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.

FIG. 1.

Changes in concentrations of dissolved organic carbon (DOC) and glycolic acid in bioreactor cultures of (a) L. ferriphilum, (b) At. ferrooxidans, and (c) At. caldus. Symbols: •, total soluble iron concentration (L. ferriphilum); ○, sulfate S concentration (At. ferrooxidans and At. caldus); ▪, DOC concentration; ⧫, glycolic acid concentration (colorimetric assay); ▵, total number of cells. The arrows indicate the time at which the “pH shock” (rapid lowering of the bioreactor pH to 1.0) was imposed.

FIG. 2.

Changes in total numbers (solid bars) and viable counts (cross-hatched bars) of (a) L. ferriphilum, (b) At. ferrooxidans, and (c) At. caldus immediately before (day 0) and 1 and 2 days following acidification of the bioreactors to pH 1.0.

FIG. 3.

Growth of Fp. acidiphilum on spent At. caldus medium. Symbols: ▾, Fe²⁺ concentration; ▪, DOC concentration; ▴, glycolic acid concentration; ○, number of cells. The symbols indicate means for duplicate cultures, and the error bars indicate ranges.

FIG. 4.

Degradation of glycolic acid by acidophilic Firmicutes. Symbols: ▴, S. thermosulfidooxidans strain TH1; ▪, S. acidophilus strain YTF1; ▾, S. benefaciens type strain; •, isolate SLC1; ○, noninoculated control. The symbols indicate the mean glycolic acid concentrations for duplicate cultures, and the error bars (where visible) indicate the ranges.