Abstract
Elemental sulfur (S(0)) is an important intermediate of the sulfur cycle and is generated by chemical and biological sulfide oxidation. Raman spectromicroscopy can be applied to environmental samples for the detection of S(0), as a practical non-destructive micron-scale method for use on wet material and living cells. Technical advances in filter materials enable the acquisition of ultra-low frequency (ULF) Raman measurements in the 10–100 cm−1 range using a single-stage spectrometer. Here we demonstrate the potency of ULF Raman spectromicroscopy to harness the external vibrational modes of previously unrecognized S(0) structures present in environmental samples. We investigate the chemical and structural nature of intracellular S(0) granules stored within environmental mats of sulfur-oxidizing γ-Proteobacteria (Thiothrix). In vivo intracellular ULF scans indicate the presence of amorphous cyclooctasulfur (S8), clarifying enduring uncertainties regarding the content of microbial sulfur storage globules. Raman scattering of extracellular sulfur clusters in Thiothrix mats furthermore reveals an unexpected abundance of metastable β-S8 and γ-S8, in addition to the stable α-S8 allotrope. We propose ULF Raman spectroscopy as a powerful method for the micron-scale determination of S(0) structure in natural and laboratory systems, with a promising potential to shine new light on environmental microbial and chemical sulfur cycling mechanisms.
Highlights
Elemental sulfur S(0) is prevalent across a diverse number of sulfide-rich environments from geothermal[1] and cold sulfidic springs[2], deep-sea hydrothermal vents[3], to salt marshes[4], marine sediments[5,6], pyroclastic ash emissions[7], mangrove swamps[8], deep-sea hydrocarbon seeps[9], and waste-water treatment plants[10]
The present study highlights the advantages of using low-frequency Raman in the detection and characterization of elemental sulfur in microbial and environmental samples
This work supplements the limited literature on sulfur Raman external modes[33,34,35,44], and presents the first low-frequency Raman spectrum of γ-S8 published so far
Summary
Elemental sulfur S(0) is prevalent across a diverse number of sulfide-rich environments from geothermal[1] and cold sulfidic springs[2], deep-sea hydrothermal vents[3], to salt marshes[4], marine sediments[5,6], pyroclastic ash emissions[7], mangrove swamps[8], deep-sea hydrocarbon seeps[9], and waste-water treatment plants[10]. The allotropic enantiotropy of S8, its ability to catenate[16], and many possible reduction or oxidation reactions make sulfur a difficult material for analysis These difficulties have so far prevented a consensus on the speciation and structure of biological S(0) stored as intracellular globules in bacteria, which has been described as “S8 rings”17, “solid S8”18, “microcrystalline S8”19, “liquid sulfur”20, “a mixture of polysulfides and cyclooctasulfur”[21], and sulfur chains associated with “unidentified organic residues”[22]. A series of publications has debated the application of synchrotron-based X-ray absorption near edge structure spectroscopy (XANES) at the sulfur K-edge to investigate the nature of intracellular sulfur globules[17,18,22,23,24] Shortcomings of this method include possible distortions of the XANES spectra due to experimental artifacts[17], the inability to discriminate different crystal structures of S8, and the infeasibility of in vivo measurements. Characteristic internal vibrational (molecular) spectra make S(0) easy to detect and characterize with Raman scattering[9,19,21,25,26,27]
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