Low-temperature formation and stabilization of rare allotropes of cyclooctasulfur (β-S8 and γ-S8) in the presence of organic carbon at a sulfur-rich glacial site in the Canadian High Arctic

Abstract

Large-scale deposits of elemental sulfur form annually on a glacier's surface at Borup Fiord Pass in the Canadian High Arctic. However, the mechanisms of mineralization and stabilization of elemental sulfur at this site are currently unknown. Here we show that x-ray diffraction (XRD) data for fresh sulfur precipitates collected from the surface of a melt pool over sulfide-rich ice reveal the presence of three sulfur allotropes, α-S 8 , β-S 8 , and γ-S 8 (the three solid forms of cyclooctasulfur (S 8 )). The detection of the β-S 8 allotrope of elemental sulfur is notable, since β-S 8 typically only forms in high temperature environments (>96°C). The γ-S 8 allotrope is also rare in natural settings and has previously been implicated as a signature of microbial sulfur cycling. Using combustion and infrared spectroscopy approaches, organic carbon is also detected within the sample bearing the three allotropes of elemental sulfur. Electron microscopy and scanning transmission x-ray microscopy (STXM) at the C K-edge show that the sulfur precipitates are intimately associated with the organic carbon at the submicron scale. The occurrence of β-S 8 and γ-S 8 in this low-temperature setting indicates that there are unknown pathways for the formation and stabilization of these rare allotropes of elemental sulfur. In particular, we infer that the occurrence of these allotropes is related to their association with organic carbon. The formation of carbon-associated sulfur globules may not be a direct by-product of microbial activity; however, a potential role of direct or indirect microbial mediation in the formation and stabilization of β-S 8 and γ-S 8 remains to be assessed. 48 of sulfide (e.g. Helz, 2014;Pjevac et al., 2014) and can also serve as a reactant for microbially 49 mediated sulfur reduction, oxidation, and disproportionation reactions (e.g. Zopfi et al., 2004; 50 Boyd and Druschel, 2013;Pjevac et al., 2014). Even though S 0 has a low solubility in water 51 (Kamyshny, 2009) and is far less reactive than sulfides, polysulfides, and other sulfur 52 intermediates, the range of environments in which S 0 is stable are limited (e.g. anoxic marine 53 settings, volcanic and fumarolic systems, and in caprock deposits overlying salt domes; Davis 54 and Kirkland, 1979;Zopfi et al., 2004;Sievert et al., 2007;and Mandeville, 2010). 55

Metagenomic Insights into Microbial Metabolisms of a Sulfur-Influenced Glacial Ecosystem

“…In addition to the cycling of sulfur identified at BFP we also searched for core metabolic processes including carbon and nitrogen metabolism. Low levels of nitrate were previously reported at BFP (0.0030 mM in 2009; Wright et al, 2013, and 0.0908 mM in 2014; Lau et al, 2017) within both spring and melt pool fluids, respectively, possibly due to microbial nitrate reduction. Based on gene presence/absence we infer that some MAGs from BFP may be capable of nitrate/nitrite reduction.…”

“…The spring system of interest is approximately 210-240 m above sea level and occurs at the toe of two coalesced glaciers (Grasby et al, 2003). The spring system is thought to be perennial in nature but discharges from different locations along the glacier from year to year (Grasby et al, 2003; Gleeson et al, 2010; Wright et al, 2013; Lau et al, 2017). However, given lack of winter observations it is not confirmed that spring flow is year-round or limited to the warmer spring and summer months.…”

“…A lateral fault approximately 100 m south of the toe of the glacier has been implicated in playing a role in the subsurface hydrology, in that this may be one of the areas where subsurface fluid flow is focused to the surface (Grasby et al, 2003; Scheidegger, et al 2012). Furthermore, organic matter in shales along this fault may be a source of carbon for subsurface microbial processes such as biological sulfate reduction (BSR; Lau et al, 2017).…”

“…Previous research at BFP has surveyed microbial activity, redox bioenergetics (Wright et al, 2013), the biomineralization of elemental sulfur (S 0 ; (Grasby et al, 2003; Gleeson et al, 2011; Lau et al, 2017), and the cryogenic carbonate vaterite (Grasby, 2003). Wright et al (2013) produced a metagenome in the context of geochemical data to constrain the bioenergetics of microbial metabolism from a mound of elemental sulfur sampled at the site in 2012, and found that at least in surface mineral deposits, sulfur oxidation was likely the dominant metabolism present among Epsilonproteobacteria .…”

Transcriptomic and proteomic insight into the mechanism of cyclooctasulfur‐ versus thiosulfate‐oxidation by the chemolithoautotroph Sulfurimonas denitrificans

“…As an essential component of biomass, sulfur is assimilated into organic compounds and is also involved in energy yielding processes either as electron acceptor or donor in chemolithautotrophic, photolithoautotrophic and heterotrophic microorganisms (Kletzin et al, 2004;Friedrich et al, 2005;Robertson and Kuenen, 2006;Sievert et al, 2007;Finster, 2008;Dahl et al, 2008;Sievert and Vetriani, 2012). In various environments, elemental sulfur accumulates in amounts visible to the naked eye, including marine sediments, microbial mats, hydrothermal vents, glacial shields, oxygen minimum zones and volcanic soils (Ruby et al, 1981;Woodruff and Shanks, 1988;Raiswell, 1992;Taylor and Wirsen, 1997;Alonso-Azcárate et al, 2001;Engel et al, 2004;Zopfi et al, 2004;Lavik et al, 2009;Cosmidis and Templeton, 2016;Lau et al, 2017). In these cases, sulfur occurs mostly in the form of zero-valence sulfur (S 0 ) with cyclooctasulfur (S 8 ) as the most stable and common form (Steudel and Eckert, 2003).…”