Sunita R. Shah Walter

A substantial fraction of marine sediments experience elevated temperatures due to burial or hydrothermal activity. These conditions can fundamentally reshape both microbial activity and the chemical nature of sedimentary organic matter (OM). Laboratory incubations have demonstrated that moderate heating of marine sediments can lead to the production of labile organic compounds such as acetate, however, it remains unclear whether heating alters the bioavailability of the remaining OM pool. In this study, we experimentally tested the effect of temperature on the bioavailability of OM through a series of bioreactor experiments using deeply buried sediment collected from Guaymas Basin (Gulf of California). We measured acetate concentrations in sterilized Guaymas Basin sediments before and after artificial heating (70°C for 7 days) to quantify abiotic acetate generation. We then conducted incubations of a model marine bacterium with sterilized, artificially heated sediment and tracked respired CO₂ production and its associated ¹³C and ¹⁴C signatures. Our study revealed that sediment depth and hydrothermal history strongly control abiotic acetate production, with higher acetate yields from shallower, cooler sediments. Respiration rates in control and heated sediment incubations were nearly identical, indicating that heating does not measurably alter the bioavailability of bulk sedimentary OM. Moreover, the δ¹³C values of respired CO₂ were indistinguishable between control and heated sediment incubations while the Δ¹⁴C values were more depleted in the first 24 hours in incubations with heated sediment. This transient offset suggests that low-temperature heating mobilizes a small pool of older material due to desorption of mineral-bound OM without altering overall bioavailability. Our findings shed light on the role of thermal alteration in shaping carbon cycling in marine sediments by influencing how OM is made available to sedimentary microorganisms.

Most of Earth’s eruptive volcanism occurs along mid-ocean ridges (MORs), yet direct observations of eruptions and their immediate hydrothermal consequences remain rare. On 29 April 2025, the science party of R/V Atlantis Expedition AT50-36 directly observed a long-anticipated eruption at the East Pacific Rise (EPR) 9°N only hours after its onset, representing the most rapidly detected and documented deep-sea MOR eruption to date. The expedition occupied the area from 10 April to 3 May 2025, allowing characterization of hydrothermal and water-column conditions from ~14 days before the eruption to ~96 hours afterward. In this communication we present on-board fluid geochemistry measurements documenting both pre-eruption vent fluid chemistry and post-eruption water-column responses. High-temperature and diffuse-flow fluids were sampled at the Bio9, P Vent, Tica, BioVent, and YBW vent fields during DSV Alvin dives conducted in the days preceding the eruption. Post-eruption bottom waters were investigated using a CTD–rosette system equipped with an in situ electrochemical analyzer. Pre-eruption measurements of dissolved Fe (dFe), H₂S, dissolved Mn (dMn), and pH show elevated H₂S:T and H₂S:dFe ratios relative to previous years at EPR 9°N, consistent with subsurface phase separation and volatile-enriched hydrothermal fluids prior to eruption. Immediately following the eruption, high-temperature vent sources could not be accessed due to aborted Alvin dives; however, CTD profiles revealed pronounced bottom-water anomalies in at least one of pH (up to 0.8 units), H₂S (up to 70 µM), or dFe (up to 841 nM) at CTD stations conducted over Bio9, P Vent, Tica, BioVent vent fields. These geochemical anomalies were spatially widespread along the ridge axis and extended to at least 10 m above the seafloor, with pH and temperature perturbations closely coupled to elevated H₂S concentrations. Although temperature anomalies in bottom waters decayed within four days of the eruption, pH, H₂S, and dFe anomalies persisted. A CTD cast conducted four days post-eruption revealed, via the rosette-mounted electrochemical analyzer, H₂S concentrations of up to 40 µM coincident with turbidity and redox potential anomalies extending to at least 600 m above the seafloor, indicating the development of a vertically extensive hydrothermal megaplume. Such concentration ranges are typically confined to the immediate vicinity of black smoker orifices, highlighting the exceptional spatial scale of hydrothermal discharge following this eruptive event. The near-real-time observation of this MOR eruption provides new constraints on eruption-triggered hydrothermal fluxes, plume formation, and the role of episodic volcanic events in modulating ridge-axis hydrothermal systems.

