Biodiversity and ecology of acidophilic microorganisms

Microbial life in extremely low pH (< 3) natural and man-made environments may be considerably diverse. Prokaryotic acidophiles (eubacteria and archae a) have been the focus of much of the research activity in this area, prima rily because of the importance of these microorganisms in biotechnology (pr edominantly the commercial biological processing of metal ores) and in envi ronmental pollution (genesis of 'acid mine drainage'); however, obligately acidophilic eukaryotes (fungi, yeasts, algae and protozoa) are also known, and may form stable microbial communities with prokaryotes, particularly in lower temperature (< 35 degrees C) environments. Primary production in aci dophilic environments is mediated by chemolitho-autotrophic prokaryotes (ir on and sulfur oxidisers), and may be supplemented by phototrophic acidophil es (predominantly eukaryotic microalgae) in illuminated sites. The most the rmophilic acidophiles are archaea (Crenarchaeota) whilst in moderately ther mal (40-60 degrees C) acidic environments archaea (Euryarchaeota) and bacte ria (mostly Gram-positives) may co-exist. Lower temperature (mesophilic) ex tremely acidic environments tend to be dominated by Gram-negative bacteria, and there is recent evidence that mineral oxidation may be accelerated by acidophilic bacteria at very low (ca. 0 degrees C) environments. Whilst mos t acidophiles have conventionally been considered to be obligately aerobic, there is increasing evidence that many isolates are facultative anaerobes, and are able to couple the oxidation of organic or inorganic electron dono rs to the reduction of ferric iron. PI variety of interactions have been de monstrated to occur between acidophilic microorganisms, as in other environ ments; these include competition, predation, mutualism and synergy. Mixed c ultures of acidophiles are frequently more robust and efficient (e.g. in ox idising sulfide minerals) than corresponding pure cultures. In view of the continuing expansion of microbial mineral processing ('biomining') as a cos t-effective and environmentally sensitive method of metal extraction, and t he ongoing concern of pollution from abandoned mine sites, acidophilic micr obiology will continue to be of considerable research interest well into th e new millennium. (C) 1998 Federation of European Microbiological Societies . Published by Elsevier Science B.V. All rights reserved.