Evidence

Stable rosickyite was found in samples from Borup Fiord Pass, a low-temperature glacial site, and was thought to be potentially biogenic in origin.

Sulfur samples from Borup Fiord Pass, which is an Arctic glacial site with low temperature conditions, showed the presence of stable rosickyite crystals. Its presence could potentially be due to the abundance of EPS (extracellular polymeric substances) and microbial activity at this site, which could stabilize liquid sulfur droplets and allow time for the slow crystallization of rosickyite.

Evidence is Sourced from:

Title

Biosignature detection at an Arctic analog to Europa

Authors

Gleeson, Damhnait F, Pappalardo, RT, Anderson, MS, Grasby, SE, Mielke, RE, Wright, KE, Templeton, AS

Abstract

The compelling evidence for an ocean beneath the ice shell of Europa makes it a high priority for astrobiological investigations. Future missions to the icy surface of this moon will query the plausibly sulfur-rich materials for potential indications of the presence of life carried to the surface by mobile ice or partial melt. However, the potential for generation and preservation of biosignatures under cold, sulfur-rich conditions has not previously been investigated, as there have not been suitable environments on Earth to study. Here, we describe the characterization of a range of biosignatures within potentially analogous sulfur deposits from the surface of an Arctic glacier at Borup Fiord Pass to evaluate whether evidence for microbial activities is produced and preserved within these deposits. Optical and electron microscopy revealed microorganisms and extracellular materials. Elemental sulfur (S0), the dominant mineralogy within field samples, is present as rhombic and needle-shaped mineral grains and spherical mineral aggregates, commonly observed in association with extracellular polymeric substances. Orthorhombic α-sulfur represents the stable form of S0, whereas the monoclinic (needle-shaped) γ-sulfur form rosickyite is metastable and has previously been associated with sulfide-oxidizing microbial communities. Scanning transmission electron microscopy showed mineral deposition on cellular and extracellular materials in the form of submicron-sized, needle-shaped crystals. X-ray diffraction measurements supply supporting evidence for the presence of a minor component of rosickyite. Infrared spectroscopy revealed parts-per-million level organics in the Borup sulfur deposits and organic functional groups diagnostic of biomolecules such as proteins and fatty acids. Organic components are below the detection limit for Raman spectra, which were dominated by sulfur peaks. These combined investigations indicate that sulfur mineral deposits may contain identifiable biosignatures that can be stabilized and preserved under low-temperature conditions. Borup Fiord Pass represents a useful testing ground for instruments and techniques relevant to future astrobiological exploration at Europa.

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Evidence

The presence of rosickyite in a microbial community in Death Valley suggests the activity of microorganisms is responsible for maintaining this mineral in stable form.

Rosickyite crystals were found within the cyanobacterial layer of an endoevaporite (within a natural salt or mineral deposit left behind after the evaporation of water) microbial community in Death Valley. As rosickyite is not normally stable at Earth’s surface conditions, it was thought that microbial activity was providing the appropriate geochemical conditions (water, sulfur, alkaline, pH) for rosickyite to form and remain stable.

Evidence is Sourced from:

Title

Mineral biosignatures in evaporites: Presence of rosickyite in an endoevaporitic microbial community from Death Valley, California

Authors

Douglas, Susanne, Yang, Heixong

Abstract

Rosickyite is a rare form of sulfur (γ-sulfur; monoclinic symmetry) that is not thermodynamically predicted to be stable at Earth's surface temperatures; instead, it reverts to the more common α-sulfur form (orthorhombic symmetry). Here we show, for the first time, that rosickyite exists and is stably maintained within an endoevaporitic microbial community from the salt pan of Death Valley, California. We hypothesize that this mineral is formed by a cycle of microbial dissolution of gypsum (CaSO4·2H2O) to sulfide and reoxidation of the sulfide to elemental sulfur (rosickyite) within a stable oxygen-sulfide gradient maintained by the organisms. Furthermore, we report a microstratigraphic layering of mineral types that correlates with layering of the microbial community. Knowledge of how microbial communities can affect the mineral assemblages of evaporite deposits on Earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes.

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Evidence

Stable rosickyite and ꞵ-sulfur found in the low-temperature setting of Borup Fiord Pass are likely due to the presence of complex organic matter/carbon.

