The role of intermediate sulfur species (ISS) for isotopic fractionation processes during abiotic and chemolithoautotrophic sulfide oxidation in a natural environment
Final Report Abstract
Sulfur isotope fractionation (34S/32S) is widely used to study the chemical and biological sulfur cycle. However, for sulfide oxidation, inconsistencies exist about the extent and direction of fractionation between the two end-members sulfide and sulfate. Within the present project, we investigated how formation and transformation of intermediate sulfur species, specifically thiosulfate and arsenic (As)-sulfide complexes, so called thioarsenates (As(V)OnS4-n^3-, n = 0-3), might contribute to fractionation. Since these intermediate sulfur species cannot be precipitated separately, a method for species-selective isotope determination had to be developed. We first investigated sample enrichment via solid phase extraction (SPE) tubes, followed by separation of a 30 mL aliquot on a preparative chromatographic column with collection of chromatographic fractions which were further processed offline with H2O2 and BaCl2 to precipitate BaSO4 which was then analyzed by traditional isotope ratio mass spectrometry (IR-MS). While the SPE method itself was successfully established for stabilization and enrichment of all intermediate sulfur species in the field, the whole further process was time-consuming, prone to chromatographic problems due to the large volumes and high pressures applied and detection limits were often insufficient for natural samples. In an alternative approach, we coupled our analytical chromatographic separation to a multi-collector ICP-MS for direct online species-selective sulfur isotope determination. We confirmed that no artifact fractionation occurred during chromatographic separation and applied the new IC-MC-ICP-MS method to investigate isotope fraction during oxidative thioarsenate transformation. While tetrathioarsenate showed no fractionation during stepwise transformation to tri-, di-, and monothioarsenate, a normal isotope effect was observed for monothioarsenate. From an enrichment of up to 15 ‰, about 6 ‰ resulted from direct abiotic oxidation of monothioarsenate to sulfate while sulfide oxidation to sulfate introduced an additional enrichment of up to 9 ‰ to monothioarsenate through intermolecular isotope exchange between arsenic-bound sulfur and sulfide in solution. Besides showing that monothioarsenate contributes significantly to isotope fractionation during sulfide oxidation, we were also able to show that routine sulfide precipitation with ZnAc2 followed by IR-MS analysis will lead to co-determination of monothioarsenate. While the common recommendation is just to use “excess” ZnAc2, the extent of monothioarsenate co-precipitation is highly sensitive to the exact surplus used. IC-MC-ICP-MS was further employed to investigate the question, whether the reduced sulfur in thioarsenates can be utilized directly as an electron donor by chemolithotrophic bacteria. Incubation experiments with the hyperthermophile Thermocrinis ruber performed in this study revealed a pronounced inverse isotope effect. The combination of δ34S values observed during abiotic and biotic oxidation (sulfate enrichment +1.2 ‰ and + 4.4 ‰, respectively) indicated that arsenic-bound sulfur can in fact not be used directly as a substrate for microbial metabolism. An alternative model was proposed, in which monothioarsenate is first disproportionated abiotically to elemental sulfur and arsenite, followed by microbial oxidation of these intermediates to sulfate and arsenate. In a side project, we used offline chromatographic fraction collection and isotope determination by MC-ICP-MS to confirm previously theoretically calculated isotope fractionation between different thiomolybdates. We here used the more sensitive Mo isotopes, not the S isotopes, but the basic approach is the same. In summary, our project shows that species-selective isotope analysis, especially when dealing with so many reactive intermediate species like in the sulfur cycle, is required for a holistic understanding of processes during isotope fractionation.
Publications
- (2018) Sulfur isotope analysis by IC-MC-ICP-MS provides insight into fractionation of thioarsenates during abiotic oxidation. Chemical Geology 477 92–99
Ullrich, Maria K.; Gelman, Faina; Zakon, Yevgeni; Halicz, Ludwik; Knöller, Kay; Planer-Friedrich, Britta
(See online at https://doi.org/10.1016/j.chemgeo.2017.12.008) - A New Method for Thioarsenate Preservation in Iron- rich Waters by Solid Phase Extraction, Water Research, 102, 542‐550 (2016)
Ullrich MK, Misiari V, Planer-Friedrich B
(See online at https://doi.org/10.1016/j.watres.2016.07.008) - Experimental confirmation of isotope fractionation in thiomolybdates using ion chromatographic separation and detection by multi-collector ICP-MS, Analytical Chemistry, 89(5), 3123‐3129 (2017)
Kerl C, Vance D, Lohmayer R, Bura-Nakic E, Planer-Friedrich B
(See online at https://doi.org/10.1021/acs.analchem.6b04898)
