This study evaluated the long-term impact of negative hydraulic barriers, created by saline groundwater (SGW) pumping in confined coastal aquifers, on groundwater dynamics by combining multiyear field monitoring of pumping and nearby observation wells, hydrogeochemical analyses, and 3D numerical modeling. Monitoring data revealed that the fresh saline groundwater interface (FSI) dynamics was primarily governed by SGW pumping regimes. SGW pumping (180-260 m3/h) reduced subaquifer salinity (5-10%) over two years in observation wells 100 m inland. Six years of pumping achieved 12% salinity reduction in the pumped water, at a gradually decreasing rate. Geochemical analyses of the pumped water revealed an evolution of individual elements that deviate from trends associated with salinity decrease, specifically, a nonconservative 20% increase in the Ca2+/Cl- ratio alongside a 15% decrease in the B/Cl- ratio. Numerical model simulations fitted short-term hydrological observations and provided long-term prediction of the negative hydraulic barrier efficiency, demonstrating that the FSI becomes restrained at the pumping site. These findings validate the negative hydraulic barrier as a viable management strategy for confined coastal aquifers, offering a scalable framework for optimizing negative hydraulic barrier design to facilitate seawater intrusion mitigation and sustainable SGW production with potential downstream industrial applications.

Marine sediments host a vast deep biosphere, yet how microorganisms persist under severe energy limitation and govern long-term organic carbon (OC) preservation remains poorly understood. Here we show that archaea, primarily Bathyarchaeia, systematically displace bacteria with depth and form net growth zones across East China Sea shelf deep sediments. Multi-omics analyses and bioenergetic modelling reveal that this transition is driven by sustained archaeal metabolism of diverse recalcitrant OC compounds, and a physiological trade-off that prioritizes cellular maintenance over growth, minimizing mortality in deep sediments. This strategy triggers a fundamental shift in sedimentary carbon turnover: from rapid bacterial degradation of labile OC near the surface to persistent archaea-driven turnover of recalcitrant OC at depth. We estimate that Bathyarchaeia mediate ~77% of total OC degradation after 1,000 years of burial, corresponding to ~18% (~11.4 Pg C) of millennial OC degradation in global shelf sediments. These findings identify subsurface archaea as key microbial regulators of long-term OC preservation and reveal how physiological trade-offs sustain life and carbon turnover in the energy-limited deep biosphere.

Chemical signaling between soil inhabitants is of tremendous importance to the health of many ecosystems, and at the same time detailed molecular knowledge underlying these conversations is surprisingly scarce. One of the major bacterial genera inhabiting the rhizosphere is Pseudomonas, of which most species are known to produce phenazines, which carry antibiotic properties. Pseudomonas chlororaphis, a common rhizosphere dwelling species with plant growth-promoting traits, produces phenazine-1-carboxamide (PCN). This study examines how the production of PCN by Pseudomonas affects another common species in soil that it often encounters, namely Bacillus subtilis. When both species were cultured at close distance, distinct and visible changes in colony morphologies were observed without changes in growth rates. Interestingly, a clear transformation occurred in the morphology of B. subtilis colonies in the presence of supplemented PCN, indicating the role of phenazines in affecting colony morphology. In addition, untargeted metabolomics analyses showed a decrease in the production of plipastatin and surfactin by B. subtilis in the presence of P. chlororaphis. Our results indicate that PCN induces changes in morphology and signaling of B. subtilis without significantly affecting its growth. We hypothesize that P. chlororaphis and B. subtilis sense one another and act to conserve energy while avoiding competition.

