Biodegradation of thiocyanate by a native groundwater microbial consortium

Abstract

Gold ore processing typically generates large amounts of thiocyanate (SCN^-)-contaminated effluent. When this effluent is stored in unlined tailings dams, contamination of the underlying aquifer can occur. The potential for bioremediation of SCN^--contaminated groundwater, either in situ or ex situ, remains largely unexplored. This study aimed to enrich and characterise SCN^--degrading microorganisms from mining-contaminated groundwater under a range of culturing conditions. Mildly acidic and suboxic groundwater, containing ∼135 mg L^-1 SCN^-, was collected from an aquifer below an unlined tailings dam. An SCN^--degrading consortium was enriched from contaminated groundwater using combinatory amendments of air, glucose and phosphate. Biodegradation occurred in all oxic cultures, except with the sole addition of glucose, but was inhibited by NH₄ ⁺ and did not occur under anoxic conditions. The SCN^--degrading consortium was characterised using 16S and 18S rRNA gene sequencing, identifying a variety of heterotrophic taxa in addition to sulphur-oxidising bacteria. Interestingly, few recognised SCN^--degrading taxa were identified in significant abundance. These results provide both proof-of-concept and the required conditions for biostimulation of SCN^- degradation in groundwater by native aquifer microorganisms.

Keywords: Bioremediation; Contamination; Geomicrobiology; Groundwater; Microorganisms; Mining; Sulphurs-oxidation; Thiocyanate.

Figure 1. Concentration profiles of SCN⁻ (red) and NH₄⁺ (green) in SCN⁻ metabolising culturing experiments.

The profiles are shown alongside OD₆₀₀measurement (blue) during SCN⁻ removal from filter sterilised groundwater, inoculated with the groundwater culture enriched by addition of DOC and PO₄³⁻. Error bars are equal to 1 standard deviation within each triplicate.

Figure 2. Concentration profile of SCN⁻ and NH₄⁺ in groundwater SCN⁻ biodegradation culturing experiments.

Profiles represent the inoculated (with the DOC and PO₄³⁻ enriched community) filter sterilised groundwater, in the presence of increasing NH₄⁺ concentrations. Error bars are equal to 1 standard deviation within triplicates of each experiment.

Figure 3. SCN⁻—degradation profiles with increasing NH₄⁺ concentrations by the enriched microbial consortium.

(A) With unamended NH₄⁺ levels, (B) with moderate NH₄⁺ levels, and (C) with high NH₄⁺ levels.

Figure 4. The relative abundance of 16S rRNA gene sequence assignments from the Greengenes database (A) and 18S rRNA gene sequence assignments from the SILVA database (B).

Relative abundances are shown for the re-cultured groundwater community amended with DOC and PO₄³⁻. 16S rRNA gene Taxonomic assignments are defined at the phylum (inner circle), family (middle) and genus (outer) levels, while 18S rRNA gene taxonomic assignments are classified at the phylum (inner), class (middle) and species (outer) levels. Classified taxa comprising ≥1% total abundance are labelled, and grey areas represent unclassified taxa.