Ferric hydroxide and ferric hydroxysulfate precipitation by bacteria in an acid mine drainage lagoon

The spontaneous precipitation of amorphous iron hydroxide and ferric hydroxysulfate has generally been considered to be an inorganic process involving the oxidation of ferrous iron with or without the presence of sulfate. However, our study of bacterial communities growing in an acid mine drainage lagoon sediment has confirmed that microorganisms were also capable of facilitating this mineral precipitation. Transmission electron microscopy revealed that bacteria growing at the surface had iron-rich capsules, along with detectable amounts of Zn, Ti, Mn and K incorporated into the mineralised matrix. In the subsurface, more cells were associated with granular, fine-grained mineral precipitates, composed almost exclusively of iron and sulfur. Pore water profiles indicated that no discernible sulfate reduction had taken place, suggesting that these authigenic minerals were 'ferric hydroxysulfate', and not iron sulfide. Energy dispersive X-ray spectroscopy further indicated that the subsurface minerals had variable composition, with the Fe:S ratio decreasing with depth from 3.5:1 at 15 cm to 1.9:1 at 30 cm. This indicates the high reactivity of ferric hydroxide for dissolved sulfate. Because iron reduction was limited to sediment depths between 3-10 cm, it is conceivable that these minerals are not amenable to bacterial reduction, and hence, the ability of bacteria to bind and form such precipitates may provide a natural solution to cleansing acidified waters with a high dissolved metal content.