Orit Sivan

Iron oxides stimulate sulfate-driven anaerobic methane oxidation in seeps

Significance Anaerobic oxidation of methane (AOM) coupled to sulfate reduction has been shown to consume up to 90% of the greenhouse gas methane produced within the subseafloor environment; however, the mechanism of this process has remained enigmatic. Here, we provide geochemical evidence based on sulfur, oxygen, and carbon isotopes for the involvement of iron oxides in sulfate-driven AOM in methane seeps. Our results suggest that, beyond the function of iron as nutrient, the presence of iron oxides stimulates sulfate-driven AOM to a greater extent than in sediments with low concentrations of iron oxides. The isotope analyses further indicate that sulfate reduction in methane seep habitats differs than sulfate reduction in diffusive profiles in and above the sulfate–methane transition zone. Seep sediments are dominated by intensive microbial sulfate reduction coupled to the anaerobic oxidation of methane (AOM). Through geochemical measurements of incubation experiments with methane seep sediments collected from Hydrate Ridge, we provide insight into the role of iron oxides in sulfate-driven AOM. Seep sediments incubated with 13C-labeled methane showed co-occurring sulfate reduction, AOM, and methanogenesis. The isotope fractionation factors for sulfur and oxygen isotopes in sulfate were about 40‰ and 22‰, respectively, reinforcing the difference between microbial sulfate reduction in methane seeps versus other sedimentary environments (for example, sulfur isotope fractionation above 60‰ in sulfate reduction coupled to organic carbon oxidation or in diffusive sedimentary sulfate–methane transition zone). The addition of hematite to these microcosm experiments resulted in significant microbial iron reduction as well as enhancing sulfate-driven AOM. The magnitude of the isotope fractionation of sulfur and oxygen isotopes in sulfate from these incubations was lowered by about 50%, indicating the involvement of iron oxides during sulfate reduction in methane seeps. The similar relative change between the oxygen versus sulfur isotopes of sulfate in all experiments (with and without hematite addition) suggests that oxidized forms of iron, naturally present in the sediment incubations, were involved in sulfate reduction, with hematite addition increasing the sulfate recycling or the activity of sulfur-cycling microorganisms by about 40%. These results highlight a role for natural iron oxides during bacterial sulfate reduction in methane seeps not only as nutrient but also as stimulator of sulfur recycling.

Hydrogeochemical Tool to Identify Salinization or Freshening of Coastal Aquifers Determined from Combined Field Work, Experiments, and Modeling

This study proposes a hydrogeochemical tool to distinguish between salinization and freshening events of a coastal aquifer and quantifies their effect on groundwater characteristics. This is based on the chemical composition of the fresh-saline water interface (FSI) determined from combined field work, column experiments with the same sediments, and modeling. The experimental results were modeled using the PHREEQC code and were compared to field data from the coastal aquifer of Israel. The decrease in the isotopic composition of the dissolved inorganic carbon (delta(13)C(DIC)) of the saline water indicates that, during seawater intrusion and coastal salinization, oxidation of organic carbon occurs. However, the main process operating during salinization or freshening events in coastal aquifers is cation exchange. The relative changes in Ca(2+), Sr(2+), and K(+) concentrations during salinization and freshening events are used as a reliable tool for characterizing the status of a coastal aquifer. The field data suggest that coastal aquifers may switch from freshening to salinization on a seasonal time scale.

Radiocarbon in seawater intruding into the Israeli Mediterranean coastal aquifer.

