Spatial evidence of cryptic methane cycling and methylotrophic metabolisms along a land–ocean transect in salt marsh sediment

Journal Article · Wed Jul 23 00:00:00 EDT 2025 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2025.07.019· OSTI ID:3003766

Krause, Sebastian J. E. ^[1]; Wipfler, Rebecca L. ^[2]; Liu, Jiarui ^[3]; Yousavich, David J. ^[3]; Robinson, DeMarcus ^[3]; Hoyt, David W. ^[4]; Orphan, Victoria J. ^[2]; Treude, Tina ^[3]

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1. Univ. of California, Los Angeles, CA (United States); Univ. of California, Santa Barbara, CA (United States)
2. California Institute of Technology (CalTech), Pasadena, CA (United States)
3. Univ. of California, Los Angeles, CA (United States)
4. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)

Methylotrophic methanogenesis in the sulfate-rich zone of coastal and marine sediments couples with anaerobic oxidation of methane (AOM), forming the cryptic methane cycle. This study provides evidence of cryptic methane cycling in the sulfate-rich zone across a land–ocean transect of four stations–two brackish, one marine, and one hypersaline–within the Carpinteria Salt Marsh Reserve (CSMR), southern California, USA. Samples from the top 20 cm of sediment from the transect were analyzed through geochemical and molecular (16S rRNA) techniques, in-vitro methanogenesis incubations, and radiotracer incubations utilizing ³⁵S-SO₄, ¹⁴C-mono-methylamine, and ¹⁴C-CH₄. Sediment methane concentrations were consistently low (3 to 28 µM) at all stations, except for the marine station, where methane increased with depth reaching 665 µM. Methanogenesis from mono-methylamine was detected throughout the sediment at all stations with estimated CH₄ production rates in the sub-nanomolar to nanomolar range per cm³ sediment and day. 16S rRNA analysis identified methanogenic archaea (Methanosarcinaceae, Methanomassiliicoccales, and Methanonatronarchaeacea) capable of producing methane from methylamines in sediment where methylotrophic methanogenesis was found to be active. Metabolomic analysis of porewater showed mono-methylamine was mostly undetectable (<3 µM) or present in trace amounts (<10 µM) suggesting rapid metabolic turnover. In-vitro methanogenesis incubations of natural sediment showed no linear methane buildup, suggesting a process limiting methane emissions. AOM activity, measured with ¹⁴C-CH₄, overlapped with methanogenesis from mono-methylamine activity at all stations, with rates ranging from 0.03 to 19.4 nmol cm⁻³ d⁻¹. Geochemical porewater analysis showed the CSMR sediments are rich in sulfate and iron. Porewater sulfate concentrations (9–91 mM) were non-limiting across the transect, supporting sulfate reduction activity (1.5–2,506 nmol cm⁻³ d⁻¹). Porewater sulfide and iron (II) profiles indicated that the sediment transitioned from a predominantly iron-reducing environment at the two brackish stations to a predominantly sulfate-reducing environment at the marine and hypersaline stations, which coincided with the presence of phyla (Desulfobacterota) involved in these processes. AOM activity overlapped with sulfate reduction and porewater iron (II) concentrations suggesting that AOM is likely coupled to sulfate and possibly iron reduction at all stations. However, 16S rRNA analysis identified anaerobic methanotrophs (ANME-2) only at the marine and hypersaline stations while putative methanogens were found in sediment across all stations. In one sediment horizon at the marine station, methanogen families (Methanosarcinaceae, Methanosaetaceae, Methanomassiliicoccales, and Methanoregulaceae) and ANME 2a,2b, and 2c groups were found together. Collectively, our data suggest that at the brackish stations methanogens alone may be involved in cryptic methane cycling, while at the marine and hypersaline stations both groups may be involved in the process. Differences in rate constants from incubations with ¹⁴C-labeled methane and mono-methylamine suggest a non-methanogenic process oxidizing mono-methylamine to inorganic carbon, likely mediated by sulfate-reducing bacteria. Understanding the potential competition of sulfate reducers with methanogens for mono-methylamine needs further investigation as it might be another important process responsible for low methane emissions in salt marshes.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); National Aeronautics and Space Administration (NASA); National Science Foundation (NSF); USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: AC05-76RL01830; SC0022991
• OSTI ID: 3003766
• Report Number(s): PNNL-SA--202988
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 2025 Vol. 404; ISSN 0016-7037
• Publisher: Elsevier; The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

Anaerobic oxidation of methane
Iron reduction
Methanogenesis
Methanogenic archaea
Methanotrophic archaea
Mono-methylamine
NMR metabolomics
Sulfate reduction
Sulfate-reducing bacteria
methylotrophy