12 Summary SUMMARY Coastal ecosystems are dynamic hotspots for methane. They are highly sensitive to anthropogenic impacts and climate-change derived challenges. This PhD dissertation investigated the mechanisms by which anaerobic methanotrophs, important microorganisms in counteracting methane emissions, responds to coastal ecosystem stressors such as sulfide, nitric oxide, and salinity-induced osmotic pressure. Additionally, sulfide was studied in the context of sulfatedependent anaerobic oxidation of methane (S-AOM) in meromictic Lake Cadagno. To conduct this investigation, we used a complementary approach, combining co-cultures of anaerobic methanotrophs in bioreactors, enrichments started with anoxic coastal sediments, along with biogeochemical measurements, meta-omics analysis, stable isotope activity tests, molecular assays, and physiological studies (Figure 1). In Chapter 1, we introduce the environmental relevance of methane oxidation in coastal ecosystems and present the potential of employing a culture of anaerobic methane oxidizing archaea (ANME) from the genus “Candidatus (Ca.) Methanoperedens” as a study model. Ultimately, we identify current knowledge gaps and outline the research questions that guided this thesis. In Chapter 2, we concluded that a new genus of ANME archaea might be able to act as a methane biofilter in the coastal Stockholm Archipelago under hypoxic but not euxinic conditions due to sulfide toxicity. ANME-2 Metagenome Assembled Genome (MAG) 011 was first classified as ANME-2b on the original paper and later updated with the latest Genome Taxonomy Database (GTBD) classification for this thesis (September 2024) as genus QBUR01 closely related to “Ca. Methanocomedens” (canonical ANME-2a). For this investigation, we defined sulfide inhibition for sulfate S-AOM with half inhibitory thresholds at ~ 1 mM sulfide. We also hypothesized that the sulfite reductase from the Fsr Group II belonging to the identified ANME MAG could aid in sulfite detoxification. In Chapter 3, we monitored two coastal anoxic sediment microbiomes for over a year under distinct methane, nitrogen (nitrate and ammonium), and sulfide
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