Production of Glycolic Acid by Chemolithotrophic Iron- and Sulfur-Oxidizing Bacteria and Its Role in Delineating and Sustaining Acidophilic Sulfide Mineral-Oxidizing Consortia

Abstract

Glycolic acid was detected as an exudate in actively growing cultures of three chemolithotrophic acidophiles that are important in biomining operations, Leptospirillum ferriphilum, Acidithiobacillus (At.) ferrooxidans, and At. caldus. Although similar concentrations of glycolic acid were found in all cases, the concentrations corresponded to ca. 24% of the total dissolved organic carbon (DOC) in cultures of L. ferriphilum but only ca. 5% of the total DOC in cultures of the two Acidithiobacillus spp. Rapid acidification (to pH 1.0) of the culture medium of At. caldus resulted in a large increase in the level of DOC, although the concentration of glycolic acid did not change in proportion. The archaeon Ferroplasma acidiphilum grew in the cell-free spent medium of At. caldus; glycolic acid was not metabolized, although other unidentified compounds in the DOC pool were metabolized. Glycolic acid exhibited levels of toxicity with 21 strains of acidophiles screened similar to those of acetic acid. The most sensitive species were chemolithotrophs (L. ferriphilum and At. ferrivorans), while the most tolerant species were chemoorganotrophs (Acidocella, Acidobacterium, and Ferroplasma species), and the ability to metabolize glycolic acid appeared to be restricted (among acidophiles) to Firmicutes (chiefly Sulfobacillus spp.). Results of this study help explain why Sulfobacillus spp. rather than other acidophiles are the main organic carbon-degrading bacteria in continuously fed stirred tanks used to bioprocess sulfide mineral concentrates and also why temporary cessation of pH control in these systems, resulting in rapid acidification, often results in a plume of the archaeon Ferroplasma.Extremely acidic environments (generally considered environments that have a pH of Ͻ3) are unusual in that chemoautotrophs rather than photoautotrophs are often the dominant (and sometimes exclusive) primary production agents (15). There are two major reasons for this: (i) inorganic electron donors (ferrous iron and reduced forms of sulfur) are often very abundant, as many of the most extremely acidic environments are in sulfur-rich (e.g., solfatara springs) or sulfide mineral-rich (e.g., metal mine wastes) locations, and (ii) in general, cyanobacteria, algae, and higher plants are more sensitive than chemotrophic bacteria and archaea to the elevated concentrations of soluble transition metals and other solutes often present in acidic waters. Lithotrophy-based primary production may be evident in subterranean chambers (caves and worked-out mines) in the form of slimes and massive "acid streamer" growths (2, 22, 30). In lower-temperature (Ͻ40°C) environments, the microorganisms that use energy derived from the oxidation of ferrous iron and/or reduced sulfur to fix CO 2 are predominantly bacteria and include a number of species (e.g., bacteria belonging to the genera Acidithiobacillus and Leptospirillum, as well as the archaeon Ferroplasma) that are known to be fundamentally important in biomining operations (35) and in the generation of ac...

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