Bioenergetics parameters and transport in obligate acidophiles

Abstract

Obligate acidophiles, which grow optimally in the pH range of 2-4, are an intriguing group of organisms. They span the taxonomic and physiological groupings from archaebacteria (e.g., Sulfolobus acidocaldarius) to eukaryotes (e.g., Cyanidium caldarius) and strict autotrophs to strict heterotrophs. The acidic environments they inhabit abound: vast areas of low-lying soils, mining regions, diverse aquatic environments, and sulfur rich geothermal areas [1]. The acidophilic bacteria have considerable economic importance in bioremediation of environmental acid pollution [2], biodesulfurization of fossil fuels [3], and biomining [3,4]. The latter technologies depend on biological activities possible only at very low pH values. The most striking bioenergetic characteristic of acidophiles is their ability to maintain a large pH gradient, even under highly stressful conditions of pH and nutrition. Thus, these organisms possess to an exquisite degree a vital property of all living cells, namely the ability to maintain a different ionic milieu compared to the external environment. This magnified perspective makes it likely that the relevant mechanisms would be more easily amenable to study and dissection in these bacteria. The acidophilic way of life, since it depends on the existence of pH gradients, is also of interest in the context of evolution of chemiosmotic mechanisms.