Stable rosickyite and ꞵ-sulfur were found in the low-temperature setting of Borup Fiord Pass. Given that the sulfur globules appeared to have formed in association with dissolved organic matter/carbon, it was concluded that these typically unstable allotropes formed and persisted due to the interactions between the sulfur and organic carbon.

Evidence is Sourced from:

Title

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

Authors

Lau, Graham E, Cosmidis, Julie, Grasby, Stephen E, Trivedi, Christopher B, Spear, John R, Templeton, Alexis S

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-S8, β-S8, and γ-S8 (the three solid forms of cyclooctasulfur (S8)). The detection of the β-S8 allotrope of elemental sulfur is notable, since β-S8 typically only forms in high temperature environments (>96 °C). The γ-S8 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-S8 and γ-S8 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-S8 and γ-S8 remains to be assessed.

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Evidence

In experiments reacting sulfide with complex dissolved organics (yeast extract, peptone), elemental sulfur (S(0)) was shown to organomineralize into various microstructures similar to those created directly by microbial activity.

Laboratory experiments were conducted using sterile sulfide/oxygen gradient environments with complex organic compounds such as yeast extract and/or peptone (both biologically derived). It was shown that S(0) organomineralized into various microstructures that were morphologically similar to those which have been observed to form in situ due to microbial activity/biomineralization.

Evidence is Sourced from:

Title

Self-assembly of biomorphic carbon/sulfur microstructures in sulfidic environments

Authors

Cosmidis, Julie, Templeton, Alexis S

Abstract

In natural and laboratory-based environments experiencing sustained counter fluxes of sulfide and oxidants, elemental sulfur (S0)—a key intermediate in the sulfur cycle—can commonly accumulate. S0 is frequently invoked as a biomineralization product generated by enzymatic oxidation of hydrogen sulfide and polysulfides. Here we show the formation of S0 encapsulated in nanometre to micrometre-scale tubular and spherical organic structures that self-assemble in sulfide gradient environments in the absence of any direct biological activity. The morphology and composition of these carbon/sulfur microstructures so closely resemble microbial cellular and extracellular structures that new caution must be applied to the interpretation of putative microbial biosignatures in the fossil record. These reactions between sulfide and organic matter have important implications for our understanding of S0 mineralization processes and sulfur interactions with organic carbon in the environment. They furthermore provide a new pathway for the synthesis of carbon-sulfur nanocomposites for energy storage technologies.

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Evidence

S(0) formation by chemolithoautotrophic sulfur-oxidizing bacteria S. kujiense produced rosickyite and ꞵ-sulfur, which formed and was stabilized due to the presence of organics secreted by the bacteria.

S(0) formation by chemolithoautotrophic sulfur-oxidizing and nitrate-reducing bacteria Sulfuricurvum kujiense (Epsilonproteobacteria) produced rosickyite and ꞵ-sulfur. It was hypothesized that organics secreted by this bacteria could help form and stabilize these allotropes via organomineralization. The morphologies of these minerals were similar to those formed in other sulfur organomineralization experiments. Further supporting the necessity of organics in helping form/stabilize these compounds was the fact that no S(0) particles were formed in abiotic controls that lacked organics.

Evidence is Sourced from:

Title

Elemental sulfur formation by Sulfuricurvum kujiense is mediated by extracellular organic compounds

Authors

Cron, Brandi, Henri, Pauline, Chan, Clara S, Macalady, Jennifer L, Cosmidis, Julie

Abstract

Elemental sulfur [S(0)] is a central and ecologically important intermediate in the sulfur cycle, which can be used by a wide diversity of microorganisms that gain energy from its oxidation, reduction, or disproportionation. S(0) is formed by oxidation of reduced sulfur species, which can be chemically or microbially mediated. A variety of sulfur-oxidizing bacteria can biomineralize S(0), either intracellularly or extracellularly. The details and mechanisms of extracellular S(0) formation by bacteria have been in particular understudied so far. An important question in this respect is how extracellular S(0) minerals can be formed and remain stable in the environment outside of their thermodynamic stability domain. It was recently discovered that S(0) minerals could be formed and stabilized by oxidizing sulfide in the presence of dissolved organic compounds, a process called S(0) organomineralization. S(0) particles formed through this mechanism possess specific signatures such as morphologies that differ from that of their inorganically precipitated counterparts, encapsulation within an organic envelope, and metastable crystal structures (presence of the monoclinic β- and γ-S8 allotropes). Here, we investigated S(0) formation by the chemolithoautotrophic sulfur-oxidizing and nitrate-reducing bacterium Sulfuricurvum kujiense (Epsilonproteobacteria). We performed a thorough characterization of the S(0) minerals produced extracellularly in cultures of this microorganism, and showed that they present all the specific signatures (morphology, association with organics, and crystal structures) of organomineralized S(0). Using “spent medium” experiments, we furthermore demonstrated that soluble extracellular compounds produced by S. kujiense are necessary to form and stabilize S(0) minerals outside of the cells. This study provides the first experimental evidence of the importance of organomineralization in microbial S(0) formation. The prevalence of organomineralization in extracellular S(0) precipitation by other sulfur bacteria remains to be investigated, and the biological role of this mechanism is still unclear. However, we propose that sulfur-oxidizing bacteria could use soluble organics to stabilize stores of bioavailable S(0) outside the cells.

