Sarah P. Slotznick

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.

The last eukaryotic common ancestor (LECA) is widely thought to have been an oxygen-respiring organism, arising through endosymbiosis when a free-living bacterium became the mitochondrion. Owing to the mitochondrion s central metabolic task of oxidative phosphorylation, oxygen availability has long been a hypothesized driver of eukaryogenesis. However, this hypothesis is challenged by a temporal disconnect, spanning several hundred million years, between the earliest geochemical evidence for oxygen in the environment (~3.2-2.5 Ga) and the oldest widely accepted eukaryotic fossils (~1.7 Ga). Notably, the earliest candidate eukaryotes appear contemporaneous with the cessation of major iron deposits and rise of sulfide- and sulfate-rich marine sediments in coastal environments. Here, we integrate Proterozoic surface geochemical records with the microbial biochemistry of iron to examine potential environmental constraints on early eukaryotic evolution. Iron bioavailability exerts complex and often antagonistic effects on both aerobic and anaerobic microbial lineages that contributed to LECA. Elevated iron levels likely disrupted cellular homeostasis, particularly by destabilizing labile iron pools and promoting oxidative damage to bacterial lipids. The programmed cell death pathways known as ferroptosis, which is widespread among eukaryotic lineages, may trace its origins to iron-rich conditions in Archaean and Paleoproterozoic seawater and LECA. Our findings challenge oxygen-centered paradigms of eukaryogenesis and reframes the long-recognized temporal gap as a consequence of iron-mediated physiological constraints.

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.

Magnetofossils are magnetic nanoparticles that represent the fossil remains of microorganisms that biomineralize magnetic minerals in a genetically controlled manner. Most magnetofossils found in the geologic record are produced by magnetotactic bacteria, which use them for navigating within their living environment. Magnetofossils can be identified using a combination of magnetic and imaging techniques. A common attribute of magnetofossils, although not pervasive, is that they are arranged in chains, which determines their specific magnetic properties. Magnetofossil signatures have been reported from ancient rocks to modern sediments and even in extraterrestrial materials. They provide a window into biomineralization, past environments, and ancient magnetic fields, as well as supplying fuel for questions on the origin of life in the Solar System.

Figure S1: Low-temperature rock magnetic experiment data for Facies 1 specimens. Figure S2: Low-temperature rock magnetic experiment data for Facies 2 specimens. Figure S3: Low-temperature rock magnetic experiment data for Facies 3 specimens. Figure S4: Cross-plots of rock magnetic parameters delineating magnetic facies. Figure S5: Petrographic data from PC-1 and PRF18. Figure S6: Rock magnetic data from core WPB4. Figure S7: XRD data from bulk specimens. Figure S8: XRD data from clay peels. Figure S9: XRD minerals versus iron speciation oxalate extraction. Figure S10: Thermal demagnetization data for specimen with “mht” component. Figure S11: Inclination shallowing correction applied to Nonesuch paleomagnetic data. Figure S12: Example images of categorized organic material. Figure S13: Provisional palynomorph taxa in stratigraphic context. Table S1: Mean paleomagnetic poles for the Nonesuch Formation. Table S2: Nonesuch rock magnetic data. Table S3: Summary of palynological data.

Figure S1: Low-temperature rock magnetic experiment data for Facies 1 specimens. Figure S2: Low-temperature rock magnetic experiment data for Facies 2 specimens. Figure S3: Low-temperature rock magnetic experiment data for Facies 3 specimens. Figure S4: Cross-plots of rock magnetic parameters delineating magnetic facies. Figure S5: Petrographic data from PC-1 and PRF18. Figure S6: Rock magnetic data from core WPB4. Figure S7: XRD data from bulk specimens. Figure S8: XRD data from clay peels. Figure S9: XRD minerals versus iron speciation oxalate extraction. Figure S10: Thermal demagnetization data for specimen with “mht” component. Figure S11: Inclination shallowing correction applied to Nonesuch paleomagnetic data. Figure S12: Example images of categorized organic material. Figure S13: Provisional palynomorph taxa in stratigraphic context. Table S1: Mean paleomagnetic poles for the Nonesuch Formation. Table S2: Nonesuch rock magnetic data. Table S3: Summary of palynological data.