International Journal of Marine Science, 2024, Vol.14, No.5, 312-320 http://www.aquapublisher.com/index.php/ijms 316 in Methylococcaceae was crucial for the initial oxidation of methane to methanol (Taubert et al., 2019). In anaerobic environments, the key enzymes and genes are associated with the ANME archaea and their SRB partners. Metatranscriptomic analysis has revealed significant upregulation of genes such as flaB in ANME-2d and pilA in Desulfococcus, which are likely involved in the formation of nanonets for electron transfer between ANME and SRB (Nie et al., 2021). Additionally, in nitrite-dependent methane oxidation, genes related to nitrite reduction and stress response systems are critical for the niche differentiation of n-DAMO bacteria like "Candidatus Methylomirabilis oxyfera" and "Candidatus Methylomirabilis sinica" (Nie et al., 2023). 5 Factors Controlling Methane Flux 5.1 Physical factors (temperature, pressure, salinity) Physical factors such as temperature, pressure, and salinity play crucial roles in controlling methane flux in marine environments. Temperature influences the metabolic rates of methanogenic and methanotrophic microorganisms, with higher temperatures generally enhancing microbial activity and methane production (Klasek et al., 2019). Pressure, particularly in deep-sea environments, affects the solubility of gases and the stability of methane hydrates, which can act as significant methane reservoirs (Aromokeye et al., 2020). Salinity impacts the availability of electron acceptors and the composition of microbial communities, with lower salinity environments often exhibiting reduced sulfate concentrations, thereby affecting sulfate-dependent anaerobic oxidation of methane (S-AOM) (Wallenius et al., 2021). 5.2 Biological controls (microbial activity, community composition) Biological factors, including microbial activity and community composition, are pivotal in regulating methane flux. Methanotrophic bacteria, such as those from the Methylococcaceae family, play a significant role in oxidizing methane before it escapes to the atmosphere (Bornemann et al., 2016; Taubert et al., 2019). The presence and activity of anaerobic methanotrophic archaea (ANME) and their interactions with sulfate-reducing bacteria (SRB) are critical in the sulfate-methane transition zone (SMTZ), where a substantial portion of methane is oxidized. The community composition of methanotrophs, including niche separation between Alpha- and Gamma-MOB, also influences methane oxidation rates and overall methane flux (Reis et al., 2019). 5.3 Human impacts on methane emission and uptake Human activities, such as eutrophication and climate change, significantly impact methane emissions and uptake in marine environments. Eutrophication, driven by nutrient runoff, increases organic matter inputs, leading to hypoxia and enhanced methanogenesis in coastal sediments. Climate change, through global warming, reduces oxygen solubility in surface waters, promoting water column stratification and further enhancing methanogenesis (He et al., 2019). Additionally, anthropogenic disturbances, such as gas blowouts, can create high methane flux sites, where microbial communities rapidly adapt to oxidize methane, mitigating its release to the atmosphere (Steinle et al., 2016). 6 Global Significance of Methane Flux in Marine Systems 6.1 Contribution of marine methane emissions to global greenhouse gas levels Methane (CH4) is a potent greenhouse gas with a global warming potential significantly higher than carbon dioxide (CO2) over a 100-year period. Marine systems, particularly coastal areas, contribute substantially to global methane emissions. Coastal sediments are major sources of methane due to the activity of methanogenic archaea in anoxic conditions (Mai et al., 2024). However, a significant portion of this methane is oxidized before it reaches the atmosphere, primarily through sulfate-dependent anaerobic oxidation of methane (S-AOM) in the sulfate-methane transition zone (SMTZ). Vegetated coastal ecosystems (VCEs) such as mangroves, salt marshes, and seagrasses also play a critical role, contributing approximately 0.33~0.39 Tmol CH4-C/year, which increases the global marine methane budget by more than 60% (Arnold et al., 2023). Despite the mitigation by microbial oxidation, methane emissions from marine systems remain a significant component of the global greenhouse gas inventory. 6.2 Role of marine systems in global methane cycling Marine systems are integral to the global methane cycle, acting both as sources and sinks of methane. Methane is
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