Salt marshes can emit significant methane to the atmosphere. These emissions are highly variable, but the cause of this variability is not well understood. Although methanogenesis should be inhibited by sulfate reduction where sulfate is present due to its thermodynamic unfavorableness, methane emissions are not well predicted by sulfate concentrations; thus, other controls on methane production must be active. One hypothesis is that where sulfate is present, salt marsh methanogens are fueled by methylated carbon substrates that sulfate reducers do not compete for. It has also been shown that crystalline iron minerals can facilitate increased methane production in low-salinity wetlands, but this has not been explored in salt marshes. This study documents how different organic carbon sources (monomethylamine and ethanol) and Fe(III) minerals (ferrihydrite, magnetite, and hematite) influence methane production by microbial communities from a polyhaline tidal marsh creek in the Great Marsh Preserve, DE, USA. Carbon source had a strong influence on microbial community composition by the end of the incubations. More methane was produced with monomethylamine amendment than with ethanol, and the highest methane production rates were in incubations supplied with both monomethylamine and magnetite or hematite. This increased methane production in the presence of (semi)conductive iron minerals could indicate that interspecies electron transfer was active in some of our treatments. However, instead of the more commonly described syntrophic partners, this interaction appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron-reducing bacterial group, possibly Candidatus Omnitrophus. Much less methane was measured with ethanol and ferrihydrite amendments. However, in ethanol-amended incubations, a small proportion of anaerobic methane oxidizers was detected, which suggests that both methane production and re-oxidation may have occurred, leading to low measured methane production. Although some iron reduction and Fe²⁺ production was observed in all treatments, significant transformation of ferrihydrite to magnetite was observed only with ethanol amendment. If microbially mediated magnetite formation occurs in salt marsh sediment, our observations indicate that the resulting magnetite could enhance methane production by methylotrophic methanogens. This study highlights the importance of methylated compounds to salt marsh methane production as well as the potential importance of iron mineral composition for predicting methane production and iron reduction rates.

Salt marshes can emit significant methane to the atmosphere, but these emissions are highly variable and not well-predicted by sulfate concentrations. The cause of this variability is unclear, but one hypothesis is that rather than being outcompeted where sulfate is present, salt marsh methanogens use carbon substrates that sulfate reducers do not. In low-salinity wetlands, crystalline iron minerals have also been found to facilitate increased methane production, but this has not been explored in salt marshes. This study documents how different organic carbon sources (monomethylamine and ethanol) and Fe(III) minerals (ferrihydrite, magnetite or hematite) influence methane production by microbial communities from a polyhaline tidal marsh creek in the Great Marsh Preserve, DE, USA. Carbon source was the major influence on methane production and microbial community composition at the end of microcosm incubations. Microcosms amended with monomethylamine produced more methane than those with ethanol. The highest methane production rates were in microcosms with both monomethylamine and magnetite or hematite amendments. Less methane was produced with monomethylamine and ferrihydrite, a non-conductive iron oxide, and without iron supplementation. This increased methane production in the presence of (semi)conductive iron minerals could indicate that interspecies electron transfer was active in some of our treatments. Instead of the more commonly described syntrophic partners, however, this interaction appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron reducing bacterial group, possibly Ca. Omnitrophus. Much less measured methane overall was measured with ethanol amendment and there was no discernible effect of iron treatment. A small proportion of anaerobic methane oxidizers was detected in ethanol-amended incubations, which suggests cryptic methane cycling may have occurred, however. Although some iron reduction and Fe²⁺ production was observed in all treatments, significant transformation of ferrihydrite to magnetite was observed only with ethanol amendment. If such microbially-mediated magnetite formation occurs in salt marsh sediment, our observations indicate that the resulting magnetite could enhance methane production by methylotrophic methanogens. This study highlights the importance of methylated compounds such as those released to sediments by Spartina grasses to salt marsh methane production as well as the potential importance of iron mineral composition for predicting methane production and iron reduction rates.

