Editorial: Recent Advances in Acidophile Microbiology: Fundamentals and Applications

Acidophilic microorganisms thrive in extremely low pH natural and man-made environments such as acidic lakes, some hydrothermal systems, acid sulfate soils, sulfidic regoliths and ores, as well as metal and coal mine-impacted environments. The most widely studied acidophiles, prokaryotes that oxidize reduced iron and/or sulfur, are able to catalyze the oxidative dissolution of metal sulfide minerals such as pyrite (FeS 2), thereby severely acidify the environment (often to pH <3) in which they thrive. At such low pH values the ferric iron generated by their activities is soluble and serves as the chemical oxidant of sulfide minerals. One the one hand, this is highly beneficial, and is the core process in the biotechnology known generically as " biomining, " where acidophiles are used to facilitate the extraction and recovery of base (e.g., copper, cobalt, nickel, and zinc) and precious metals (principally gold), and also uranium. On the other hand, uncontrolled microbial metal sulfide oxidation in abandoned mines and mine spoils can generate highly noxious waste-waters (acid mine/rock drainage) which, because of their low pH, elevated concentrations of potential toxic metals and metalloids, and high osmotic potentials, pose severe threats to the environment. Recent research, however, has shown that some species of acidophilic microorganisms could also be used not only to mitigate mine water pollution but also to recover metals from acidic waste-waters via selective biomineralization. This Research Topic issue comprises 10 original research articles and presents novel data on molecular/ genomic, biochemical, physiological, and applied aspects of acidophilic prokaryotes. These extremophiles may be divided into " extreme acidophiles, " which have pH growth optima at or below pH 3.0, moderate acidophiles, which grow optimally between pH 3.0 and 5.0, and acid-tolerant species which grow optimally above pH 5.0, but which also grow reasonably well at lower pH values. Eight of the papers in this Research Topic focus on extreme acidophiles, and most of these describe advances in our knowledge and understanding of the most widely researched class of acidophiles, the Acidothiobacillia. New insights into the phylogenetic structure and diversification of Acidithiobacillus species revealed by combining analyses of 16S rRNA gene-based ribotyping, oligotyping, and multi-locus sequencing analysis (MLSA) is described in the report of Nuñez et al., who investigated 580 strains of the seven recognized species of the genus (Acidithiobacillus thiooxidans, A. ferrooxidans, A. albertensis, A. caldus, A. ferrivorans, A. ferridurans, and A. ferriphilus) in their study. Another paper describes how …