Thermokarst lakes, formed by permafrost thaw in the Arctic, are ubiquitous with abrupt permafrost thaw and large atmospheric source "hotspots" of methane (CH4) and carbon dioxide (CO2) emissions, which are expected to double permafrost carbon emissions by the end of the century. While the implications of ongoing permafrost thaw on CH4 dynamics within these lakes have been modeled, here we provide empirical data on CH4 production dynamics as lakes evolve from young recently formed lakes to older lakes that have been present for hundreds of years. Sediment cores were collected from the centers and thermokarst margins of a new thermokarst lake and from an older thermokarst lake from the same interior Alaskan watershed. The highest CH4 production rates were observed in the uppermost sediments near the sediment-water interface at the thermokarst margins of both lakes, with a steep decrease with sediment depth into the talik. The young lake exhibited elevated CH4 production rates, correlated with higher carbon lability. The integrated sediment-column CH4 production rates were similar, primarily due to the thinner talik at the young lake. Our data support the predictions that formation and expansion of thermokarst lakes over the next centuries will increase CH4 production in newly thawed Yedoma permafrost sediments, while CH4 production will decrease as taliks mature and labile organic carbon is used up. Our results also suggest important controls of methane production and oxidation in the sediments.

Yedoma-permafrost holds disproportionately large carbon and nitrogen pools, concentrated in icy, Pleistocene-aged silt deposits in the Arctic. Upon thaw, these undergo microbial mineralization, releasing greenhouse gases (GHGs) including carbon-dioxide (CO2), methane (CH4) and nitrous-oxide (N2O). Here we present combined geochemical data with microbial function and community dynamics from deep-talik soil boreholes in an unsaturated yedoma upland. Our results reveal significant in-situ spatio-temporal seasonal shifts in microbial functional, community composition and diversity within 7-m deep upland talik. In situ methanogenesis persisted in the soil talik throughout the year due to the permafrost thaw. In the winter methanotrophy was negligible within and above the methanogenic zone, leading to elevated CH4 emissions to the atmosphere. This is likely due to reduced microbial methanotrophic activity, associated with lower temperatures and nitrogen availability. During summer, at and above the anoxic methanogenic zone, nitrate/nitrite mediated anaerobic oxidation of methane (N-AOM) by ANME2d and the NC-10 phylum, together with aerobic methanotrophy near the soil surface, significantly attenuated CH4 emissions. Nitrous-oxide concentrations peaked at 10 cm (7.2 uM) and 105 cm (6.7 uM) and were associated with denitrification and N-AOM by Methanoperedens (ANME2d). In the summer only and within the top 1 m of soil, high expression of nitrogen related genes (narG, norB, amoA, Annamox, and Feammox) indicated active redox dynamics, potentially providing nitrogen species for N-AOM. The potential N2O emissions in summer may imply higher net GHGs emission from yedoma uplands as climate warming leads to longer summers and warmer soils in the future. ### Competing Interest Statement The authors have declared no competing interest.

Wastewater reuse is essential to water supply and conservation efforts. Anaerobic membrane bioreactors (AnMBR) are emerging as an efficient treatment, enabling high effluent quality and low energy consumption. Subsequent reverse osmosis (RO) treatment can achieve potable water quality. Although mineral fouling in RO has been extensively studied, research on iron-based minerals, particularly under anaerobic conditions, remains notably scarce. Specifically, the potential fouling caused by Vivianite (Fe3(PO4)28H2O) - a mineral prone to precipitate and clog pipes of anaerobic wastewater streams - during high recovery RO applied to anaerobic effluents (e.g., following AnMBR) had not been systematically studied. This study used geochemical modeling and filtration experiments to test the potential mineral fouling of ferrous ion-bearing minerals in RO of synthetic and real AnMBR effluents. The geochemical model calculated mineral precipitation potential at pH 5-10 and an 85-95 % recovery ratio. Vivianite and amorphous calcium phosphate (ACP) were the dominant precipitants. Filtration experiments at varying feed Fe2+ concentrations (0-10 mg/L) and pH (6.0-7.1) revealed the significant effect of these parameters on mineral fouling type and extent, agreeing with the geochemical model. Vivianite precipitated at all pH values, significantly reducing (>90 %) at pH 6 compared to pH 6.75 & 7.1. Higher Fe2+ feed concentrations were correlated with a higher extent of vivianite mineral fouling. ACP was observed only at pH 7.1 since it competed with vivianite over P utilization. This research presents the first systematic investigation of vivianite mineral fouling in RO processes, informing process design of anaerobic effluent water reuse processes.