Salt marshes can emit significant methane to the atmosphere, but these emissions are highly variable and not well-predicted by sulfate concentrations. The cause of this variability is unclear, but one hypothesis is that rather than being outcompeted where sulfate is present, salt marsh methanogens use carbon substrates that sulfate reducers do not. In low-salinity wetlands, crystalline iron minerals have also been found to facilitate increased methane production, but this has not been explored in salt marshes. This study documents how different organic carbon sources (monomethylamine and ethanol) and Fe(III) minerals (ferrihydrite, magnetite or hematite) influence methane production by microbial communities from a polyhaline tidal marsh creek in the Great Marsh Preserve, DE, USA. Carbon source was the major influence on methane production and microbial community composition at the end of microcosm incubations. Microcosms amended with monomethylamine produced more methane than those with ethanol. The highest methane production rates were in microcosms with both monomethylamine and magnetite or hematite amendments. Less methane was produced with monomethylamine and ferrihydrite, a non-conductive iron oxide, and without iron supplementation. This increased methane production in the presence of (semi)conductive iron minerals could indicate that interspecies electron transfer was active in some of our treatments. Instead of the more commonly described syntrophic partners, however, this interaction appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron reducing bacterial group, possibly Ca. Omnitrophus. Much less measured methane overall was measured with ethanol amendment and there was no discernible effect of iron treatment. A small proportion of anaerobic methane oxidizers was detected in ethanol-amended incubations, which suggests cryptic methane cycling may have occurred, however. Although some iron reduction and Fe²⁺ production was observed in all treatments, significant transformation of ferrihydrite to magnetite was observed only with ethanol amendment. If such microbially-mediated magnetite formation occurs in salt marsh sediment, our observations indicate that the resulting magnetite could enhance methane production by methylotrophic methanogens. This study highlights the importance of methylated compounds such as those released to sediments by Spartina grasses to salt marsh methane production as well as the potential importance of iron mineral composition for predicting methane production and iron reduction rates.

Salt marshes can emit significant methane to the atmosphere, but these emissions are highly variable and not well-predicted by sulfate concentrations. The cause of this variability is unclear, but one hypothesis is that rather than being outcompeted where sulfate is present, salt marsh methanogens use carbon substrates that sulfate reducers do not. In low-salinity wetlands, crystalline iron minerals have also been found to facilitate increased methane production, but this has not been explored in salt marshes. This study documents how different organic carbon sources (monomethylamine and ethanol) and Fe(III) minerals (ferrihydrite, magnetite or hematite) influence methane production by microbial communities from a polyhaline tidal marsh creek in the Great Marsh Preserve, DE, USA. Carbon source was the major influence on methane production and microbial community composition at the end of microcosm incubations. Microcosms amended with monomethylamine produced more methane than those with ethanol. The highest methane production rates were in microcosms with both monomethylamine and magnetite or hematite amendments. Less methane was produced with monomethylamine and ferrihydrite, a non-conductive iron oxide, and without iron supplementation. This increased methane production in the presence of (semi)conductive iron minerals could indicate that interspecies electron transfer was active in some of our treatments. Instead of the more commonly described syntrophic partners, however, this interaction appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron reducing bacterial group, possibly Ca. Omnitrophus. Much less measured methane overall was measured with ethanol amendment and there was no discernible effect of iron treatment. A small proportion of anaerobic methane oxidizers was detected in ethanol-amended incubations, which suggests cryptic methane cycling may have occurred, however. Although some iron reduction and Fe²⁺ production was observed in all treatments, significant transformation of ferrihydrite to magnetite was observed only with ethanol amendment. If such microbially-mediated magnetite formation occurs in salt marsh sediment, our observations indicate that the resulting magnetite could enhance methane production by methylotrophic methanogens. This study highlights the importance of methylated compounds such as those released to sediments by Spartina grasses to salt marsh methane production as well as the potential importance of iron mineral composition for predicting methane production and iron reduction rates.

Salt marshes can emit significant methane to the atmosphere, but these emissions are highly variable and not well-predicted by sulfate concentrations. The cause of this variability is unclear, but one hypothesis is that rather than being outcompeted where sulfate is present, salt marsh methanogens use carbon substrates that sulfate reducers do not. In low-salinity wetlands, crystalline iron minerals have also been found to facilitate increased methane production, but this has not been explored in salt marshes. This study documents how different organic carbon sources (monomethylamine and ethanol) and Fe(III) minerals (ferrihydrite, magnetite or hematite) influence methane production by microbial communities from a polyhaline tidal marsh creek in the Great Marsh Preserve, DE, USA. Carbon source was the major influence on methane production and microbial community composition at the end of microcosm incubations. Microcosms amended with monomethylamine produced more methane than those with ethanol. The highest methane production rates were in microcosms with both monomethylamine and magnetite or hematite amendments. Less methane was produced with monomethylamine and ferrihydrite, a non-conductive iron oxide, and without iron supplementation. This increased methane production in the presence of (semi)conductive iron minerals could indicate that interspecies electron transfer was active in some of our treatments. Instead of the more commonly described syntrophic partners, however, this interaction appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron reducing bacterial group, possibly Ca. Omnitrophus. Much less measured methane overall was measured with ethanol amendment and there was no discernible effect of iron treatment. A small proportion of anaerobic methane oxidizers was detected in ethanol-amended incubations, which suggests cryptic methane cycling may have occurred, however. Although some iron reduction and Fe²⁺ production was observed in all treatments, significant transformation of ferrihydrite to magnetite was observed only with ethanol amendment. If such microbially-mediated magnetite formation occurs in salt marsh sediment, our observations indicate that the resulting magnetite could enhance methane production by methylotrophic methanogens. This study highlights the importance of methylated compounds such as those released to sediments by Spartina grasses to salt marsh methane production as well as the potential importance of iron mineral composition for predicting methane production and iron reduction rates.

