Biological Abatement of Acid Mine Drainage: The Role of Acidophilic Protozoa and Other Indigenous Microflora

Abstract

Extremely acidic, metal-rich environments such as acid mine drainage and industrial leachate liquors may be populated by a considerable diversity of obligate acidophilic microorganisms. These include the familiar metal-mobilising chemolithotrophic bacteria (Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, etc.) as well as other bacteria and eukaryotes, some of which have received relatively little attention, but which have considerable potential either in controlling the production of acidic mine effluents, or in treating this form of pollution once it has formed. Several isolates of acidophilic protozoa have been shown to graze mineral-oxidising and other acidophilic bacteria and, in some cases, thereby to decrease the rate of pyrite oxidation in coal samples. The predator-prey relationship which exists between acidophilic bacteria and grazing protozoa has been found to suppress rather than to eliminate metal-mobilising bacteria and, at present, prospects for biological control of acid mine drainage using protozoa would appear to be somewhat remote. Other indigenous acidophilic microflora include some heterotrophic bacteria which essentially reverse the reactions of pyrite oxidation, by inducing either the dissimilatory reduction of ferric iron or of sulphate. These reactions generate net alkalinity, and also cause chalcophilic metals present in acidic effluents to precipitate as highly insoluble sulphides; they therefore have considerable potential in the development of novel bioremediation schemes for acid mine water pollution.