pH and thiosulfate dependent microbial sulfur oxidation strategies across diverse environments.

Sulfur oxidizing bacteria (SOB) play a key role in sulfur cycling in mine tailings impoundment (TI) waters, where sulfur concentrations are typically high. However, our understanding of SOB sulfur cycling via potential S oxidation pathways (sox, rdsr, and S4I) in these globally ubiquitous contexts, remains limited. Here, we identified TI water column SOB community composition, metagenomics derived metabolic repertoires, physicochemistry, and aqueous sulfur concentration and speciation in four Canadian base metal mine, circumneutral-alkaline TIs over four years (2016 - 2019). Identification and examination of genomes from nine SOB genera occurring in these TI waters revealed two pH partitioned, metabolically distinct groups, which differentially influenced acid generation and sulfur speciation. Complete sox (csox) dominant SOB (e.g., Halothiobacillus spp., Thiomonas spp.) drove acidity generation and S2O3 2- consumption via the csox pathway at lower pH (pH ~5 to ~6.5). At circumneutral pH conditions (pH ~6.5 to ~8.5), the presence of non-csox dominant SOB (hosting the incomplete sox, rdsr, and/or other S oxidation reactions; e.g. Thiobacillus spp., Sulfuriferula spp.) were associated with higher [S2O3 2-] and limited acidity generation. The S4I pathway part 1 (tsdA; S2O3 2- to S4O6 2-), was not constrained by pH, while S4I pathway part 2 (S4O6 2- disproportionation via tetH) was limited to Thiobacillus spp. and thus circumneutral pH values. Comparative analysis of low, natural (e.g., hydrothermal vents and sulfur hot springs) and high (e.g., Zn, Cu, Pb/Zn, and Ni tailings) sulfur systems literature data with these TI results, reveals a distinct TI SOB mining microbiome, characterized by elevated abundances of csox dominant SOB, likely sustained by continuous replenishment of sulfur species through tailings or mining impacted water additions. Our results indicate that under the primarily oxic conditions in these systems, S2O3 2- availability plays a key role in determining the dominant sulfur oxidation pathways and associated geochemical and physicochemical outcomes, highlighting the potential for biological management of mining impacted waters via pH and [S2O3 2-] manipulation.

Cascading sulfur cycling in simulated oil sands pit lake water cap mesocosms transitioning from oxic to euxinic conditions.

Base Mine Lake (BML), the first full-scale demonstration of oil sands tailings pit lake reclamation technology, is experiencing expansive, episodic hypolimnetic euxinia resulting in greater sulfur biogeochemical cycling within the water cap. Here, Fluid Fine Tailings (FFT)-water mesocosm experiments simulating the in situ BML summer hypolimnetic oxic-euxinic transition determined sulfur biogeochemical processes and their controlling factors. While mesocosm water caps without FFT amendments experienced limited geochemical and microbial changes during the experimental period, FFT-amended mesocosm water caps evidenced three successive stages of S speciation in ∼30 days: (S1) rising expansion of water cap euxinia from FFT to water surface; enabling (S2) rapid sulfate (SO42-) reduction and sulfide production directly within the water column; fostering (S3) generation and subsequent consumption of sulfur oxidation intermediate compounds (SOI). Identified key SOI, elemental S and thiosulfate, support subsequent SOI oxidation, reduction, and/or disproportionation processes in the system. Dominant water cap microbes shifted from methanotrophs and denitrifying/iron-reducing bacteria to functionally versatile sulfur-reducing bacteria (SRB) comprising sulfate-reducing bacteria (Desulfovibrionales) and SOI-reducing/disproportionating bacteria (Campylobacterales and Desulfobulbales). The observed microbial shift is driven by decreasing [SO42-] and organic aromaticity, with putative hydrocarbon-degrading bacteria providing electron donors for SRB. Comparison between unsterile and sterile water treatments further underscores the biogeochemical readiness of the in situ water cap to enhance oxidant depletion, euxinia expansion and establishment of water cap SRB communities aided by FFT migration of anaerobes. Results here identify the collective influence of FFT and water cap microbial communities on water cap euxinia expansion associated with sequential S reactions that are controlled by concentrations of oxidants, labile organic substrates and S species. This emphasizes the necessity of understanding this complex S cycling in assessing BML water cap O2 persistence.

O2 partitioning of sulfur oxidizing bacteria drives acidity and thiosulfate distributions in mining waters.

The acidification of water in mining areas is a global environmental issue primarily catalyzed by sulfur-oxidizing bacteria (SOB). Little is known about microbial sulfur cycling in circumneutral pH mine tailing impoundment waters. Here we investigate biological sulfur oxidation over four years in a mine tailings impoundment water cap, integrating aqueous sulfur geochemistry, genome-resolved metagenomics and metatranscriptomics. The microbial community is consistently dominated by neutrophilic, chemolithoautotrophic SOB (relative abundances of ~76% in 2015, ~55% in 2016/2017 and ~60% in 2018). Results reveal two SOB strategies alternately dominate across the four years, influencing acid generation and sulfur speciation. Under oxic conditions, novel Halothiobacillus drive lower pH conditions (as low as 4.3) and lower [S2O32-] via the complete Sox pathway coupled to O2. Under anoxic conditions, Thiobacillus spp. dominate in activity, via the incomplete Sox and rDSR pathways coupled to NO3-, resulting in higher [S2O32-] and no net significant acidity generation. This study provides genomic evidence explaining acidity generation and thiosulfate accumulation patterns in a circumneutral mine tailing impoundment and has significant environmental applications in preventing the discharge of sulfur compounds that can impact downstream environments. These insights illuminate opportunities for in situ biotreatment of reduced sulfur compounds and prediction of acidification events using gene-based monitoring and in situ RNA detection.