Magnetite (Fe3O4), a ubiquitous sedimentary iron mineral, is crucial for paleomagnetic records preservation. However, reactive ferric iron minerals, including magnetite, can undergo reduction in aquatic sediments above and within the sulfidic zone and at the Sulfate-Methane Transition Zone (SMTZ), resulting in the production of dissolved ferrous iron. Partial reoxidation of the reduced iron at the oxic-anoxic interface can lead to authigenic magnetite precipitation. Yet, magnetite persistence and behavior in deeper methanic sediments have remained poorly understood. Here we explore magnetite dynamics in different sub-methanic zones (deeper, middle and upper) of Mediterranean continental shelf sediments, including the potential for its authigenic precipitation. Sequential extractions revealed increasing magnetite concentrations accompanied by low-temperature magnetization (Verwey transition). First-order reversal curve (FORC) analyses supported nanoscale authigenic magnetite presence. The results highlight a net increase in single-domain magnetite in the middle methanic zone with declines in the upper and deeper zones. Microbial analyses pointed to iron reduction throughout the methanic zone, alongside potential precipitation of magnetite and a decline in methanogenesis functional genes. Sediment incubations with spiked 57Fe-ferrihydrite showed gross precipitation of isotopically enriched 57Fe-magnetite in the upper methanic zone. Our combined findings distinguish between gross and net magnetite precipitation, suggesting authigenic magnetite formation within the methanic zone, with reshaping and smoothing of the original magnetic signal. They also emphasize the limitations of relying on a single-method approach to unravel such complex processes. We propose that the methanic zone plays a critical role in the early diagenesis of magnetic minerals, driven by dynamic cycles of magnetite dissolution and authigenic precipitation. ### Competing Interest Statement The authors have declared no competing interest.

Chemical conversations between soil inhabitants are of tremendous importance to the health of many ecosystems, and at the same time detailed molecular knowledge underlying these conversations is surprisingly scarce. One of the major bacterial genera inhabiting the rhizosphere is Pseudomonas , of which most species are known to produce phenazines, which carry antibiotic properties. Pseudomonas chlororaphis , a common rhizosphere dwelling species with plant growth-promoting traits, produces phenazine-1-carboxamide (PCN). This study examines how the production of PCN by Pseudomonas affects another common species in soil that it often encounters, namely Bacillus subtilis . When both species were cultured at close distance, distinct and visible changes in colony morphologies were observed without changes in growth rates. Interestingly, a clear transformation occurred in the morphology of B. subtilis colonies in the presence of supplemented PCN, indicating the role of phenazines in affecting colony morphology. In addition, untargeted metabolomics analyses showed a decrease in the production of plipastatin and surfactin by B. subtilis in the presence of P. chlororaphis . Our results indicate that PCN induces changes in morphology and signaling of B. subtilis without significantly affecting its growth. We hypothesize that P. chlororaphis and B. subtilis sense one another and act to conserve energy while avoiding competition. ### Competing Interest Statement The authors have declared no competing interest. Israel Science Foundation, https://ror.org/04sazxf24, 2170/24 European Research Council, https://ror.org/0472cxd90, 818450

