Sulfur and oxygen isotope insights into sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece

Abstract

Shallow-sea (5 m depth) hydrothermal venting off Milos Island provides an ideal opportunity to target transitions between igneous abiogenic sulfide inputs and biogenic sulfide production during microbial sulfate reduction. Seafloor vent features include large (>1 m2) white patches containing hydrothermal minerals (elemental sulfur and orange/yellow patches of arsenic-sulfides) and cells of sulfur oxidizing and reducing microorganisms. Sulfide-sensitive film deployed in the vent and non-vent sediments captured strong geochemical spatial patterns that varied from advective to diffusive sulfide transport from the subsurface. Despite clear visual evidence for the close association of vent organisms and hydrothermalism, the sulfur and oxygen isotope composition of pore fluids did not permit delineation of a biotic signal separate from an abiotic signal. Hydrogen sulfide (H2S) in the free gas had uniform δ34S values (2.5 ± 0.28‰, n = 4) that were nearly identical to pore water H2S (2.7 ± 0.36‰, n = 21). In pore water sulfate, there were no paired increases in δ34SSO4 and δ18OSO4 as expected of microbial sulfate reduction. Instead, pore water δ34SSO4 values decreased (from approximately 21‰ to 17‰) as temperature increased (up to 97.4°C) across each hydrothermal feature. We interpret the inverse relationship between temperature and δ34SSO4 as a mixing process between oxic seawater and 34S-depleted hydrothermal inputs that are oxidized during seawater entrainment. An isotope mass balance model suggests secondary sulfate from sulfide oxidation provides at least 15% of the bulk sulfate pool. Coincident with this trend in δ34SSO4, the oxygen isotope composition of sulfate tended to be 18O-enriched in low pH (<5), high temperature (>75°C) pore waters. The shift toward high δ18OSO4 is consistent with equilibrium isotope exchange under acidic and high temperature conditions. The source of H2S contained in hydrothermal fluids could not be determined with the present dataset; however, the end-member δ34S value of H2S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of ~300°C. Any biological sulfur cycling within these hydrothermal systems is masked by abiotic chemical reactions driven by mixing between low-sulfate, H2S-rich hydrothermal fluids and oxic, sulfate-rich seawater.

The Role of Hydrogen Sulfide in Evolution and the Evolution of Hydrogen Sulfide in Metabolism and Signaling

“…Heat deep within the smokers keeps metal sulfides in solution, and acidity favors their dissociation and elevates H 2 S concentrations (51). In the prebiotic earth, it is quite likely that H 2 S and reduced metal sulfides remained in solution in anoxic seawater for prolonged periods and could have spread considerable distances (35). Present-day vents contain the densest biomass on earth, evidence of their abundant energy and the ability of living organisms to use that energy.…”

Arsenic and high affinity phosphate uptake gene distribution in shallow submarine hydrothermal sediments

“…A number of studies have identified and successfully characterized microbial communities thriving in hydrothermal vent fields impacted by some of the most elevated As concentrations (e.g., Dando et al 1995; Bayraktarov et al 2013; Giovannelli et al 2013; Price et al 2013a, b; Ruiz-Chancho et al 2013; Gilhooly et al 2014; Godelitsas et al 2015; Callac et al 2017). Nonetheless, a mechanistic understanding of how life copes with high As conditions, the chemical and physical processes that regulate dissolved As content, and the potentially biolimiting phosphate concentrations caused by coprecipitation with hydrothermal Fe(III)(oxyhydr)oxides (e.g., Feely et al 1996, 1998; Wheat et al 1996; Schaller et al 2000; Hawkes et al 2014), remain to be clarified for these environments.…”

Another chemolithotrophic metabolism missing in nature—sulfur comproportionation

“…For example, at Vulcano Island (Italy) mixed fluids (~40-90°C) have been described with pH as low as ~2, sulfate levels up to ~60 mM, and sulfide up to ~0.4 mM (28,29). At Milos Island (Greece) fluids are lower in sulfate (up to ~33 mM), but more sulfidic (up to 3 mM) (30,31 Caption for Figure 1. Values of ∆ !…”