Concurrent Methane Production and Oxidation in Surface Sediment from Aarhus Bay, Denmark

Area of Science:

• Marine Geochemistry
• Microbial Ecology
• Biogeochemical Cycles

Background:

• Marine surface sediments are typically sulfate-rich and presumed to lack methanogenesis.
• However, the presence of methanogenic archaea suggests a potential for methane production.

Purpose of the Study:

• To quantify methane (CH4) turnover rates in sulfate-rich marine surface sediments.
• To investigate the abundance and types of methanogenic archaea in these environments.

Main Methods:

• Isotope dilution method using 13C-CH4 spiking.
• Sediment bag incubation and analysis of CH4 production and oxidation rates.
• Molecular analyses including quantitative PCR and 16S rRNA gene sequencing.

Main Results:

• Highest CH4 production and oxidation rates (>200 pmol cm-3 d-1) were observed in the top 0-2 cm of sediment.
• Methane turnover and organic matter mineralization decreased with sediment depth.
• Methanogenic archaeal abundance peaked in the top sediment layer, dominated by hydrogenotrophic and methylotrophic pathways.

Conclusions:

• Elevated methanogenic activity and dynamic CH4 recycling occur in sulfate-rich marine surface sediments.
• Methanogenesis in these environments is likely driven by CO2 reduction and fermentation of methylated compounds.
• Cryptic methane cycling may be widespread in similar coastal systems worldwide.