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

Salt wedges and trapped brines of low-latitude endoreic saline lakes as potential modulators of GHG emission

Large salt lakes are long-term witnesses to climatic conditions and land use in their basins. The majority are experiencing a drastic drop in water levels due to climate change and human impact. Endoreic Lake Urmia (NW Iran), the sixth largest salt lake worldwide, is a striking example of this decline. Quantification of the relative contributions of natural variability and human impact on the lake's water supply is therefore essential. Here we present isotopic and radiocarbon analyses of surface and groundwater from the Shahr Chay River catchment, entering Lake Urmia on its western shore, and radiocarbon dating of a sedimentary core. Lake Urmia behaves like a large saltwater wedge almost entirely fed by the river and shallow groundwater. This leads to trapping of residual brines and formation of CH4 and secondary CO2 greenhouse gases, impacting sediment geochemical records and corresponding time scales for paleoenvironmental reconstructions. We conclude that (1) salt lakes functioning like a saline wedge, allowing organic matter oxidation, could contribute to increasing methane sources or reducing carbon sinks globally, and (2) endoreic basins worldwide need to be monitored before aridification-related salinization leads to the establishment of a saline wedge precluding any possibility of return to an equilibrium state.

Chloride-salinity as indicator of the chemical composition of groundwater: empirical predictive model based on aquifers in Southern Quebec, Canada

The present study first describes the variations in concentrations of 12 chemical elements in groundwater relative to salinity levels in Southern Quebec (Canada) groundwater systems, and then uses this data to develop an empirical predictive model for evaluating groundwater chemical composition relative to salinity levels. Data is drawn from a large groundwater chemistry database containing 2608 samples. Eight salinity classes were established from lowest to highest chloride (Cl) concentrations. Graphical analyses were applied to describe variations in major, minor, and trace element concentrations relative to salinity levels. Results show that the major elements were found to be dominant in the lower salinity classes, whereas Cl becomes dominant at the highest salinity classes. For each of the major elements, a transitional state was identified between domination of the major elements and domination of Cl. This transition occurred at a different level of salinity for each of the major elements. Except for Si, the minor elements Ba, B, and Sr generally increase relative to the increase of Cl. The highest Mn concentrations were found to be associated with only the highest levels of Cl, whereas F was observed to be more abundant than Mn. Based on this analysis of the data, a correlation table was established between salinity level and concentrations of the chemical constituents. We thus propose a predictive empirical model, identifying a profile of the chemical composition of groundwater relative to salinity levels, to help homeowners and groundwater managers evaluate groundwater quality before resorting to laborious and costly laboratory analyses.

Intensified microbial sulfate reduction in the deep Dead Sea during the early Holocene Mediterranean sapropel 1 deposition

The hypersaline Dead Sea and its sediments are natural laboratories for studying extremophile microorganism habitat response to environmental change. In modern times, increased freshwater runoff to the lake surface waters resulted in stratification and dilution of the upper water column followed by microbial blooms. However, whether these events facilitated a microbial response in the deep lake and sediments is obscure. Here we investigate archived evidence of microbial processes and changing regional hydroclimate conditions by reconstructing deep Dead Sea chemical compositions from pore fluid major ion concentration and stable S, O, and C isotopes, together with lipid biomarkers preserved in the hypersaline deep Dead Sea ICDP-drilled core sediments dating to the early Holocene (ca. 10,000 years BP). Following a significant negative lake water balance resulting in salt layer deposits at the start of the Holocene, there was a general period of positive net water balance at 9500-8300 years BP. The pore fluid isotopic composition of sulfate exhibit evidence of intensified microbial sulfate reduction, where both δ34S and δ18O of sulfate show a sharp increase from estimated base values of 15.0‰ and 13.9‰ to 40.2‰ and 20.4‰, respectively, and a δ34S vs. δ18O slope of 0.26. The presence of the n-C₁₇ alkane biomarker in the sediments suggests an increase of cyanobacteria or phytoplankton contribution to the bulk organic matter that reached the deepest parts of the Dead Sea. Although hydrologically disconnected, both the Mediterranean Sea and the Dead Sea microbial ecosystems responded to increased freshwater runoff during the early Holocene, with the former depositing the organic-rich sapropel 1 layer due to anoxic water column conditions. In the Dead Sea prolonged positive net water balance facilitated primary production and algal blooms in the upper waters and intensified microbial sulfate reduction in the hypolimnion and/or at the sediment-brine interface.