Large carbon and nitrogen pools are disproportionately concentrated in the icy, Pleistocene-aged silt deposits of Arctic Yedoma permafrost. Upon thaw, these undergo microbial mineralization, releasing greenhouse gases (GHGs) including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Here, we present combined geochemical data with microbial function and community dynamics from deep (7-m) talik soil boreholes in water-unsaturated yedoma upland in interior Alaska. Our results reveal significant in situ seasonal shifts in microbial function, community composition, and diversity throughout the talik. Methanogenesis persisted in the deep talik year-round. Winter methanotrophy was negligible within and above the methanogenic zone, leading to elevated CH4 production and emission to the atmosphere. This is likely due to reduced microbial methanotrophic activity associated with lower temperatures and nitrogen availability. During summer, strong aerobic methanotrophy near the soil surface reduced CH4 emissions. Nitrate/nitrite-mediated anaerobic oxidation of methane (AOM) by both archaea (ANME-2d clade) and bacteria (NC10 phylum) occurred at and above the anoxic methanogenic zone, further offsetting CH4 production. In contrast to CH4 production potentials, which were higher in surface soils in winter compared to summer, we observed higher N2O production potentials in summer compared to winter. Nitrous oxide concentrations peaked at 10 cm (7.2 μM) and 105 cm (6.7 μM) and were associated with denitrification; nitrogen-mediated AOM by Methanoperedens (ANME2d). In the summer only and within the top 1 m of soil, high expression of nitrogen-related genes (narG, norB, amoA, Annamox, and Feammox) indicated active redox dynamics, potentially providing nitrogen species for AOM. The potential N2O production in summer may imply higher net GHG emissions from yedoma uplands as climate change leads to longer summers and warmer soils in the future.

Thermokarst lakes, formed by permafrost thaw in the Arctic, are hotspots for methane (CH4) and carbon dioxide (CO2) emissions and are expected to double permafrost carbon emissions by the end of the century. While the implications of ongoing permafrost thaw on CH4 dynamics in these lakes have been modeled, here we provide empirical data on CH4 production dynamics as lakes evolve from young recently formed lakes to older lakes that have been present for hundreds of years. Sediment cores (up to 4 m long) were collected from the centers and thermokarst margins of a new thermokarst lake (Big Trail Lake (BTL), < 70 years old) and from an older thermokarst lake (Goldstream Lake (GSL), ∼ 900 years old) from the same interior Alaskan watershed. The highest CH4 production rates were observed in the uppermost sediments near the sediment–water interface at the thermokarst margins of both lakes, with a steep decrease with sediment depth into the talik. BTL exhibited elevated CH4 production rates, correlated with higher carbon lability for thermal-induced reactions measured by Rock-Eval analyses, suggesting its potential use as a proxy for organic carbon breakdown by methanogenesis. In contrast, GSL displayed lower CH4 production rates, likely due to a longer period of organic carbon degradation and reduced carbon lability. The integrated sediment-column CH4 production rates were similar (around 7 to 10 mol m−2 yr−1), primarily due to the thinner talik at BTL. Our data support the predictions that the formation and expansion of thermokarst lakes over the next centuries will increase CH4 production in newly thawed Yedoma permafrost sediments, while CH4 production will decrease as taliks mature and labile organic carbon is used up.

Microbial iron and manganese respiration processes have been observed in deep methanic sediments of lacustrine and marine environments, challenging the "classical" model of microbial respiration in aquatic systems. Nonetheless, assessments of the type and relative role of these respiration processes in the methanic zone are lacking. Here, we quantify both the thermodynamic and the kinetic controls of potential iron and manganese respiration processes in the diffusive controlled steady state methanic sediments of lacustrine and marine sites - Lake Kinneret (LK) and the Southeastern Mediterranean Sea (MedS). We consider the substrates (electron donors) and iron and manganese oxides (electron acceptors) at concentrations that have been measured at these sites. Using theoretical bioenergetic methods, we develop a nominal model to calculate catabolic rates, considering both kinetic and thermodynamic parameters. Then, we estimate the biomass growth rates from the catabolic rates, the energy generated in each reduction-oxidation (redox) reaction, the biomass yield from a given amount of energy, the number of cells participating in each reaction, and the energetic needs of the cells. Lastly, we estimate the microbial community sizes of expected iron and manganese reducers. Additionally, we perform a Monte Carlo simulation to account for variations in uncertain parameter values, along with a sensitivity analysis. Together, these calculations enable estimation of the expected total reaction rates of the various metabolic processes. Our results indicate that the type of iron or manganese oxide, which determines its thermodynamic and kinetic properties, is more significant in influencing bioreaction rates than its concentration. Thus, bioreactions with amorphous manganese oxides are more favorable than those with highly reactive iron oxides. Among the iron oxides, the reduction of amorphous iron oxyhydroxide and ferrihydrite are the only reactions capable of generating biomass in the methanic sediments at both sites. In both environments, manganese oxide reduction by ammonium and methane oxidation are expected to be significant, while manganese oxide reduction by hydrogen and acetate oxidation are expected to be considerable only in LK. The most probable iron oxide reduction process in LK is hydrogen oxidation, followed by methane oxidation. In the MedS iron oxide reduction is most probably coupled to the oxidation of ammonium (Feammox) to molecular nitrogen (N2), and in a few cases may be coupled to methane oxidation. The Monte Carlo simulation agrees with the nominal model results for manganese reduction, and additionally predicts that iron reduction may be possible with some combinations of parameter values. These findings improve our understanding of the thermodynamic and kinetic controls on the composition of microbial communities and their effect on the geochemistry of methanic sediments.

