Environmental Context. It is well known that bacteria can significantly accelerate oxidation rates of copper sulfide minerals under acidic conditions. However, this process is not thought to be important at higher pH. This study reports the bacterially assisted oxidation of copper sulfide minerals in slightly alkaline (pH 8.0–8.3) tropical river waters downstream of a large copper mine. This process leads to significant solubilization of copper from particulate phases.

Abstract. Field and laboratory experiments are reported which demonstrate the bacterially facilitated oxidation of copper sulfide minerals in the water column of tropical rivers. When river water samples, collected downstream of a large copper mine, were incubated under controlled laboratory conditions (pH 8.0–8.3) significant dissolved copper release was observed. This was accompanied by an increase in cold acid soluble particulate copper, indicating oxidation of copper sulfides. Dissolved copper release and copper sulfide oxidation were markedly lower in sterile control samples demonstrating biological mediation. In samples collected close to the mine discharge, dissolved copper release ceased after the first 150 h of incubation, presumably following the consumption of easily oxidizable solid copper sulfide phases or armouring of particles with insoluble oxidation products. Attempts to isolate the bacteria responsible were unsuccessful. However, oxidation rates could be enhanced by culturing aliquots of unfiltered river water samples in simple mineral media (pH 7.0) amended only with sulfide. This provided strong evidence that the copper sulfide oxidizing bacteria were chemolithotrophs. Our results suggest that such bacteria are naturally present in mineralized areas and are actively involved in the cycling of particulate copper between sulfide and hydroxy-carbonate forms, thus influencing the solubility and bioavailability of copper.