Salt marshes can emit significant methane to the atmosphere, but these emissions are highly variable and not well-predicted by sulfate concentrations. The cause of this variability is unclear, but one hypothesis is that rather than being outcompeted where sulfate is present, salt marsh methanogens use carbon substrates that sulfate reducers do not. In low-salinity wetlands, crystalline iron minerals have also been found to facilitate increased methane production, but this has not been explored in salt marshes. This study documents how different organic carbon sources (monomethylamine and ethanol) and Fe(III) minerals (ferrihydrite, magnetite or hematite) influence methane production by microbial communities from a polyhaline tidal marsh creek in the Great Marsh Preserve, DE, USA. Carbon source was the major influence on methane production and microbial community composition at the end of microcosm incubations. Microcosms amended with monomethylamine produced more methane than those with ethanol. The highest methane production rates were in microcosms with both monomethylamine and magnetite or hematite amendments. Less methane was produced with monomethylamine and ferrihydrite, a non-conductive iron oxide, and without iron supplementation. This increased methane production in the presence of (semi)conductive iron minerals could indicate that interspecies electron transfer was active in some of our treatments. Instead of the more commonly described syntrophic partners, however, this interaction appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron reducing bacterial group, possibly Ca. Omnitrophus. Much less measured methane overall was measured with ethanol amendment and there was no discernible effect of iron treatment. A small proportion of anaerobic methane oxidizers was detected in ethanol-amended incubations, which suggests cryptic methane cycling may have occurred, however. Although some iron reduction and Fe²⁺ production was observed in all treatments, significant transformation of ferrihydrite to magnetite was observed only with ethanol amendment. If such microbially-mediated magnetite formation occurs in salt marsh sediment, our observations indicate that the resulting magnetite could enhance methane production by methylotrophic methanogens. This study highlights the importance of methylated compounds such as those released to sediments by Spartina grasses to salt marsh methane production as well as the potential importance of iron mineral composition for predicting methane production and iron reduction rates.

Though archaea are ubiquitous in the oceans, at times present in quantities equal to bacteria, in general, archaea are underreported when compared with bacteria, and little is known about archaeal connectivity in the ocean. Here, we present the first time series data of size‐fractionated archaea with temporal resolution of days across a depth transect in the oligotrophic Atlantic Ocean. Archaeal communities were variable from day to day, both in regards to community composition and relative abundance. In the context of the total prokaryotic community, we found that water depth and particle size were both determinants of the archaeal share of microbial populations, though the archaeal portion of the community was less discriminating between size and depth classes than the rest of the community. Quantitative PCR shows lower relative abundances of archaea in the deep Atlantic Ocean than previously reported. Overall, we find far lower connectivity between depths in the Atlantic Ocean than suggested by previous studies.

Coastal bays, such as Delaware Bay, are highly productive, ecologically important transitions between rivers and the coastal ocean. They offer opportunities to investigate archaeal assemblages across seasons, with the exchange of water masses that occurs with tidal cycles, and in the context of variable organic matter quality. For a year‐long estuarine, size‐fractionated time series, we used amplicon sequencing, chemical measurements, and qPCR to follow archaeal groups through the seasons. We detected seasonally high abundances of Marine Group II archaea in summer months which correlate with indicators of phytoplankton production, although not phytoplankton biomass. Although previous studies have reported associations between Marine Group II archaea and particles, here they are almost entirely found in very small particles (0.22–0.7 μm), suggesting they are free‐living cells. Populations of Nitrososphaeria did not vary with particle size or environmental conditions. Methanogens were significant fractions of archaeal sequences in large particles at low tide during winter months. Contrary to expectations, Nanoarchaeia were found predominantly in the free‐living fraction despite the previous observation that they require an association with hosts. These results underscore the utility of time series studies in shallow, tidally mixed estuarine environments that capture variable conditions for understanding the ecology and biogeochemistry of planktic archaea.