Background Microbial methane oxidation, methanotrophy, plays a crucial role in mitigating the release of the potent greenhouse gas methane from aquatic systems. While aerobic methanotrophy is a well-established process in oxygen-rich environments, emerging evidence suggests their activity in hypoxic conditions. However, the adaptability of these methanotrophs to such environments has remained poorly understood. Here, we explored the genetic adaptability of aerobic methanotrophs to hypoxia in the methanogenic sediments of Lake Kinneret (LK). These LK methanogenic sediments, situated below the oxidic and sulfidic zones, were previously characterized by methane oxidation coupled with iron reduction via the involvement of aerobic methanotrophs. Results In order to explore the adaptation of the methanotrophs to hypoxia, we conducted two experiments using LK sediments as inoculum: i) an aerobic "classical" methanotrophic enrichment with ambient air employing DNA stable isotope probing (DNA-SIP) and ii) hypoxic methanotrophic enrichment with repeated spiking of 1% oxygen. Analysis of 16S rRNA gene amplicons revealed the enrichment of Methylococcales methanotrophs, being up to a third of the enriched community. Methylobacter , Methylogaea , and Methylomonas were prominent in the aerobic experiment, while hypoxic conditions enriched primarily Methylomonas . Using metagenomics sequencing of DNA extracted from these experiments, we curated five Methylococcales metagenome-assembled genomes (MAGs) and evaluated the genetic basis for their survival in hypoxic environments. A comparative analysis with an additional 62 Methylococcales genomes from various environments highlighted several core genetic adaptations to hypoxia found in most examined Methylococcales genomes, including high-affinity cytochrome oxidases, oxygen-binding proteins, fermentation-based methane oxidation, motility, and glycogen use. We also found that some Methylococcales, including LK Methylococcales, may denitrify, while metals and humic substances may also serve as electron acceptors alternative to oxygen. Outer membrane multi-heme cytochromes and riboflavin were identified as potential mediators for the utilization of metals and humic material. These diverse mechanisms suggest the ability of methanotrophs to thrive in ecological niches previously thought inhospitable for their growth. Conclusions Our study sheds light on the ability of enriched Methylococcales methanotrophs from methanogenic LK sediments to survive under hypoxia. Genomic analysis revealed a spectrum of genetic capabilities, potentially enabling these methanotrophs to function. The identified mechanisms, such as those enabling the use of alternative electron acceptors, expand our understanding of methanotroph resilience in diverse ecological settings. These findings contribute to the broader knowledge of microbial methane oxidation and have implications for understanding and potential contribution methanotrophs may have in mitigating methane emissions in various environmental conditions.