Saline groundwaters from the Israeli coastal aquifer were analyzed for their radiocarbon and tritium content to assess the rate of seawater penetration. The low (super 14) C values (28-88 pMC versus 100-117 pMC in seawater) imply an apparent non-recent seawater source, or water-rock interactions along the penetration route. The latter process is supported by measurable tritium values at some locations, which imply a relatively rapid rate of seawater intrusion. In other locations, low tritium values (

Anaerobic oxidation of methane by sulfate in hypersaline groundwater of the Dead Sea aquifer

lt;2 T.U.) indicate that recent seawater (

A unique isotopic fingerprint of sulfate-driven anaerobic oxidation of methane

lt;50 yr) did not penetrate inland. The low delta (super 13) C values in saline groundwater (average of -5.3 per mil versus 0 per mil in seawater) indicate that the dissolved carbon pool is comprised of a significant fraction of organic carbon. A linear negative correlation between delta (super 13) C and (super 14) C implies that this organic source is old (low (super 14) C values).

The dynamic redox chemistry of iron in the epilimnion of Lake Kinneret (Sea of Galilee)

Geochemical and microbial evidence points to anaerobic oxidation of methane (AOM) likely coupled with bacterial sulfate reduction in the hypersaline groundwater of the Dead Sea (DS) alluvial aquifer. Groundwater was sampled from nine boreholes drilled along the Arugot alluvial fan next to the DS. The groundwater samples were highly saline (up to 6300 mm chlorine), anoxic, and contained methane. A mass balance calculation demonstrates that the very low δ13CDIC in this groundwater is due to anaerobic methane oxidation. Sulfate depletion coincident with isotope enrichment of sulfur and oxygen isotopes in the sulfate suggests that sulfate reduction is associated with this AOM. DNA extraction and 16S amplicon sequencing were used to explore the microbial community present and were found to be microbial composition indicative of bacterial sulfate reducers associated with anaerobic methanotrophic archaea (ANME) driving AOM. The net sulfate reduction seems to be primarily controlled by the salinity and the available methane and is substantially lower as salinity increases (2.5 mm sulfate removal at 3000 mm chlorine but only 0.5 mm sulfate removal at 6300 mm chlorine). Low overall sulfur isotope fractionation observed (34ε = 17 ± 3.5‰) hints at high rates of sulfate reduction, as has been previously suggested for sulfate reduction coupled with methane oxidation. The new results demonstrate the presence of sulfate-driven AOM in terrestrial hypersaline systems and expand our understanding of how microbial life is sustained under the challenging conditions of an extremely hypersaline environment.

CHARACTERIZATION AND DATING OF SALINE GROUNDWATER IN THE DEAD SEA AREA

The largest reservoir of the powerful greenhouse gas methane is in marine sediments, and catastrophic release of this methane has been invoked to explain climate perturbations throughout Earth history. Marine methane oxidation is mainly coupled anaerobically to microbial sulfate reduction, which both limits and controls the release of methane from this sedimentary reservoir to the rest of Earth’s surface. Methane can be transported within the pore space of marine sediments either via diffusion or as bubbles. When methane travels in bubbles, these bubbles often are not completely oxidized and reach the overlying water where the methane emerges from the sediment in cold seeps. Although paleo–cold seeps can be identified by geological features such as carbonate mounds, a geochemical signature for cold seeps remains elusive. We demonstrate, using the sulfur and oxygen isotope composition of sulfate, that a unique isotopic signature emerges during microbial sulfate reduction coupled to methane oxidation in bubbling cold seeps. This isotope signature differs from that when sulfate is reduced by either organic matter oxidation or by the slower, diffusive flux of methane within marine sediments. We also show, through a comparison with the literature, that this unique isotope fingerprint is preserved in the rock record in authigenic buildups of barite associated with methane cold seeps.

Co-existence of Methanogenesis and Sulfate Reduction with Common Substrates in Sulfate-Rich Estuarine Sediments

