Organic sulfones in the brine of Lake Vida, East Antarctica

Abstract

Located in a closed basin in the McMurdo Dry Valleys, East Antarctica, Lake Vida brine is a cold (−13 °C), hypersaline, interstitial, anoxic, and aphotic ecosystem trapped within 27+ m of ice. This brine is not in contact with the atmosphere, and currently hosts a slow-growing, cold-limited bacteria-dominated ecosystem that have persisted for at least ∼2800 years. Analysis of the dichloromethane extractable fraction of dissolved organic matter of Lake Vida brine revealed the presence of seven novel organic sulfones, tentatively identified on the basis of mass spectral fragmentation, as well as eight sulfones that have been structurally described in previous chemistry studies. A total of fifteen organic sulfones, and others that have yet to be identified, were observed in Lake Vida brine, most of which have never been previously detected in any other natural ecosystems. Results suggest that these compounds may be derived from a lake system in which a dissolved organic carbon pool may have been oxidized in lake waters in contact with the atmosphere. Alternative hypotheses on the origin of these sulfones, involving potential abiotic alterations of dissolved organic sulfur as a consequence of long-term brine-rock reactions that generated reactive oxygen species, are also considered. Understanding the origin and formation mechanisms of these organic sulfones may reveal an important pathway that influence the dissolved organic sulfur constituent, not only of Antarctic aquatic systems, but also potentially the global marine environment, as well as extraterrestrial hypersaline, subzero habitats.

Introduction

Dissolved organic sulfur (DOS) plays an important role in the sulfur cycle and can be the main source of sulfur for biological growth in aquatic environments (Kertesz, 1999, Tripp et al., 2008, Ksionzek et al., 2016). Not only is sulfur an essential element in the amino acids, cysteine and methionine, it is also found in sulfolipids, vital enzyme cofactors such as biotin, coenzyme A, coenzyme M, lipoic acid, S-adenosyl methionine, and iron-sulfur clusters. The reduction of oxidized inorganic sulfur such as sulfate (SO₄²⁻) and sulfite (SO₃²⁻) can be either assimilatory, in which organic sulfur compounds (OSCs) such as cysteine and methionine are synthesized by organisms using energy, or dissimilatory, in which electron acceptors like SO₄²⁻ are reduced to yield energy while forming reduced byproducts such as H₂S. Aside from being incorporated in metal sulfides such as pyrite, H₂S can also be methylated to form methanethiol (CH₃SH) or dimethylsulfide (DMS), through both biological and abiotic processes (Drotar et al., 1987, Schulz and Dickschat, 2007, Sela-Adler et al., 2015).

Though our understanding of the sulfur cycle has increased significantly in the last few decades, partially due to the recognized role of DMS in the global sulfur cycle, little is known about the chemical pathway between biosynthesized OSCs and recalcitrant dissolved organic sulfur (DOS) that may persist in aquatic systems or may eventually be deposited in sedimentary environments. This DOS chemical inventory hold critical information on the complex interactions between the pathways through which organic sulfur compounds are biologically utilized, and the pathways through which volatile sulfur compounds are released into the atmosphere, the latter being crucial for climate regulation. Marine phytoplankton play a role in both pathways. Biological sulfur assimilation results in proteinogenic synthesis of methionine and cysteine. These amino acids can subsequently be used as precursors for other important sulfur-containing compounds such as S-adenosyl-methionine (SAM), a cofactor that typically facilitates the catalysis of methyl group transfers in biochemical pathways, or dimethylsulfoniopropionate (DMSP), an osmolyte, cryoprotectant, and antioxidant agent that enters the DOS carbon pool and is further metabolized by other marine microbiota (Stefels, 2000, Sunda et al., 2002, Asher et al., 2011).

DMSP is one of the most abundant OSCs on the Earth’s surface (Kirst et al., 1991, Yoch, 2002, Dickschat et al., 2015). Though DMSP production is commonly attributed to phytoplankton, micro and macroalgae, as well as other photosynthetic eukaryotes (Blunden et al., 1982, Van Diggelen et al., 1986, Kasamatsu et al., 2004), it has been recently shown that marine heterotrophic bacteria also have the genetic potential to synthesize DMSP, likely via the same methionine transamination pathway found in microalgae and phytoplankton (Curson et al., 2017). This discovery potentially broadens the range of environments where DMSP production and utilization may occur. DMSP is released into the water column upon cell lysis (via grazing, or due to viral infections) and can be taken up by marine bacteria and further degraded into volatile OSCs (VOSC’s) such as DMS or MeSH. There are two different pathways for which marine bacteria can catabolize DMSP: (1) the cleavage of DMSP to DMS and acrylate and (2) the demethylation of DMSP into MeSH and 3-(methylmercapto)propionate, and further into 3-mercaptopropionate (Taylor and Gilchrist, 1991). In addition, bacterial methylation to form MeSH occurs in anoxic fresh water systems via dissimilatory sulfate reduction (Schulz and Dickschat, 2007, Sela-Adler et al., 2015). Downstream alteration (i.e. microbial degradation, photodegradation, photooxidation) of VOSC’s or other non-volatile byproducts produced by DMSP catabolism may contribute significantly to the total DOS pool in marine environments.

