Methane leakage through the sulfate–methane transition zone of the Baltic seabed

UNC Libraries · 2025-12-02

Anaerobic oxidation of methane at the sulfate–methane transition in marine sediments is generally considered to be a near-perfect barrier against methane release from the seabed, but the mechanisms involved are not well understood. On the basis of a survey of Baltic Sea sediments we show that a highly variable amount (0–100%) of subseafloor methane leaks through the sulfate–methane transition. The diffusive methane flux to the sediment–water interface is often high, reaching over 2 mmol m−2 d−1. Even though anaerobic methane oxidation is thermodynamically and kinetically favoured where methane fluxes are high, there is no evidence of methane oxidation in concentration, isotope and modelling results. Cores that lacked anaerobic methane oxidation had high modelled organic matter mineralization rates, suggesting that a possible mechanism could be high electron donor availability due to elevated H2 concentrations, as has been predicted by laboratory studies. We show that methane leakage across the sulfate–methane transition is widespread in organic-rich marine sediments.

Methane leakage through the sulfate–methane transition zone of the Baltic seabed

Nature Geoscience · 2024-12-01 · 22 citations

Zonation of the active methane-cycling community in deep subsurface sediments of the Peru trench

UNC Libraries · 2023-06-16

The production and anaerobic oxidation of methane (AOM) by microorganisms is widespread in organic-rich deep subseafloor sediments. Yet, the organisms that carry out these processes remain largely unknown. Here we identify members of the methane-cycling microbial community in deep subsurface, hydrate-containing sediments of the Peru Trench by targeting functional genes of the alpha subunit of methyl coenzyme M reductase (mcrA). The mcrA profile reveals a distinct community zonation that partially matches the zonation of methane oxidizing and –producing activity inferred from sulfate and methane concentrations and carbon-isotopic compositions of methane and dissolved inorganic carbon (DIC). McrA appears absent from sulfate-rich sediments that are devoid of methane, but mcrA sequences belonging to putatively methane-oxidizing ANME-1a-b occur from the zone of methane oxidation to several meters into the methanogenesis zone. A sister group of ANME-1a-b, referred to as ANME-1d, and members of putatively aceticlastic Methanothrix (formerly Methanosaeta) occur throughout the remaining methanogenesis zone. Analyses of 16S rRNA and mcrA-mRNA indicate that the methane-cycling community is alive throughout (rRNA to 230 mbsf) and active in at least parts of the sediment column (mRNA at 44 mbsf). Carbon-isotopic depletions of methane relative to DIC (−80 to −86‰) suggest mostly methane production by CO2 reduction and thus seem at odds with the widespread detection of ANME-1 and Methanothrix. We explain this apparent contradiction based on recent insights into the metabolisms of both ANME-1 and Methanothricaceae, which indicate the potential for methanogenetic growth by CO2 reduction in both groups.

Zonation of the active methane-cycling community in deep subsurface sediments of the Peru trench

Frontiers in Microbiology · 2023 · 7 citations

• Environmental chemistry
• Chemistry
• Ecology

reduction in both groups.

Understanding the isotopic composition of sedimentary sulfide: A multiple sulfur isotope diagenetic model for Aarhus Bay

American Journal of Science · 2022 · 11 citations

• Geology
• Chemistry
• Mineralogy

Measurement of the multiple sulfur isotopes (³²S/³³S/³⁴S) enables the calibration of microbial biosignatures and provides a unique diagnosis of S-based metabolic processes: sulfate reduction, disproportionation, and sulfide oxidation. All three metabolisms carry distinct geochemical consequences for S cycling in modern systems, and are particularly powerful for paleoenvironmental interpretations if their respective contributions can be separated. To hone those interpretations and to further develop a quantitative context for understanding early diagenetic sulfur cycling, we constructed a multiple S isotope reactive transport model for the sediments of a geochemically well-characterized system (Aarhus Bay, Denmark). The model reconciles pore water and solid phase concentration profiles of the major species associated with Fe/S/C cycling, and uses multiple S isotope systematics to predict the isotope profiles of the major S species, including pore water sulfate, free sulfide and solid phase pyrite. We note that very large fractionations associated with sulfate reduction (³⁴ε_sr = 70‰) are required to reproduce the observed pore water profiles, and we reconcile these fractionations with low temperature theoretical predictions for isotope equilibrium fractionation. The minor sulfur isotope values (noted as Δ³³S) of sulfate increase at shallow depths within the Aarhus Bay core, and decrease when sulfate drops below 10 mM. Values (Δ³³S) for sulfide decrease nearly monotonically towards seawater sulfate values near the zone of sulfate depletion. Pyrite Δ³³S values are nearly uniform downcore (0.170 ± 0.010‰) despite a ∼10‰ enrichment in surface versus deep pyrite δ³⁴S values. Sulfate reduction is the most important process controlling S isotope pore water distributions, with modest contributions from oxidative S cycling. Further, microbial sulfate reduction demonstrates large fractionations typically not expected for shallow, organic rich (TOC ∼ 4%) continental margin systems.