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Evidence

Abiogenic greigite formed in experiments where formaldehyde (HCHO) was added to iron sulfide (FeS) and hydrogen sulfide (H2S) was produced at a much slower rate than biogenic greigite in magnetotactic bacteria.

Experiments observing the formation of pyrite through the oxidation of FeS by H2S in aqueous solutions at temperatures between 40-100°C had HCHO and other aldehydes added to observe their effects on the amount of pyrite and/or greigite formed. A larger amount of aldehydes in the system result in a larger proportion of greigite formed.

Evidence is Sourced from:

Title

A novel iron sulphide mineral switch and its implications for Earth andplanetary science

Authors

David Rickard and Ian B Butler and Anthony Oldroyd

Abstract

We report that iron sulphide mineral products of different compositions and oxidation states are produced by the catalytic activities of aldehydic carbonyl groups. We show that the product of the oxidation of iron(II) monosulphide (FeS) by aqueous H2S at temperatures between 40 and 100°C under anoxic conditions is dependent on the presence of catalytic quantities of aldehydes. The products switch between the ferrimagnetic thiospinel, greigite (Fe3S4) and the iron disulphide, pyrite (FeS2). Compared with other mineral switches, the effect of this first observed switch in the iron sulphide system is quite dramatic. In the presence of aldehydic carbonyls, the Fe(II) in iron(II) monosulphide is oxidised, but the S(−II) is unaffected; in the absence of aldehydic carbonyls, the S(−II) in the iron(II) monosulphide is oxidised but the Fe(II) remains unchanged. The effectiveness of the aldehyde switch is such that it suggests that the presence of trace organics can determine the stoichiometry and oxidation state of the iron sulphide product. The results have general implications for the coupling of iron sulphides with prebiotic organic systems in the early development of life. They provide a potential new pathway for the development of biogenic greigite, with implications for the development of remanent magnetism in sediments and suggest possible constraints on the use of greigite as a biomarker in Earth and planetary sciences.

Citation

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Evidence

The presence of rosickyite in a microbial community in Death Valley suggests the activity of microorganisms provide the appropriate geochemical conditions for it to remain stable.

Rosickyite crystals were found within the cyanobacterial layer of an endoevaporite (within a natural salt or mineral deposit left behind after the evaporation of water) microbial community in Death Valley. The site in question is a “wet zone” due to the water-holding capacity of the microbial cells, gypsum provided a source of sulfate, the higher pH (8.2) and alkalization of the environment by the microbes favored the dissolution of gypsum, therefore providing necessary conditions for sulfide-oxidizing bacteria that could produce the rosickyite in question.

Evidence is Sourced from:

Title

Mineral biosignatures in evaporites: Presence of rosickyite in an endoevaporitic microbial community from Death Valley, California

Authors

Douglas, Susanne, Yang, Heixong

Abstract

Rosickyite is a rare form of sulfur (γ-sulfur; monoclinic symmetry) that is not thermodynamically predicted to be stable at Earth's surface temperatures; instead, it reverts to the more common α-sulfur form (orthorhombic symmetry). Here we show, for the first time, that rosickyite exists and is stably maintained within an endoevaporitic microbial community from the salt pan of Death Valley, California. We hypothesize that this mineral is formed by a cycle of microbial dissolution of gypsum (CaSO4·2H2O) to sulfide and reoxidation of the sulfide to elemental sulfur (rosickyite) within a stable oxygen-sulfide gradient maintained by the organisms. Furthermore, we report a microstratigraphic layering of mineral types that correlates with layering of the microbial community. Knowledge of how microbial communities can affect the mineral assemblages of evaporite deposits on Earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes.