Marine continental shelf sediments with high deposition rates may provide useful archives of rapid geomagnetic secular variation as long as the primary magnetization is not altered substantially by diagenesis. To quantify the effects of sulfate (SO42-) reduction, which is a dominant early diagenetic process in such sediments, on paleomagnetic recording, we analyzed four 6-m long sediment cores from the eastern Mediterranean shelf. Two cores did not reach the methanogenic zone and are characterized by continuous organoclastic sulfate reduction (OSR), while the other two have a distinctive shallow sulfate-methane transition zone (SMTZ). Age models based on 28 radiocarbon ages indicate steady deposition rates with spatially varying age spans, which suggest that different parts of the shelf stopped accumulating sediments at different times during the Holocene. The upper sediment column in all cores is dominated by detrital titanomagnetite and biogenic magnetite. OSR-affected sediments record continuous (titano) magnetite dissolution, which resulted in steady magnetic susceptibility and remanence decreases. For cores that reach the methanogenic zone, similar behavior is observed at or above the STMZ, but magnetic properties stabilize at greater depths. Paleomagnetic directions in these sediments are more coherent, with better agreement with geomagnetic models than sediments affected by OSR. We suggest that methane-rich sediments with a shallow SMTZ and high sedimentation rates can better preserve primary paleomagnetic signals than OSR-dominated sediments due to a lack of dissolved sulfide in the main methanogenic zone, and that a susceptibility decline with depth should be a warning sign for paleomagnetic studies.

Abstract Landscape drying associated with permafrost thaw is expected to enhance microbial methane oxidation in arctic soils. Here we show that ice-rich, Yedoma permafrost deposits, comprising a disproportionately large fraction of pan-arctic soil carbon, present an alternate trajectory. Field and laboratory observations indicate that talik (perennially thawed soils in permafrost) development in unsaturated Yedoma uplands leads to unexpectedly large methane emissions (35–78 mg m−2 d−1 summer, 150–180 mg m−2 d−1 winter). Upland Yedoma talik emissions were nearly three times higher annually than northern-wetland emissions on an areal basis. Approximately 70% emissions occurred in winter, when surface-soil freezing abated methanotrophy, enhancing methane escape from the talik. Remote sensing and numerical modeling indicate the potential for widespread upland talik formation across the pan-arctic Yedoma domain during the 21st and 22nd centuries. Contrary to current climate model predictions, these findings imply a positive and much larger permafrost-methane-climate feedback for upland Yedoma.

The ongoing global temperature rise enhances permafrost thaw in the Arctic, allowing Pleistocene-aged frozen soil organic matter to become available for microbial degradation and production of greenhouse gases, particularly methane. Here, we examined the extent and mechanism of anaerobic oxidation of methane (AOM) in the sediments of four interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice-rich permafrost thaw. In cores of surface (similar to 1 m) lake sediments we quantified methane production (methanogenesis) and AOM rates using anaerobic incubation experiments in low (4 degrees C) and high (16 degrees C) temperatures. Methanogenesis rates were measured by the accumulation of methane over similar to 90 d, whereas AOM rates were measured by adding labeled-(CH4)-C-13 and measuring the produced dissolved inorganic C-13. Our results demonstrate that while methanogenesis was vigorous in these anoxic sediments, AOM was lower by two orders of magnitude. In almost all sediment depths and temperatures, AOM rates remained less than 2% of the methanogenesis rates. Experimental evidence indicates that the AOM is strongly related to methanogens, as the addition of a methanogens' inhibitor prevented AOM. Variety of electron acceptor additions did not stimulate AOM, and methanotrophs were scarcely detected. These observations suggest that the AOM signals in the incubation experiments might be a result of enzymatic reversibility ("back-flux") during CH4 production, rather than thermodynamically favorable AOM. Regardless of the mechanism, the quantitative results show that near surface (< 1-m) thermokarst sediments in interior Alaska have little to no buffer mechanisms capable of attenuating methane production in a warming climate.

&lt;p&gt;About 40% of the annual methane emissions originate from natural, non-anthropogenic sources. These include mainly freshwater sediments, in which significant increase in methane emissions has been observed throughout the past decades with the ongoing global temperature rise. Thermokarst lakes, formed by abrupt thawing of permafrost, play a significant role in this observed increase in methane emissions. However, methane production rates and natural consumption controls there are not well constrained, as well as their response to global warming. &amp;#160;&lt;/p&gt;&lt;p&gt;We explore the rates and mechanisms of methane production and anaerobic oxidation (AOM) processes several interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice-rich permafrost thaw. This is mainly through geochemical and microbial profiles combined with slurry incubation experiments with labeled isotopes, potential electron acceptors and several inhibitors in different temperatures. Our manipulated experiments shed insight on the controls of methanogenesis onset and the mechanisms of both methanogenesis and AOM. Direct rate measurements using two isotope methods and modeling provide robust rate estimations for methanogenesis and AOM. They indicate that the role of AOM in these lakes is less significant than previous estimations, and that AOM will probably not attenuate the methanogenesis increase in a warmer climate.&amp;#160;&lt;/p&gt;