Abstract The redox chemistry of Fe was investigated in Lake Kinneret (Sea of Galilee), a mesotrophic, monomictic lake in the central part of the Jordan Rift Valley. The concentrations of Fe(II) and Fe(tot) in the epilimnion and in the hypolimnion were measured, and the relationships between Fe(II) and other parameters (e.g., light, pH) were investigated. In addition, laboratory experiments were conducted where filtered (biota-free) Lake Kinneret waters, sterile unfiltered Lake Kinneret waters, and distilled waters were spiked with various concentrations of Fe(III) and Fe(II). The concentrations of Fe(III) and Fe(II) were measured as a function of time in water samples under a variety of pH, O2, and radiation conditions. Iron(II) concentrations in the epilimnion were below detection limit (0.04 μM) during nighttime, whereas in daytime Fe(II) concentrations were always above the detection limit and changed significantly around the year (0.05–0.15 μM). Fe(II)/Fe(tot) ratios measured in the lake (3–99%) are higher than the expected values for a high pH (pH ∼ 8), low ionic strength (∼10 mM) aquatic system. In addition to photo-induced reduction of Fe(III), there is a strong evidence that Fe(II) is stabilized in the O2-saturated lake water, as Fe(II) concentrations can be detected at a depth of 10 m, where less than 10% of the light penetrates. The results of the oxidation and photo-reduction experiments suggest that the observed rates of Fe redox reactions in sterile lake water are consistent with known chemical (abiotic) rates, but that these rates cannot account for the observed Fe(II) concentrations in the epilimnion waters. Therefore, we propose that the photo-induced redox cycle of Fe in the epilimnion of Lake Kinneret is largely controlled by biological activity and that abiotic photo-reduction of Fe accounts for only a small fraction of the observed Fe(II) in the epilimnion.

Hydrocarbon-related microbial processes in the deep sediments of the Eastern Mediterranean Levantine Basin

This work presents an attempt to date brines and determine flow rates of hypersaline groundwater in the extremely dynamic system of the Dead Sea (DS), whose level has dropped in the last 30 yr by ~20 m. The processes that affect the carbon species and isotopes of the groundwater in the DS area were quantified in order to estimate their flow rate based on radiocarbon and tritium methods. In contrast to the conservative behavior of most ions in the groundwater, the carbon system parameters indicate additional processes. The dissolved inorganic carbon (DIC) content of most saline groundwater is close to that of the DS, but its stable isotopic composition (δ13CDIC) is much lower. The chemical composition and carbon isotope mass balance suggest that the low δ13CDIC of the saline groundwater is a result of anaerobic organic matter oxidation by bacterial sulfate reduction (BSR) and methane oxidation. The radiocarbon content (14CDIC) of the saline groundwater ranged from 86 pMC (greater than the ~82 pMC value of the DS in the 2000s) to as low as 14 pMC. The similarity between the 14CDIC value and Na/Cl ratio of the groundwater at the DS shore and that of the 1980s DS brine indicates that the DS penetrated to the aquifer at that time. The low 14CDIC values in some of the saline groundwater suggest the existence of ancient brine in the subaquifer.

Saline Groundwater from Coastal Aquifers As a Source for Desalination

The competition between sulfate reducing bacteria and methanogens over common substrates has been proposed as a critical control for methane production. In this study, we examined the co-existence of methanogenesis and sulfate reduction with shared substrates over a large range of sulfate concentrations and rates of sulfate reduction in estuarine systems, where these processes are the key terminal sink for organic carbon. Incubation experiments were carried out with sediment samples from the sulfate-methane transition zone of the Yarqon (Israel) estuary with different substrates and inhibitors along a sulfate concentrations gradient from 1 to 10 mM. The results show that methanogenesis and sulfate reduction can co-exist while the microbes share substrates over the tested range of sulfate concentrations and at sulfate reduction rates up to 680 μmol L-1 day-1. Rates of methanogenesis were two orders of magnitude lower than rates of sulfate reduction in incubations with acetate and lactate, suggesting a higher affinity of sulfate reducing bacteria for the available substrates. The co-existence of both processes was also confirmed by the isotopic signatures of δ34S in the residual sulfate and that of δ13C of methane and dissolved inorganic carbon. Copy numbers of dsrA and mcrA genes supported the dominance of sulfate reduction over methanogenesis, while showing also the ability of methanogens to grow under high sulfate concentration and in the presence of active sulfate reduction.