The role of intermediate sulfur species (ISS) for isotopic fractionation processes during abiotic and chemolithoautotrophic sulfide oxidation in a natural environment

自20世纪40年代末以来，硫同位素分馏(34S/32S)被用于研究化学和生物硫循环。虽然细菌硫酸盐还原过程中的大同位素分馏被成功地用来解释古代海洋中硫酸盐的积累或早期生命的演化，但对硫化物氧化过程中的分馏作用却知之甚少。硫化物和硫酸盐的两个端元之间的分馏通常要小得多，无论是底物还是产物都存在不一致。这些不一致的原因是缺乏对氧化途径和速率以及中间硫物种的了解，如元素硫、多硫化物、硫代硫酸盐、亚硫酸盐或金属硫化物络合物(如硫代砷酸盐)，它们可能起到34S汇的作用。在拟议的项目中，我们将开发一种基于制备色谱分离的硫物种选择性同位素分析方法。SN2-和S0的分离将在C18柱上用三氟化甲酯衍生化，在AS16柱上分离碱性洗脱液中的其他硫物种。收集的馏分中的硫将直接用活性铜片(Sn2-，S0)提取，或以ZnS(S2-)或BaSO4的形式沉淀，然后以SO2的形式进行常规分析。这种物种选择方法的结果将与以前的端员池测定和顺序沉淀技术的结果进行比较。该方法将应用于美国黄石国家公园中性到碱性温泉的硫化物氧化剖面，在那里我们发现中间硫物种是重要的物种。只测定硫化物和硫酸盐中的34S/32S，我们先前的研究显示了两个硫化物氧化剖面的温泉排水的不同分馏模式，从地球化学角度看似乎相似。造成不同同位素趋势的原因尚不清楚。在本项目中，我们将以化学结石型硫氧化细菌Thermocrinis ruber为模式生物，通过现场培养实验区分物种选择性非生物分馏和生物分馏。对于选定的样品，我们将测试33S和36S是否进一步阐明物种选择性硫化物氧化模式。我们预计，较低的源硫化物浓度会增加元素硫的歧化，从而增加氧化还原循环和同位素分馏。我们还预计，包括硫代砷酸盐在内的中间硫物种的浓度越大，同位素分馏就越大。在物种选择性池中进行分馏后，我们将能够澄清之前报道的底物或产物中34S浓缩的不一致之处，阐明硫化物的氧化途径和速率，并揭示硫代谢的细节。我们的新方法和现场数据将为未来更一致的硫化物氧化研究奠定基础。

Sulfur isotope fractionation (34S/32S) has been used since the late 1940s to study the chemical and biological sulfur cycle. While large isotope fractionations during bacterial sulfate reduction were used successfully to interpret, e.g., accumulation of sulfate in ancient oceans or the evolution of early life, much less is known about fractionation during sulfide oxidation. The fractionation between the two end-members sulfide and sulfate is commonly much smaller and inconsistencies exist whether substrate or product are enriched. These inconsistencies are explained by a lack of knowledge on oxidation pathways and rates as well as intermediate sulfur species, such as elemental sulfur, polysulfides, thiosulfate, sulfite, or metalloid-sulfide complexes (e.g. thioarsenates), potentially acting as 34S sinks.In the proposed project, we will develop a method for sulfur species-selective isotope analysis based on separation by preparative chromatography. Separation of Sn2- and S0 will be achieved after derivatization with methyl triflate on a C18 column, separation of the other sulfur species in an alkaline eluent on an AS16 column. Sulfur in the collected fractions will be extracted directly with activated copper chips (Sn2-, S0), or precipitated as ZnS (S2-) or BaSO4 and analyzed by routine methods as SO2. Results of this species-selective approach will be compared to those from previous techniques of end-member pool determinations and sequential precipitations.The method will be applied to sulfide oxidation profiles at neutral to alkaline hot springs at Yellowstone National Park, USA, where we detected intermediate sulfur species as important species. Determining 34S/32S only in sulfide and sulfate, our previous study has shown different fractionation patterns for two hot spring drainages with sulfide oxidation profiles that seemed similar from a geochemical perspective. The reasons for the different isotopic trends are unclear. In the present project, we will differentiate species-selective abiotic versus biotic fractionation using on-site incubation experiments with the chemolithotrophic sulfur-oxidizing bacteria Thermocrinis ruber as model organism. For selected samples, we will test whether 33S and 36S further elucidate species-selective sulfide oxidation patterns. We expect that lower source sulfide concentrations increase elemental sulfur disproportionation, thus increase redox cycling and isotope fractionation. We also expect that the larger the concentration of intermediate sulfur species, including thioarsenates, the larger the isotope fractionation. Following fractionation in species-selective pools, we will be able to clarify previously reported inconsistencies of 34S enrichment in substrate or product, elucidate sulfide oxidation pathways and rates, and reveal details about sulfur metabolism. Our new methodology and field-based data will be a basis for more consistent studies on sulfide oxidation in the future.