36 Chapter 1 COASTAL ECOSYSTEM STRESSORS: EUTROPHICATION AND SEA-LEVEL RISE Coastal ecosystems are subject to a variety of anthropogenic stressors, with eutrophication and sea-level rise being particularly induced by climate change (Figure 4) (Howarth et al., 2011; Malone & Newton, 2020; Wallenius et al., 2021). Eutrophication in coastal ecosystems leads to an excess of organic carbon degradation coupled with sulfate reduction, resulting in the buildup of sulfide (Żygadłowska et al., 2024a). Elevated sulfide levels can disrupt microbial community functions, leading to reduced methane oxidation rates (Chapter 2 and 4). Rising sulfide concentrations can also inhibit key metabolic processes by damaging copper- and iron-containing cofactors (Jin et al., 1998) and suppressing methanogenesis (Karhadkar et al., 1987). It also poses toxicity risks to aquatic life, especially fish and invertebrates, and negatively impacts water quality for human recreational use (Riesch et al., 2015) (Figure 4, A-B). Sea-level rise, driven by climate change, poses a significant threat to coastal methane biofilters by altering methane dynamics (IPCC, 2023; Kaushal et al., 2021) (Figure 4, C-D). As with sea levels rise, saltwater intrusion into freshwater and brackish systems imposes hyperosmotic stress on freshwater microbial communities and can reduce methane oxidation rates (Ho et al., 2018; Osudar et al., 2017). While some freshwater “Ca. Methanoperedens” have been observed in marine environments (Chapter 7), the long-term effects of salinity on their methaneoxidizing capacity and physiological adaptations remain poorly understood. A common microbial strategy for coping with osmotic stress involves the accumulation of intracellular osmolytes. Osmolytes are water-soluble organic compounds that accumulate within cells to balance hyperosmotic pressure. These compounds include sugars, amino acids, polyols, and their derivatives (BebloVranesevic et al., 2017; Gregory & Boyd, 2021; Guan et al., 2017).
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