Evaluating the Metal Tolerance Capacity of Microbial Communities Isolated from Alberta Oil Sands Process Water

Abstract

Anthropogenic activities have resulted in the intensified use of water resources. For example, open pit bitumen extraction by Canada's oil sands operations uses an estimated volume of three barrels of water for every barrel of oil produced. The waste tailings-oil sands process water (OSPW)-are stored in holding ponds, and present an environmental concern as they are comprised of residual hydrocarbons and metals. Following the hypothesis that endogenous OSPW microbial communities have an enhanced tolerance to heavy metals, we tested the capacity of planktonic and biofilm populations from OSPW to withstand metal ion challenges, using Cupriavidus metallidurans, a known metal-resistant organism, for comparison. The toxicity of the metals toward biofilm and planktonic bacterial populations was determined by measuring the minimum biofilm inhibitory concentrations (MBICs) and planktonic minimum inhibitory concentrations (MICs) using the MBEC ™ assay. We observed that the OSPW community and C. metallidurans had similar tolerances to 22 different metals. While thiophillic elements (Te, Ag, Cd, Ni) were found to be most toxic, the OSPW consortia demonstrated higher tolerance to metals reported in tailings ponds (Al, Fe, Mo, Pb). Metal toxicity correlated with a number of physicochemical characteristics of the metals. Parameters reflecting metal-ligand affinities showed fewer and weaker correlations for the community compared to C. metallidurans, suggesting that the OSPW consortia may have developed tolerance mechanisms toward metals present in their environment.

Fig 1. Heat map analysis of relative metal toxicity to (a) the OSPW consortia and (B) C. metallidurans.

The heat map colors represent average minimum inhibitory concentrations (MIC) and minimum biofilm inhibitory concentrations (MBIC) based on average values obtained from two to nine trials, where red reflects the most toxic metals and green represents the least toxic. The Hard Soft Acid Base (HSAB) designation describes the behaviour of metal ions based on preferential donor ligands. Soft acids prefer to bind with thiol (S-group) ligands, hard acids with N and O, and borderline acids have varied preference for S, N, and O-containing ligands.

Fig 2. Linear regression analysis of MBIC values plotted against physicochemical parameters for OSPW consortia.

MBIC correlations with physicochemical parameters illustrate a typical metal susceptibility profile of the OSPW community. Parameters include: metal-sulfide solubility product (pK_sp), electronegativity (X_m), standard reduction-oxidation potentials (ΔE₀), Pearson’s softness index (σ_p), first ionization energy (I₁), and first hydrolysis constant (|Log K_OH|). Trend lines and 95% confidence bands (dashed lines) shown on linear regressions that correlate with significance.

Fig 3. Planktonic minimal inhibitory concentrations plotted against pK_sp.

Significant correlation is evident with the OSPW community (A), and absent with C. metallidurans (B). Dashed lines show 95% confidence bands.