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Mercury (Hg) emissions from informal gold mining operations represent a serious environmental problem in developing countries. In Amazonia, from 70 to 170 tonnes (t) of Hg are discharged annually. The extent of biota contamination is also widespread. Mercury must be renderedsoluble and then converted into methylmercury in order to accumulatein the food-chain. Generally, it is regarded that Hg must be oxidized to produce significant dissolution. In this work, we examine the stability of mercury as the unoxidized aqueous elemental Hg0 (aq.) in aquatic environments. In contact with organic acids, evidence of Hg-organic complex formation has been found from both thermodynamic analysis and electrochemical experiments. Although methylation of these soluble species is not fully understood, formation of such complexes in darkwater rivers must contribute to increased bioavailability. [ABSTRACT FROM AUTHOR]