Citation

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Evidence

The presence of organic matter from possible but unconfirmed microbial sources at Borup Fiord Pass allows the mineralization of rosickyite and ꞵ-sulfur.

Stable rosickyite and ꞵ-sulfur were found in the low-temperature environment (0-5°C) of Borup Fiord Pass. As these allotropes are expected to only persist in high temperature conditions (>96°C) and given the close association of organic carbon with the sulfur globules found, it was hypothesized that they were stable due to the direct interactions of organic matter and sulfide. The organic matter in question is likely from microbial sources, though not confirmed.

Evidence is Sourced from:

Title

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

Authors

Lau, Graham E, Cosmidis, Julie, Grasby, Stephen E, Trivedi, Christopher B, Spear, John R, Templeton, Alexis S

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-S8, β-S8, and γ-S8 (the three solid forms of cyclooctasulfur (S8)). The detection of the β-S8 allotrope of elemental sulfur is notable, since β-S8 typically only forms in high temperature environments (>96 °C). The γ-S8 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-S8 and γ-S8 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-S8 and γ-S8 remains to be assessed.

Citation

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Evidence

The abundance of EPS (particularly polysaccharides and lipids) within Borup Fiord Pass sulfur deposits provide the necessary conditions for the formation of rosickyite.

Sulfur minerals from Borup Fiord Pass samples were commonly found to be coated in EPS. In particular, the presence of organic ligands provided by polysaccharides and lipids create the necessary conditions for the formation of rosickyite - they are thought to stabilize liquid sulfur droplets and allow time for the slow crystallization of this mineral.

Evidence is Sourced from:

Title

Biosignature detection at an Arctic analog to Europa

Authors

Gleeson, Damhnait F, Pappalardo, RT, Anderson, MS, Grasby, SE, Mielke, RE, Wright, KE, Templeton, AS

Abstract

The compelling evidence for an ocean beneath the ice shell of Europa makes it a high priority for astrobiological investigations. Future missions to the icy surface of this moon will query the plausibly sulfur-rich materials for potential indications of the presence of life carried to the surface by mobile ice or partial melt. However, the potential for generation and preservation of biosignatures under cold, sulfur-rich conditions has not previously been investigated, as there have not been suitable environments on Earth to study. Here, we describe the characterization of a range of biosignatures within potentially analogous sulfur deposits from the surface of an Arctic glacier at Borup Fiord Pass to evaluate whether evidence for microbial activities is produced and preserved within these deposits. Optical and electron microscopy revealed microorganisms and extracellular materials. Elemental sulfur (S0), the dominant mineralogy within field samples, is present as rhombic and needle-shaped mineral grains and spherical mineral aggregates, commonly observed in association with extracellular polymeric substances. Orthorhombic α-sulfur represents the stable form of S0, whereas the monoclinic (needle-shaped) γ-sulfur form rosickyite is metastable and has previously been associated with sulfide-oxidizing microbial communities. Scanning transmission electron microscopy showed mineral deposition on cellular and extracellular materials in the form of submicron-sized, needle-shaped crystals. X-ray diffraction measurements supply supporting evidence for the presence of a minor component of rosickyite. Infrared spectroscopy revealed parts-per-million level organics in the Borup sulfur deposits and organic functional groups diagnostic of biomolecules such as proteins and fatty acids. Organic components are below the detection limit for Raman spectra, which were dominated by sulfur peaks. These combined investigations indicate that sulfur mineral deposits may contain identifiable biosignatures that can be stabilized and preserved under low-temperature conditions. Borup Fiord Pass represents a useful testing ground for instruments and techniques relevant to future astrobiological exploration at Europa.

Citation

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Evidence

Experiments where simple organic compounds (such as simple sugars and amino acids) were reacted with sulfides formed organomineralized elemental sulfur (S(0)), including rosickyite and ꞵ-sulfur.

Experiments were conducted where sulfides and oxygen were reacted with different types of dissolved organic compounds. In the cases where simple organic compounds such as simple sugars and amino acids (glucose, glycine) were used, S(0) organomineralization occurred and produced rosickyite and ꞵ-sulfur, which are not typically stable under 96°C.

