254 Chapter 8 Although Chapter 5 worked with the plasmid harboring “Ca. Methanoperedens BLZ2” culture, the involvement of the plasmid was not specifically analyzed under sulfide stress conditions, as the focus was on sulfide/sulfite detoxification mechanisms. In Chapter 7, we did not find sufficient evidence to conclude that the “Ca. Methanoperedens” spp. studied harbored Borgs, based on the markers used. To date, Borgs have only been identified in environmental “Ca. Methanoperedens”, while bioreactor culture representatives seem to lack them In the Borg screening of “Ca. Methanoperedens” spp. from Lake Cadagno, the 40 previously described Borg markers were derived from “Ca. Methanoperedens” in a different ecosystem (soil samples from a wetland) (Schoelmerich et al., 2024). This difference complicates the search for “Ca. Methanoperedens” with Borgs in ecosystems with distinct chemistries or niche adaptations. Chapter 7 also includes the screening of additional ECEs, but no conclusive matches were found for viruses but perhaps other potential ECEs in two of the “Ca. Methanoperedens” spp. studied. CONCLUDING REMARKS AND FUTURE PERSPECTIVES This PhD dissertation sheds light on the metabolic adaptions of complex methane cycling communities and freshwater ANME archaea “Ca. Methanoperedens” to different coastal ecosystem stressors. We describe the scope of future investigation, one chapter at a time. In Chapter 2, we utilized sediments from the brackish and eutrophied Stockholm Archipelago to examine methane dynamics under different bottom-water redox conditions. We demonstrated the critical role of sulfide in regulating methane emissions. To improve biogeochemical model predictions for S-AOM, we propose that more toxicity threshold incubations, such as the sulfide toxicity experiment presented in this chapter, are necessary. Specifically, additional site-specific (e.g., oxic-hypoxic-euxinic conditions) and coastal ecosystem-specific (e.g., Stockholm Archipelago vs. Bothnian Sea) 13C-CH4 activity assays with unamended and increasingly sulfide-exposed S-AOM incubations will be required. This approach will provide improved reference S-AOM rates for each site or ecosystem studied, leading to more accurate corrections in biogeochemical models for future changes
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