In methane (CH4) generating sediments, methane oxidation coupled with iron reduction was suggested to be catalyzed by archaea and bacterial methanotrophs of the order Methylococcales. However, the co-existence of these aerobic and anaerobic microbes, the link between the processes, and the oxygen requirement for the bacterial methanotrophs have remained unclear. Here, we show how stimulation of aerobic methane oxidation at an energetically low experimental environment influences net iron reduction, accompanied by distinct microbial community changes and lipid biomarker patterns. We performed incubation experiments (between 30 and 120 days long) with methane generating lake sediments amended with 13C-labeled methane, following the additions of hematite and different oxygen levels in nitrogen headspace, and monitored methane turnover by 13C-DIC measurements. Increasing oxygen exposure (up to 1%) promoted aerobic methanotrophy, considerable net iron reduction, and the increase of microbes, such as Methylomonas, Geobacter, and Desulfuromonas, with the latter two being likely candidates for iron recycling. Amendments of 13C-labeled methanol as a potential substrate for the methanotrophs under hypoxia instead of methane indicate that this substrate primarily fuels methylotrophic methanogenesis, identified by high methane concentrations, strongly positive δ13CDIC values, and archaeal lipid stable isotope data. In contrast, the inhibition of methanogenesis by 2-bromoethanesulfonate (BES) led to increased methanol turnover, as suggested by similar 13C enrichment in DIC and high amounts of newly produced bacterial fatty acids, probably derived from heterotrophic bacteria. Our experiments show a complex link between aerobic methanotrophy and iron reduction, which indicates iron recycling as a survival mechanism for microbes under hypoxia.

Magnetic properties of marine sediments are dictated not only by the detrital mineralogy, but also by diagenetic processes that can start instantaneously after deposition and may proceed for a long time as sediments are buried. Early diagenesis encompass a range of biochemical reactions associated with bacterial respirations. These may include oxidation at the water sediment interface, iron reduction, sulfate reduction, and anaerobic oxidation of methane (AOM) if methane is present. To investigate the link between diagenesis, sedimentary magnetic properties and quality of paleomagnetic recording, we collected eight 6m-long piston-cores from the Holocene Eastern Mediterranean continental shelf in four locations. Two locations are characterized by high concentration of methane and detectable sulfate-methane transition zone (SMTZ) at depth of 1-4 m. In the other two locations, organoclastic sulfate reduction is dominant throughout the entire cores. Sedimentation rates in this region range between 1 - 5 mm/year. In all cores, concentrations of sulfate, methane and ferrous iron were measured from the pore water. The geochemical data were compared to the mineral magnetic profile that include a range of parameters calculated from IRM, ARM, low- and high- field susceptibility, hysteresis, and FORCs. Paleomagnetic time-series of declination and inclination were obtained from demagnetization experiments carried out in 2 cm resolution. Age models were constructed from radiocarbon dating of carefully collected foraminifera. The results show a consistent link between the diagenetic zones and the magnetic mineralogy: Increase of magnetic properties in the shallow ferruginous zone, decay of magnetic parameters in the sulfate reduction zones, rapid decrease at the SMTZ and stabilization at the methanogenic zone. XHR-SEM analysis show multiple generations of greigite and pyrite framboids at all depths and unaltered detrital titanomagnetites. We find that except a short time interval below the SMTZ of one core, the paleomagnetic directions in these sediments do not represent the expected directions of the geomagnetic field. We conclude that continuous organoclastic sulfate reduction in marine sediments might have a profound effect on the quality of paleomagnetic recording, but AOM at the SMTZ may help stabilize the magnetic phase.