Evidence is Sourced from:

Title

Formation and stabilization of elemental sulfur through organomineralization

Authors

Cosmidis, Julie, Nims, Christine W, Diercks, David, Templeton, Alexis S

Abstract

Elemental sulfur (S(0)) is an important intermediate in the biogeochemical cycle of sulfur that is formed by chemical or biological oxidation of more reduced sulfur species. Given the restricted geochemical conditions under which S(0) should persist, the mechanisms whereby S(0) can be stabilized in the environment are not fully understood. Here we identify a process called “S(0) organomineralization”, by which S(0) minerals are produced and stabilized following the oxidation of hydrogen sulfide in the presence of numerous types of dissolved organics, including simple sugars and amino acids. The S(0) particles formed through this mechanism are closely associated with organics, which often form an envelope around the mineral. The organic envelopes are formed by self-assembly of the dissolved organic molecules in the presence of hydrogen sulfide and oxygen, and play in a role in the stabilization of S(0). Organic compound sulfurization probably plays an important role in the self-assembly mechanism, by causing the polymerization of relatively small dissolved organic molecules into solid, macromolecular, polymeric organics. The organomineralized S(0) particles present unique and complex morphologies, which are controlled by the type of dissolved organic compound present in the experimental media. Depending on the organics present, organomineralized S(0) can exist as different combinations of several crystal structures, including the non-thermodynamically stable β- and γ-S8 allotropes, which are most likely stabilized by their close association with the organic phase. We propose that complex particle morphology combined with the presence of metastable S(0) allotropes could be used as a signature of S(0) organomineralization in natural settings. S(0) organomineralization was obtained in the laboratory under a wide range of experimental conditions that span across geochemical conditions which can be encountered in many sulfidic environments. It is possible that the reaction between reduced sulfur species and organics may significantly affect the production and preservation of S(0) in numerous natural systems.

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Evidence

Organomineralization of S(0) has been observed in a range of geochemical conditions that encompass many sulfidic environments. 

Experiments where sulfides and oxygen were reacted with simple sugars and amino acids were performed with various concentrations of S(0) (500 𝜇M to 5 mM), initial concentrations of dissolved organics (10 mgᐧL-1 to 15gᐧL-1), and pH (5.5 to 8.5) at room temperatures (21°C). These conditions encompass many sulfidic environments such as marine sediments, euxinic (no oxygen and higher level of free hydrogen sulfide) lakes, sulfidic caves, sub-glacial springs, freshwater systems, and groundwater to name a few. Moreover, simple sugars and amino acids are not uncommon compounds.

Evidence is Sourced from:

Title

Formation and stabilization of elemental sulfur through organomineralization

Authors

Cosmidis, Julie, Nims, Christine W, Diercks, David, Templeton, Alexis S

Abstract

Elemental sulfur (S(0)) is an important intermediate in the biogeochemical cycle of sulfur that is formed by chemical or biological oxidation of more reduced sulfur species. Given the restricted geochemical conditions under which S(0) should persist, the mechanisms whereby S(0) can be stabilized in the environment are not fully understood. Here we identify a process called “S(0) organomineralization”, by which S(0) minerals are produced and stabilized following the oxidation of hydrogen sulfide in the presence of numerous types of dissolved organics, including simple sugars and amino acids. The S(0) particles formed through this mechanism are closely associated with organics, which often form an envelope around the mineral. The organic envelopes are formed by self-assembly of the dissolved organic molecules in the presence of hydrogen sulfide and oxygen, and play in a role in the stabilization of S(0). Organic compound sulfurization probably plays an important role in the self-assembly mechanism, by causing the polymerization of relatively small dissolved organic molecules into solid, macromolecular, polymeric organics. The organomineralized S(0) particles present unique and complex morphologies, which are controlled by the type of dissolved organic compound present in the experimental media. Depending on the organics present, organomineralized S(0) can exist as different combinations of several crystal structures, including the non-thermodynamically stable β- and γ-S8 allotropes, which are most likely stabilized by their close association with the organic phase. We propose that complex particle morphology combined with the presence of metastable S(0) allotropes could be used as a signature of S(0) organomineralization in natural settings. S(0) organomineralization was obtained in the laboratory under a wide range of experimental conditions that span across geochemical conditions which can be encountered in many sulfidic environments. It is possible that the reaction between reduced sulfur species and organics may significantly affect the production and preservation of S(0) in numerous natural systems.

