Microbial community structure within a weathered waste-rock pile overlain by a monolayer soil cover

Abstract

A historic waste-rock stockpile (WRS) at the Detour Lake Mine (DLM), covered with a thin layer (<1 m) of local overburden, was studied to determine the potential for microbially-mediated generation of acid rock drainage (ARD). The sulfur content of the waste rock ranged from 0 to 2.2 wt %, with pyrite and pyrrhotite identified as the principal sulfide minerals. Acidity released through sulfide oxidation in the WRS has been neutralized through carbonate mineral dissolution, and has resulted in the generation of neutral mine drainage (pH 6–8). However, the WRS is heterogeneous, and localized water samples collected from discrete zones within the stockpile were more highly oxidized and acidic (i.e., pH ≥ 2.5). Enumerations of acidophilic sulfur- (aSOM) and iron-oxidizing microorganisms (aIOM) were performed, yielding mean abundances of 1.2 × 103 and 9.0 × 105 cells g−1, respectively. The mean abundance of neutrophilic sulfur-oxidizing microorganisms (nSOM) was 5.5 × 105 cells g−1. Fungi and bacteria present in the waste rock were identified using high-throughput amplicon sequencing of 18S and 16S rRNA genes, respectively. Sequencing confirmed the presence of Thiobacillus and Acidithiobacillus species. Bacterial diversity was greatest in samples from the cover material. Unoxidized waste rock samples were characterized by neutrophilic iron- or sulfur-oxidizing genera (i.e., Thiobacillus), whereas samples collected from oxidized and acidic zones in the WRS showed greater abundances of acidophilic taxa (i.e., Acidithiobacillus). None of the fungal genera identified in this study have been shown to oxidize sulfide minerals directly, other than indirectly through creation of a suitable environment for the prokaryotes involved in the processes. Although installation of a simple, non-engineered cover is anticipated to have slowed the generation of ARD, evidence of ongoing sulfide oxidation within the covered WRS was observed. High abundances and activities of sulfur- and iron-oxidizing microorganisms indicate that the soil cover has not prevented the growth of microorganisms that catalyze sulfide-mineral oxidation.