Effects of acidophilic protozoa on populations of metal-mobilizing bacteria during the leaching of pyritic coal

Five acidophilic protozoa (three flagellates, one ciliate and one amoeba) were isolated from acid mine water and a coal biotreatment plant, and grown in mixed cultures with acidophilic bacteria. Cultures were routinely maintained in ferrous sulphate media: in media containing pyrite or pyritic coal, protozoa grew in cultures containing coarse-grain (61-200 pm) but not fine-grain (c 61 pm) minerals. In cultures of pyritic coal, protozoa grazed iron-oxidizing and heterotrophic bacteria, but to varying extents. One of the flagellates appeared only transiently in coal leachates, whilst the four others persisted through the 100 d incubation. A reduction of numbers of unattached iron-oxidizing acidophiles, due to protozoan grazing, did not always result in lower rates of pyrite oxidation. The presence of protozoa was noted to effect changes in acidophilic populations, in particular often causing Leptospirihm ferrooxidans to become the dominant iron-oxidizer at an earlier stage than in corresponding protozoa-free controls.