In the context of organic matter cycling, biogenic OSCs synthesized by primary producers are often referred to as “primary organic sulfur”. Inorganic reduced sulfur can also abiotically react with organic matter in sulfidic water columns and sediments as part of early diagenetic processes (Sinninghe Damsté et al., 1989, Vairavamurthy et al., 1994, Werne et al., 2003). These OSCs are termed “secondary organic sulfur”. Processes that promote the incorporation of reduced sulfur into sedimentary organic matter persist mainly in sulfidic environments where high concentrations of sulfides such as H₂S (Sinninghe Damsté et al., 1989), or reactive intermediates such as polysulfides (Kohnen et al., 1989), elemental sulfur or thiosulfate (S₂O₃²⁻) are present (Werne et al., 2003). Sulfur may also be incorporated into dissolved organic matter (DOM; Heitmann and Blodau, 2006). In marine environments, this mechanism has been demonstrated through laboratory simulations of abiotic sulfurization of DOM under sulfidic conditions (Pohlabeln et al., 2017) and is evidenced by the high levels of DOS that have been in contact with recirculating reduced (H₂S-rich) marine hydrothermal fluids (Gomez-Saez et al., 2016). Though attempts have been made to elucidate the chemical diversity of DOS to constrain its origin and distribution in aquatic systems (Pohlabeln and Dittmar, 2015), the mechanisms by which OSCs are transformed to recalcitrant DOS remain speculative.

Generally, the understanding of the chemical complexity of DOS is limited to marine ecosystems and sediments (Cutter et al., 2004, Sleighter et al., 2014, Ksionzek et al., 2016). Analytical methods used to characterize organic constituents within complex DOM samples utilize nuclear magnetic resonance (NMR), fluorescent spectroscopy, or more recently, electrospray ionization Fourier transform-ion cyclotron resonance-mass spectrometry (ESI FT-ICR-MS), each of which have different advantages and disadvantages. ESI FT-ICR-MS has been shown to be a powerful and sensitive approach resulting in molecular formula predictions for elucidating chemical differences in DOM chemistry (Dittmar, 2015, Cawley et al., 2016), including those that are S-bearing (Ksionzek et al., 2016, Poulin et al., 2017). However, structural predictions can be more challenging. Collision induced tandem mass spectrometry may be useful in determining S-bearing functional groups by observing neutral losses when compounds are introduced into a collision cell (Pohlabeln and Dittmar, 2015), though tandem mass spectral libraries are not as comprehensive as electron ionization mass spectral databases, limiting precise molecular structure prediction. Similarly, other approaches have characterized sulfur functional groups and oxidation states in organic matter using stable isotope probing or X-ray absorption near edge structures (XANES) spectroscopy (Eglinton et al., 1994, Vairavamurthy et al., 1997, Jokic et al., 2003, Summons et al., 2008, Manceau and Nagy, 2012, Zhu et al., 2014), though fully elucidated structures of individual dissolved OSCs were rarely reported.

Here, we report the presence of organic sulfur compounds (sulfones) in a subsurface, interstitial, hypersaline brine located in the icy matrix of frozen Lake Vida, in East Antarctica. Their tentative structures were determined on the basis of mass spectrometry analysis. We performed conventional liquid-liquid extraction of the dissolved organic matter in Lake Vida brine (LVBr) using dichloromethane (DCM). The extract was analyzed using comprehensive multidimensional gas chromatography-time of flight-mass spectrometry (GC × GC-TOF MS), which provides a high resolving power and detection sensitivity capable of elucidating small molecules (low molecular weight analytes; 5 to 1000 Daltons) in a complex environmental sample. We tentatively identify a suite of oxidized OSCs bearing sulfones functional groups using mass spectral fragmentation and by comparisons to spectra published in the literature (e.g., Bowie et al., 1966, Smakman and de Boer, 1970, Kingston et al., 1974, etc). Based on our understanding of the Lake Vida hydrological history (Dugan et al., 2015a, Dugan et al., 2015b), microbiology and geochemistry (Murray et al., 2012, Kuhn et al., 2014, Ostrom et al., 2016, Proemse et al., 2017), DOM chemistry (Cawley et al., 2016), and legacy effects on environmental metabolomics (Chou et al., 2018), we propose potential sources for the OSCs identified in this study and the mechanisms by which they may be produced, transformed, and preserved in LVBr.