increasing MeHg hair level was the impairment of lymphocyte proliferation measured as mitotic index (MI). The relation between mercury concentration in hair and MI suggests that this parameter, an indicator of changes in lymphocytes and their ability to respond to culture conditions, may be an early marker of cytotoxicity and genotoxicity in humans and should be taken into account in the preliminary evaluation of the risks to populations exposed in vivo. This is the first report showing clear cytotoxic effects of long-term exposure to MeHg. Although the results strongly suggest that, under the conditions examined here, MeHg is both a spindle poison and a clastogen, the biological significance of these observations are as yet unknown. A long-term follow-up of these subjects should be undertaken.

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Spatial and temporal variations of mercury (Hg) concentrations were monitored in the surface waters from the lower portion of the Tapajós River, the Arapiuns River, its principal tributary, and the Amazon River at its confluence with the Tapajós. In the rivers, Hg concentrations in the water column are governed by the concentration of suspended particles. Hg in the filtered water showed very little seasonal variation with low concentrations in the lower Tapajós (Ͻ1.8 ng L Ϫ1), Arapiuns (Ͻ0.8 ng L Ϫ1 ), and Amazon (Ͻ2.8 ng L Ϫ1). Concentrations of fine particulate Hg (0.6-29.7 ng L Ϫ1 ) represent 40%-90% of the total volumetric concentration of Hg. In relation to their oxyhydroxide contents, suspended particles are not richer in Hg than nearby soils where oxyhydroxides control the accumulation of Hg. The study shows that the dominant stock of Hg in the aquatic ecosystems of this region is derived from erosion of natural soils in the catchment rather than from anthropogenic pollution. The input of natural Hg coming from soils into the aquatic ecosystems may have increased over historical levels in the region. This increase of total Hg in aquatic ecosystems could potentially account for high levels of methylmercury recently reported in fish and humans in the lower Tapajós River area, but the link between the different processes that promote high levels of methylmercury exposure for the human community remains to be proved.Numerous recent studies (Bidone et al. 1997; Lebel et al. 1997;Malm et al. 1997) have documented exceedingly elevated levels of mercury in fish of the Tapajós River, one of the principal tributaries of the Amazon River. Exposure to methylmercury (MeHg) through fish consumption by the 1 Present address: IRD-Bolivia, CP 9214, La Paz, Bolivia (roulet@cict.fr). AcknowledgementsThe project would never have seen the light of day without the collaboration of Aldo Queiroz Gomes, coordinator of the Santarém campus and Cristovan Waverly Diniz, rector of the Universidade Federal do Pará (UFPa). We would also like to thank Louise Cournoyer, Gerson da Costa de Oliveira, Jean Lebel, Pierre Pichet, Everaldo Portella, Isabelle Rheault, Sewbert Rodrigues Jati, Gil Serique, Delaine Sampaio da Silva, Persio Scavone de Andrade, Nicolas Soumis, Silmara Sousa de Morales, Carlos-José Sousa Passos, Sophie Tran, and Ronilson Vasconcelos Barbosa for their assistance in the field and the laboratory. The analytical methods used for Hg were developed by Professor P. Pichet for the laboratory of the Chaire de Recherche en Environnement H-Q/CRSNG/UQAM.