Résumé
Extremophile microorganisms play a key role in iron cycling (oxidation and precipitation) in acid mine drainage. The mineral phases that precipitate in AMD are effective scavengers of toxic elements such as arsenic. The aim of this study is to better understand the role of bacterial activity in processes leading to a natural removal of Fe and As in AMD. For this, some potential fingerprints of bacterial activity were studied (speciation changes, formation of biominerals, isotopic fractionation). Field studies have been coupled to laboratory experiments. Dial and annual variations of iron and arsenic concentrations were evidenced in the Reigous AMD, due to physico-chemical and hydrological processes respectively. Tooeleite, an Fe(III)-As(III) oxyhydroxysulfate, occurs in bacterial concretions all year long. The influence of biotic factors (identity of Acidithiobacillus ferrooxidans strains and cell concentration) on the formation of this mineral was studied in laboratory experiments. The evolution of iron isotopic composition in water and sediments, monitored during the course of the experiment and in situ (Reigous and Rio Tinto streams), shows a systematic enrichment of heavy isotopes in the solid phase compared to water whatever the original Fe speciation. Even if bacteria favour iron oxidation and precipitation in these environments, the observed fractionations are equilibrium fractionations dominated by abiotic processes.