Prospects in the bioremediation of petroleum hydrocarbon contaminants from hypersaline environments: A review

Hypersaline environments are underappreciated and are frequently exposed to pollution from petroleum hydrocarbons. Unlike other environs, the high salinity conditions present are a deterrent to various remediation techniques. There is also production of hypersaline waters from oil-polluted ecosystems which contain toxic hydrophobic pollutants that are threat to public health, environmental protection, and sustainability. Currently, innovative advances are being proposed for the remediation of oil-contaminated hypersaline regions. Such advancements include the exploration and stimulation of native microbial communities capable of utilizing and degrading petroleum hydrocarbons. However, prevailing salinity in these environments is unfavourable for the growth of non-halophylic microorganisms, thus limiting effective bioremediation options. An in-depth understanding of the potentials of various remediation technologies of hydrocarbon-polluted hypersaline environments is lacking. Thus, we present an overview of petroleum hydrocarbon pollution in hypersaline ecosystems and discuss the challenges and prospects associated with several technologies that may be employed in remediation of hydrocarbon pollution in the presence of delimiting high salinities. The application of biological remediation technologies including the utilization of halophilic and halotolerant microorganisms is also discussed.

Occurrence and biodegradation of hydrocarbons at high salinities

Hypersaline environments are found around the world, above and below ground, and many are exposed to hydrocarbons on a continuous or a frequent basis. Some surface hypersaline environments are exposed to hydrocarbons because they have active petroleum seeps while others are exposed because of oil exploration and production, or nearby human activities. Many oil reservoirs overlie highly saline connate water, and some national oil reserves are stored in salt caverns. Surface hypersaline ecosystems contain consortia of halophilic and halotolerant microorganisms that decompose organic compounds including hydrocarbons, and subterranean ones are likely to contain the same. However, the rates and extents of hydrocarbon biodegradation are poorly understood in such ecosystems. Here we describe hypersaline environments potentially or likely to become contaminated with hydrocarbons, including perennial and transient environments above and below ground, and discuss what is known about the microbes degrading hydrocarbons and the extent of their activities. We also discuss what limits the microbial hydrocarbon degradation in hypersaline environments and whether there are opportunities for inhibiting (oil storage) or stimulating (oil spills) such biodegradation as the situation requires.

Redox condition of saline groundwater from coastal aquifers influences reverse osmosis desalination process

Reverse osmosis (RO) seawater desalination is a widely applied technological process to supply potable water worldwide. Recently, saline groundwater (SGW) pumped from beach wells in coastal aquifers that penetrate beneath the freshwater-seawater interface is considered as a better alternative water source to RO seawater desalination as it is naturally filtered within the sediments which reduces membrane fouling and pre-treatment costs. The SGW of many coastal aquifers is anoxic - and thus, in a low redox stage - has elevated concentrations of dissolved manganese, iron and sulfides. We studied the influence of the SGW redox stage and chemistry on the performance - permeate flux and fouling properties - of RO desalination process. SGWs from three different coastal aquifers were sampled and characterized chemically, and RO desalination experiments were performed under inert and oxidized conditions. Our results show that all three aquifers have anoxic saline groundwater and two of them have intensive anaerobic oxidation of organic matter. Two aquifers were found to be in the denitrification stage or slightly lower and the third one in the sulfate reduction stage. Our results indicate that the natural redox stage of SGWs from coastal aquifers affects the performance of RO desalination. All SGW types showed better RO performance over seawater desalination. Furthermore, air oxidation of the SGW was accompanied with pH elevation, which increased the membrane fouling. Hence, keeping the feed water unexposed to atmospheric conditions for maintaining the natural reducing stage of the SGW is crucial for low fouling potential. The observed benefits of using naturally reduced SGW in RO desalination have significant implications for reduction in overall process costs.

A review of the distribution, sources, genesis, and environmental concerns of salinity in groundwater

Awareness concerning the degradation of groundwater quality and their exacerbating adverse effects due to salinization processes is gaining traction, raising for adequate understanding of the distribution, sources, genesis, and environmental concerns of salinity in groundwater. Saline groundwater is widely distributed all over the world, with an area of 24 million km² (16% of the total land area on earth) and 1.1 billion people living in the affected areas, especially the arid/semi-arid areas in developing countries. These large-scale groundwater salinization problems are sourced from two major ways: natural and anthropogenic. The natural sources are diversified from connate saline groundwater, seawater intrusion, evaporation, dissolution of soluble salts, membrane filtration process to geothermal origin. The anthropogenic sources include irrigation return flow, road deicing salts, industrial and agricultural wastewater, and gas and oil production activities. The integrated approach of geochemical tracers and multiple isotopes (δ¹⁸O_H2O, δ²H_H2O, δ¹¹B, δ³⁶Cl, δ³⁴S_sulfate, ⁸⁷Sr/⁸⁶Sr, and δ⁷Li) is proved to be useful in the constraints of the origin and transport of solutes in groundwater. Groundwater salinization is often associated with high levels of some toxic elements like arsenic, fluoride, selenium, and boron. Four "triggers" lead to this association: salt effect, competing adsorption, microbial processes, and cation exchange.

