Effect of Organic Substrate on the Microbial Community Structure in Pilot-Scale Sulfate-Reducing Biochemical Reactors Treating Mine Drainage

Passive biological systems such as sulfate-reducing biochemical reactors have shown promise for treatment of mine drainage because of their low cost, minimal maintenance, and constructability in remote locations. However, few criteria exist for their design and operation. In particular, the impact of the choice of carbon substrate is poorly understood. This study represents the first to directly compare the effect of simple and complex organic substrate on microbial communities present in pilot-scale biochemical reactors treating mine drainage. Three organic substrates were evaluated: ethanol (ETOH), hay and pine wood chips (HYWD), and corn stover and pine wood chips (CSWD). Microbial community compositions were characterized by cloning and sequencing of 16S rRNA and apsA genes corresponding to the sulfur cycle. Quantitative polymerase chain reaction was applied to quantify Desulfovibrio-Desulfomicrobium spp. and methanogens. Results revealed differences in microbial compositions and relative quantities of total and sulfate-reducing bacteria among reactors. Notably, the greatest proportion of sulfate-reducing bacteria was observed in the ETOH reactors. HYWD and CSWD reactors contained similar bacterial communities, which were highly complex in composition relative to the ETOH reactors. Methanogens were found to be present in all reactors at low levels and were highest in the lignocellulose-based reactors. Interestingly, higher proportions of aerobic Thiobacillus spp. were detected in two reactors that experienced an oxygen exposure during operation. This study demonstrates that both substrate and environmental stress influence both microbial community composition and diversity in biochemical reactors treating mine drainage. While there were no significant differences in performance observed over the time scale of this study, potential long-term implications of the differing microbial communities on performance are discussed.