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Evidence

Abiogenically produced greigite in experiments where formaldehyde (HCHO) was added to iron sulfide (FeS) and hydrogen sulfide (H2S) is a similar size to biogenically produced greigite in magnetosomes.

Experiments observing the formation of pyrite through the oxidation of FeS by H2S in aqueous solutions at temperatures between 40-100°C had HCHO and other aldehydes added to observe their effects on the amount of pyrite and/or greigite formed. A larger amount of aldehydes in the system result in a larger proportion of greigite formed. The resulting greigite formed in these abiotic experiments resembled the size (20-100 nm) of greigite particles formed and found in magnetosomes.

Evidence is Sourced from:

Title

A novel iron sulphide mineral switch and its implications for Earth andplanetary science

Authors

David Rickard and Ian B Butler and Anthony Oldroyd

Abstract

We report that iron sulphide mineral products of different compositions and oxidation states are produced by the catalytic activities of aldehydic carbonyl groups. We show that the product of the oxidation of iron(II) monosulphide (FeS) by aqueous H2S at temperatures between 40 and 100°C under anoxic conditions is dependent on the presence of catalytic quantities of aldehydes. The products switch between the ferrimagnetic thiospinel, greigite (Fe3S4) and the iron disulphide, pyrite (FeS2). Compared with other mineral switches, the effect of this first observed switch in the iron sulphide system is quite dramatic. In the presence of aldehydic carbonyls, the Fe(II) in iron(II) monosulphide is oxidised, but the S(−II) is unaffected; in the absence of aldehydic carbonyls, the S(−II) in the iron(II) monosulphide is oxidised but the Fe(II) remains unchanged. The effectiveness of the aldehyde switch is such that it suggests that the presence of trace organics can determine the stoichiometry and oxidation state of the iron sulphide product. The results have general implications for the coupling of iron sulphides with prebiotic organic systems in the early development of life. They provide a potential new pathway for the development of biogenic greigite, with implications for the development of remanent magnetism in sediments and suggest possible constraints on the use of greigite as a biomarker in Earth and planetary sciences.

Citation

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Evidence

In the presence of simple sugars and amino acids, elemental sulfur (S(0)) organomineralized into stable ꞵ- and 𝜸-sulfur allotropes.

Experiments were conducted where sulfides and oxygen were reacted with different types of dissolved organic compounds. These experiments were performed under a wide range of geochemical conditions, including those representative of natural sulfidic environments. In the cases where simple organic compounds such as simple sugars and amino acids were used, S(0) organomineralization occurred and produced rosickyite and ꞵ-sulfur. These remained stable for more than 10 days at room temperature.

Evidence is Sourced from:

Title

Formation and stabilization of elemental sulfur through organomineralization

Authors

Cosmidis, Julie, Nims, Christine W, Diercks, David, Templeton, Alexis S

Abstract

Elemental sulfur (S(0)) is an important intermediate in the biogeochemical cycle of sulfur that is formed by chemical or biological oxidation of more reduced sulfur species. Given the restricted geochemical conditions under which S(0) should persist, the mechanisms whereby S(0) can be stabilized in the environment are not fully understood. Here we identify a process called “S(0) organomineralization”, by which S(0) minerals are produced and stabilized following the oxidation of hydrogen sulfide in the presence of numerous types of dissolved organics, including simple sugars and amino acids. The S(0) particles formed through this mechanism are closely associated with organics, which often form an envelope around the mineral. The organic envelopes are formed by self-assembly of the dissolved organic molecules in the presence of hydrogen sulfide and oxygen, and play in a role in the stabilization of S(0). Organic compound sulfurization probably plays an important role in the self-assembly mechanism, by causing the polymerization of relatively small dissolved organic molecules into solid, macromolecular, polymeric organics. The organomineralized S(0) particles present unique and complex morphologies, which are controlled by the type of dissolved organic compound present in the experimental media. Depending on the organics present, organomineralized S(0) can exist as different combinations of several crystal structures, including the non-thermodynamically stable β- and γ-S8 allotropes, which are most likely stabilized by their close association with the organic phase. We propose that complex particle morphology combined with the presence of metastable S(0) allotropes could be used as a signature of S(0) organomineralization in natural settings. S(0) organomineralization was obtained in the laboratory under a wide range of experimental conditions that span across geochemical conditions which can be encountered in many sulfidic environments. It is possible that the reaction between reduced sulfur species and organics may significantly affect the production and preservation of S(0) in numerous natural systems.

Citation

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