Formation of Large Native Sulfur Deposits Does Not Require Molecular Oxygen

Large native (i.e., elemental) sulfur deposits can be part of caprock assemblages found on top of or in lateral position to salt diapirs and as stratabound mineralization in gypsum and anhydrite lithologies. Native sulfur is formed when hydrocarbons come in contact with sulfate minerals in presence of liquid water. The prevailing model for native sulfur formation in such settings is that sulfide produced by sulfate-reducing bacteria is oxidized to zero-valent sulfur in presence of molecular oxygen (O2). Although possible, such a scenario is problematic because: (1) exposure to oxygen would drastically decrease growth of microbial sulfate-reducing organisms, thereby slowing down sulfide production; (2) on geologic timescales, excess supply with oxygen would convert sulfide into sulfate rather than native sulfur; and (3) to produce large native sulfur deposits, enormous amounts of oxygenated water would need to be brought in close proximity to environments in which ample hydrocarbon supply sustains sulfate reduction. However, sulfur stable isotope data from native sulfur deposits emplaced at a stage after the formation of the host rocks indicate that the sulfur was formed in a setting with little solute exchange with the ambient environment and little supply of dissolved oxygen. We deduce that there must be a process for the formation of native sulfur in absence of an external oxidant for sulfide. We hypothesize that in systems with little solute exchange, sulfate-reducing organisms, possibly in cooperation with other anaerobic microbial partners, drive the formation of native sulfur deposits. In order to cope with sulfide stress, microbes may shift from harmful sulfide production to non-hazardous native sulfur production. We propose four possible mechanisms as a means to form native sulfur: (1) a modified sulfate reduction process that produces sulfur compounds with an intermediate oxidation state, (2) coupling of sulfide oxidation to methanogenesis that utilizes methylated compounds, acetate or carbon dioxide, (3) ammonium oxidation coupled to sulfate reduction, and (4) sulfur comproportionation of sulfate and sulfide. We show these reactions are thermodynamically favorable and especially useful in environments with multiple stressors, such as salt and dissolved sulfide, and provide evidence that microbial species functioning in such environments produce native sulfur. Integrating these insights, we argue that microbes may form large native sulfur deposits in absence of light and external oxidants such as O2, nitrate, and metal oxides. The existence of such a process would not only explain enigmatic occurrences of native sulfur in the geologic record, but also provide an explanation for cryptic sulfur and carbon cycling beneath the seabed.

A Critical Look at the Combined Use of Sulfur and Oxygen Isotopes to Study Microbial Metabolisms in Methane-Rich Environments

Separating the contributions of anaerobic oxidation of methane and organoclastic sulfate reduction in the overall sedimentary sulfur cycle of marine sediments has benefited from advances in isotope biogeochemistry. Particularly, the coupling of sulfur and oxygen isotopes measured in the residual sulfate pool (δ¹⁸O_SO4 vs. δ³⁴S_SO4). Yet, some important questions remain. Recent works have observed patterns that are inconsistent with previous interpretations. We differentiate the contributions of oxygen and sulfur isotopes to separating the anaerobic oxidation of methane and organoclastic sulfate reduction into three phases; first evidence from conventional high methane vs. low methane sites suggests a clear relationship between oxygen and sulfur isotopes in porewater and the metabolic process taking place. Second, evidence from pure cultures and organic matter rich sites with low levels of methane suggest the signatures of both processes overlap and cannot be differentiated. Third, we take a critical look at the use of oxygen and sulfur isotopes to differentiate metabolic processes (anaerobic oxidation of methane vs. organoclastic sulfate reduction). We identify that it is essential to develop a better understanding of the oxygen kinetic isotope effect, the degree of isotope exchange with sulfur intermediates as well as establishing their relationships with the cell-specific metabolic rates if we are to develop this proxy into a reliable tool to study the sulfur cycle in marine sediments and the geological record.

Iron oxides stimulate sulfate-driven anaerobic methane oxidation in seeps

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 (13)C-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.