32 Chapter 1 THE INNER WORKINGS OF THE METHANE BIOFILTER The natural methane biofilter is a microbial network that operates in both oxic and anoxic conditions, playing a pivotal role in mitigating methane emissions from coastal ecosystems. The biofilter comprises two primary groups of microorganisms: methane-oxidizing bacteria (MOB) (Kalyuzhnaya et al., 2019) and anaerobic methane-oxidizing archaea (ANME) (Chadwick et al., 2022; Timmers et al., 2017) (Figure 2). Aerobic methane oxidation primarily occurs in the oxygen-rich layers of water columns and sediments, where MOB use oxygen to oxidize methane into carbon dioxide. The Alphaproteobacteria methanotrophs appear to be more pronounced in well-oxygenated environments, while the Gammaproteobacteria methanotrophs seem to tolerate better less-oxygenated or anoxic zones (Li et al., 2024). The adaptation of methanotrophs to also operate in (micro)oxic zones make the methane biofilter a flexible system that responds to oxygen fluctuations in sediments and water columns (Li et al., 2024; Reis et al., 2024). These metabolic adaptations might include potential for denitrification, fermentation or metal oxide reduction under oxygen-limited conditions (Kits et al., 2015; Li et al., 2023). Anaerobic oxidation of methane (AOM), mediated by ANME archaea, typically occurs in deeper anoxic sediments where electron acceptors such as sulfate, nitrate, iron-, and manganese-oxides are available (Figure 2) (Chadwick et al., 2022; Egger et al., 2018; Glodowska et al., 2022; Knittel & Boetius, 2009). In marine environments, ANME archaea from the groups ANME-1, ANME-2abc and ANME-3 act in concert with sulfate-reducing bacteria (SRB), coupling sulfate reduction to methane oxidation (Murali et al., 2023b) (Figure 3). Intriguingly, there is also evidence for iron reduction in ANME-2a enrichments (Slobodkin et al., 2023). The sedimentary interval commonly known as the sulfate-methane transition zone (SMTZ) has been extensively used to localize the AOM process in marine and brackish systems, as this is the place where the sedimentary methane biofiltering takes place and were described in Chapters 2-3 and including Chapter 7, with a particular sulfate-rich freshwater SMTZ.
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