Abstract
Methane is produced microbially in vast quantities in sediments throughout the world’s oceans. However, anaerobic oxidation of methane (AOM) provides a near-quantitative sink for the produced methane and is primarily responsible for preventing methane emissions from the oceans to the atmosphere. AOM is a complex microbial process that involves several different microbial groups and metabolic pathways. The role of different electron acceptors in AOM has been studied for decades, yet large uncertainties remain, especially in terms of understanding the processes in natural settings. This study reports whole-core incubation methane oxidation rates along an estuarine gradient ranging from near fresh water to brackish conditions, and investigates the potential role of different electron acceptors in AOM. Microbial community structure involved in different methane processes is also studied in the same estuarine system using high throughput sequencing tools. Methane oxidation in the sediments was active in three distinct depth layers throughout the studied transect, with total oxidation rates increasing seawards. We find extensive evidence of non-sulphate AOM throughout the transect. The highest absolute AOM rates were observed below the sulphate-methane transition zone (SMTZ), strongly implicating the role of alternative electron acceptors (most likely iron and manganese oxides). However, oxidation rates were ultimately limited by methane availability. ANME-2a/b were the most abundant microbial phyla associated with AOM throughout the study sites, followed by ANME-2d in much lower abundances. Similarly to oxidation rates, highest abundances of microbial groups commonly associated with AOM were found well below the SMTZ, further reinforcing the importance of non-sulphate AOM in this system.
Highlights
Methane (CH4) is a powerful greenhouse gas affecting the global climate
As sulfate reducing bacteria are involved in S-anaerobic oxidation of methane (AOM) and as we generally wanted to reveal genetic potential for anaerobic sulfate and Fe3? respiration, we focused on known sulfate and Fe3? reducing taxa (Kuever et al 2005; Youssef et al 2009; Kuever 2013; Lovley 2013; Rabus et al 2013)
At site A, CH4 was largely absent in the upper sediments, whereas in contrast C was the only site studied with clearly discernible amounts of CH4 near the sediment–water interface
Summary
Methane (CH4) is a powerful greenhouse gas affecting the global climate. Its atmospheric concentrations have more than doubled since the industrial revolution, due to anthropogenic activities (IPCC 2014). Production of CH4 primarily takes place in sediments through methanogenesis, which is the final step in anaerobic breakdown of organic matter that occurs when other electron acceptors (EA) have been depleted and carbon dioxide (CO2) is the only viable electron acceptor remaining (Thauer 1998). The exact sediment depth of the primary methanogenic zone depends on the organic matter loading of the system, spanning from a few centimeters in productive coastal systems to several meters in the oligotrophic open ocean seabed (Jørgensen et al 2001). Methanogenesis is typically more active in freshwater sediments than in marine sediments, due to the presence of sulphate (SO42-) in seawater (Capone and Kiene 1988), which provides a more energetically favorable pathway for anaerobic remineralization. Eutrophication is expected to increase methanogenesis globally due to enhanced carbon loading (Beaulieu et al 2019)
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