Microbial Sulfur Pathways and Outcomes in Tailings Impoundments: A Mesocosm Study

In mine wastewaters, three microbial sulfur oxidation pathways have the potential to cause different water quality outcomes. These outcomes can differ from abiotic models of sulfate and acidity predictions currently used to monitor potential sulfur risks. However, studies integrating microbiology and geochemistry in active mine tailings impoundments are very limited. Here, we developed a novel diagnostic approach to detect microbially driven sulfur pathways. Within this 28-day study, eight on-site, 500 L mesocosms were filled with water extracted directly from the water cap of an active Ni/Cu mine tailings impoundment. Diverse combinations of tailings, sulfur compounds, and nitrate amendments were added to the mesocosms simulating common operational variations experienced by active tailings impoundments. Mesocosm results linked complete SOx, S4I, and incomplete SOx + rDSR pathway occurrence (metagenomes, inferred from the identity, i.e. 16S rRNA) and activity (mRNA) to physiochemistry and sulfur geochemistry. By integrating the three lines of evidence, the diagnostic approach was able to identify which sulfur pathways were active under varying physiochemical conditions and how geochemical outcomes were affected. A relationship emerged between acid generation and soxCD expression (soxCD expression indicates the complete SOx pathway activity). However, observed proton yields and sulfate concentrations were less than those predicted by complete SOx pathway activity alone. This indicates other sulfur pathways, e.g. the partial S4I pathway (within Thiomonas and Halothiobacillus), and/or activity of the incomplete SOx pathway (within Thiobacillus and Desulfurivibrio) when either not coupled to rDSR, or paired with use of nitrate, influenced overall sulfur outcomes along with the complete SOx pathway.