Microbiology of Two Sulfate-Reducing Field Bioreactors Used for the Remediation of Mining Influenced Water

Sulfate-reducing permeable reactive zones (SR-PRZs), such as anaerobic wetlands, sulfate-reducing bioreactors, and permeable reactive barriers, rely on microbially-mediated processes to treat mining influenced waters (MIWs) contaminated with heavy metals. The remediation is accomplished through the precipitation of metal cations with sulfide produced by sulfate-reducing bacteria (SRB). The microbial communities in SR-PRZs are very complex and include not only SRB but also cellulose degraders and fermenters that provide SRB with the substrates necessary for their growth. Even though microorganisms are the driving force in SR-PRZs, little is known about the microbiology of these passive mine drainage treatment systems. We have recently developed a suite of molecular biology tools in our laboratory for characterizing the microbial communities present in SR-PRZs. In this study our suite of tools is used to characterize two different field bioreactors: Peerless Jenny King and Luttrell, near Helena, MT. Both bioreactors employ a compost-based substrate to promote the growth of SRB. The two bioreactors were sampled at multiple locations and depths in June, 2005, and Peerless Jenny King was sampled again in August, 2005. Cloning of polymerase chain reaction (PCR) amplified 16S rRNA genes followed by restriction digest screening and DNA sequencing provided insight regarding the overall composition of the microbial communities. To directly examine the SRB populations, two approaches were used. In the first approach, a gene specific to SRB, ApsA, was PCR-amplified, cloned, and sequenced. This revealed that two main SRB groups are prevalent in both Luttrell and Peerless Jenny King, both of which are Desulfovibrio spp, and one of which is noted to be particularly aerotolerant. This analysis also revealed the presence of Thiobacillus denitrificans, an organism that oxidizes sulfides in the presence of nitrate or oxygen, which has been considered highly undesirable for bioreactor function. In the second approach, quantitative real-time PCR (Q-PCR) was used to quantify two specific groups of SRB, Desulfovibrio and Desulfobacter. These two SRB groups were found to vary in distribution between the two bioreactors and with depth. The results of this study indicate that two different SR-PRZs also have two very different microbial communities. The ultimate goal will be to develop an improved understanding of the microbiology of SR-PRZs in order to improve design and operation of these treatment systems.