The efficacy of Phoslock® in reducing internal phosphate loading varies with bottom water oxygenation

UNC Libraries · 2022-09-14

Kinetics and identities of extracellular peptidases in subsurface sediments of the White Oak River Estuary, North Carolina

UNC Libraries · 2022-09-14

Anoxic subsurface sediments contain communities of heterotrophic microorganisms that metabolize organic carbon at extraordinarily low rates. In order to assess the mechanisms by which subsurface microorganisms access detrital sedimentary organic matter, we measured kinetics of a range of extracellular peptidases in anoxic sediments of the White Oak River Estuary, NC. Nine distinct peptidase substrates were enzymatically hydrolyzed at all depths. Potential peptidase activities (Vmax) decreased with increasing sediment depth, although Vmax expressed on a per-cell basis was approximately the same at all depths. Half-saturation constants (Km) decreased with depth, indicating peptidases that functioned more efficiently at low substrate concentrations. Potential activities of extracellular peptidases acting on molecules that are enriched in degraded organic matter (D-phenylalanine and L-ornithine) increased relative to enzymes that act on L-phenylalanine, further suggesting microbial community adaptation to access degraded organic matter. Nineteen classes of predicted, exported peptidases were identified in genomic data from the same site, of which genes for class C25 (gingipainlike) peptidases represented more than 40% at each depth. Methionine aminopeptidases, zinc carboxypeptidases, and class S24-like peptidases, which are involved in single-stranded-DNA repair, were also abundant. These results suggest a subsurface heterotrophic microbial community that primarily accesses low-quality detrital organic matter via a diverse suite of well-adapted extracellular enzymes.

Effect of seasonal changes in the pathways of methanogenesis on the δ13C values of pore water methane in a Michigan peatland

UNC Libraries · 2022-09-14 · 1 citations

The δ13C value of pore water methane produced in a Michigan peatland varied by 11% during the year. This isotopic shift resulted from large seasonal changes in the pathways of methane production. On the basis of mass balance calculations, the δ13C value of methane from CO2 reduction (average =-71.4 ± 1.8%) was depleted in 13C compared to that produced from acetate (-44.4 ± 8.2%o). The dissolved methane at the site remained heavy (approximately-51%o) during most of the year. Tracer experiments using 14C-labeled CO2 indicated that during January 110 ± 25% of the methane was produced by CO2 reduction. Because of low-methane production rates during the winter, this C-depleted methane had only a slight effect on the isotopic composition of the methane pool. In early spring when peat temperatures and methane production rates increased, the δ13C value of the dissolved methane in shallow peat was influenced by the isotopically light methane and approached-61‰. Peat incubation experiments conducted at 15°C in May and June (when the peat reaches its maximum temperature) indicated that an average of 84 ± 9% of the methane production was from acetate and had an average δ13C value of-48.7 ± 5.6‰. Rising acetate concentrations during April-May (approaching 1 mmol L-1(mM)) followed by a rapid decrease in acetate concentrations during May-June reflected the shift toward methane production dominated by acetate fermentation. During this period, dissolved methane in shallow peat at the site returned to heavier values (approximately-5 l%o) similar to that produced in the incubation experiments.

Interpreting multiple sulfur isotope signals in modern anoxic sediments using a full diagenetic model (California-Mexico margin: Alfonso basin)

Carolina Digital Repository (University of North Carolina at Chapel Hill) · 2022-09-14 · 1 citations

Recent studies targeting the metabolic, physiological, and biochemical controls of sulfur isotope fractionation in microbial systems have drawn linkages between results from culture experiments and the sulfur isotope signatures observed in natural environments. Several of those studies have used newer techniques to explore the minor isotope (33S and 36S) variability in those systems, and also have attempted to place them in an ecophysiological context. Sparingly few have incorporated this newfound understanding of minor isotope behavior into natural systems (sediment pore waters, water columns) and none of them have refined existing isotope-dependent reaction-transport models to explicitly include 33S. In this study, we construct a three-isotope (32S, 33S, and 34S) reaction-transport model of pore water sulfate for a well-characterized sedimentary system within the California-Mexico Margin (Alfonso Basin). An additional goal is placing recent laboratory culture work into a natural, physical context. The model first reproduces the measured bulk geochemical characteristics of the pore water profiles of [SO4 2], [CH4], dissolved inorganic carbon ([DIC]), and [Ca2]—and predicts bulk (non-isotope-specific but depth-dependent) rates of sulfate reduction. Next, the model uses those depth-dependent bulk rates, in combination with empirically calibrated fractionation factors, to explain the minor isotope characteristics (34S and 33S values) of the 0 to 40 cm pore water SO4 2. The down core, isotopic evolution of pore water sulfate requires a large fractionation associated with sulfate reduction (34 SR 70 5) that appears to be independent of bulk rate, but in line with low temperature thermodynamic predictions. The minor isotope characteristics (33 SR 0.5130) are also independent of rate and fall within the range expected from microbial calibrations, but differ from minor isotope predictions of thermodynamic equilibrium. The high value of 34 SR raises key questions in relating the physiological state of marine microorganisms relative to their laboratory counterparts, as well as point toward exceedingly low metabolic rates in natural marine sediments.

Inhibition experiments on anaerobic methane oxidation

UNC Libraries · 2022-09-14

Anaerobic methane oxidation is a general process important in controlling fluxes of methane from anoxic marine sediments. The responsible organism has not been isolated, and little is known about the electron acceptors and substrates involved in the process. Laboratory evidence indicates that sulfate reducers and methanogens are able to oxidize small quantities of methane. Field evidence suggests anaerobic methane oxidation may be linked to sulfate reduction. Experiments with specific inhibitors for sulfate reduction (molybdate), methanogenesis (2-bromoethanesulfonic acid), and acetate utilization (fluoroacetate) were performed on marine sediments from the zone of methane oxidation to determine whether sulfate-reducing bacteria or methanogenic bacteria are responsible for methane oxidation. The inhibition experiment results suggest that methane oxidation in anoxic marine sediments is not directly mediated by sulfate-reducing bacteria or methanogenic bacteria. Our results are consistent with two possibilities: anaerobic methane oxidation may be mediated by an unknown organism or a consortium involving an unknown methane oxidizer and sulfate-reducing bacteria.