Beyond SOx: Mining Sulfur Bacteria Produce Less Acid due to Unidentified Enzyme Pathways

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The study used 6 L benchtop microcosms inoculated with Halothiobacillus-dominated sulfur-oxidizing bacterial communities preserved from an active tailing reservoir, and exposed them to high concentrations of sulfur oxidation intermediates (thiosulfate or tetrathionate) under oxic versus suboxic conditions to investigate acid-generating processes. The authors found that thiosulfate and tetrathionate processing involved Halothiobacillus, Thiomonas, and Pandoraea, and that measured proton yields from thiosulfate oxidation were less than half of those predicted by known complete sulfur oxidation expectations, even after accounting for ion activity and phosphate buffering—suggesting additional, unidentified mechanisms beyond cSOx. They also reported that tetrathionate under oxic conditions likely triggered undescribed reactions not attributable to known sulfur enzymes, and that suboxic thiosulfate depletion occurred without nitrate/nitrite changes sufficient to identify their use as terminal electron acceptors, requiring further work. This paper does not explicitly discuss endometriosis or adenomyosis; it was included in the corpus via a keyword match in the upstream search index.

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Abstract

When thiosulfate is abundant, recent research indicates that the complete Sulfur Oxidation (cSOx) pathway is prevalent in oxic wastewater, while the incomplete Sulfur Oxidation and reverse Dissimilarity Sulfur Reduction (iSOx + rDSR) pathway has been found to be active under suboxic conditions, paired with nitrate reduction. However, both the activity of S 4 I (or other enzyme systems) and the suboxic sulfur reactions which occur when the rDSR pathway is not present in an SOB community remain unexplored. During this study, a series of 6 L benchtop microcosms were inoculated with Halothiobacillus spp. -dominated (containing cSOx and S 4 I pathways) sulfur oxidizing bacterial (SOB) communities preserved from an active tailing reservoir. The 16 microcosms were provided with high concentrations of the sulfur oxidation intermediate (SOI) compounds thiosulfate or tetrathionate under oxic and suboxic conditions to amplify the acid generating processes. The study identifies Halothiobacillus , Thiomonas, and Pandoraea , as the key SOB genera processing SOI. These microcosms demonstrate that protons yields resulting from thiosulfate oxidation by SOB are less than half of those predicted for thiosulfate oxidation, amongst the lowest reported for this genera. Despite adjustments for ion activity and PO 4 2- buffering, this difference is indicative of mechanisms beyond cSOx. Further, when offered tetrathionate under oxic conditions, undescribed reactions unmatched to known sulfur enzymes were likely employed, since disproportionation is insufficient to explain observations. Meanwhile, suboxic depletion of thiosulfate, without changes in nitrate or nitrite concentrations sufficient to indicate their use as TEA requires further investigation.
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Abstract When thiosulfate is abundant, recent research indicates that the complete Sulfur Oxidation (cSOx) pathway is prevalent in oxic wastewater, while the incomplete Sulfur Oxidation and reverse Dissimilarity Sulfur Reduction (iSOx + rDSR) pathway has been found to be active under suboxic conditions, paired with nitrate reduction. However, both the activity of S4I (or other enzyme systems) and the suboxic sulfur reactions which occur when the rDSR pathway is not present in an SOB community remain unexplored. During this study, a series of 6 L benchtop microcosms were inoculated with Halothiobacillus spp.-dominated (containing cSOx and S4I pathways) sulfur oxidizing bacterial (SOB) communities preserved from an active tailing reservoir. The 16 microcosms were provided with high concentrations of the sulfur oxidation intermediate (SOI) compounds thiosulfate or tetrathionate under oxic and suboxic conditions to amplify the acid generating processes. The study identifies Halothiobacillus, Thiomonas, and Pandoraea, as the key SOB genera processing SOI. These microcosms demonstrate that protons yields resulting from thiosulfate oxidation by SOB are less than half of those predicted for thiosulfate oxidation, amongst the lowest reported for this genera. Despite adjustments for ion activity and PO42- buffering, this difference is indicative of mechanisms beyond cSOx. Further, when offered tetrathionate under oxic conditions, undescribed reactions unmatched to known sulfur enzymes were likely employed, since disproportionation is insufficient to explain observations. Meanwhile, suboxic depletion of thiosulfate, without changes in nitrate or nitrite concentrations sufficient to indicate their use as TEA requires further investigation. Competing Interest Statement The authors have declared no competing interest.

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