International Journal of Marine Science, 2024, Vol.14, No.5, 312-320 http://www.aquapublisher.com/index.php/ijms 315 4 Methane Oxidation in Water Columns 4.1 Aerobic methane oxidation Aerobic methane oxidation is primarily facilitated by methanotrophic bacteria, which utilize methane as a carbon and energy source in the presence of oxygen. In marine environments, members of the Methylococcaceae family are key players in this process. For instance, in the sediments of a shallow seep near Elba, Italy, Methylococcaceae were found to catalyze the first step of methane oxidation to methanol, which was then further processed by methylotrophic Methylophilaceae, demonstrating a communal metabolism that significantly mitigates methane emissions (Figure 2) (Taubert et al., 2019). Additionally, aerobic methanotrophs such as Methylobacter have been identified in sub-Arctic lake sediments, where they assimilate carbon from methane, either directly or indirectly, contributing to the reduction of methane emissions in these regions (Nie et al., 2021). Figure 2 Characterization of the water column methane and MPn and MeA metabolism potential of the PMEZ microbial communities (Adopted from Wang et al., 2021) 4.2 Anaerobic methane oxidation 4.2.1 Coupling of methane oxidation with sulfate reduction AOM coupled with sulfate reduction is a well-documented pathway in marine sediments. This process involves ANME archaea and their SRB partners, which together oxidize methane and reduce sulfate to sulfide. For example, in long-term incubations of sediments from the Shenhu area of the northern South China Sea, AOM was observed alongside sulfate reduction, indicating the presence of these microbial consortia (Chai et al., 2020). Similarly, in sediment-free long-term AOM enrichments from marine seeps, ANME-2 archaea and Seep-SRB2 bacteria were found to dominate, further supporting the role of sulfate reduction in AOM (Wegener et al., 2016). 4.2.2 Nitrite-dependent methane oxidation Nitrite-dependent anaerobic methane oxidation (n-DAMO) is another significant pathway, particularly in environments where nitrite is available as an electron acceptor. This process is primarily mediated by bacteria such as "Candidatus Methylomirabilis oxyfera" and "Candidatus Methylomirabilis sinica." In deep-sea cold seeps, n-DAMO bacteria like Methylomirabilis and Methanoperedens have been identified as key performers, utilizing nitrite and nitrate, respectively, as electron acceptors. Additionally, in bioelectrochemical reactors, methane oxidation driven by nitrite reduction was observed, with Methylomirabilis oxyfera playing a significant role (Chai et al., 2020). 4.3 Enzymes and genes involved in methane oxidation pathways The enzymes and genes involved in methane oxidation pathways are critical for understanding the biochemical mechanisms underlying these processes. In aerobic methane oxidation, enzymes such as methane monooxygenase (MMO) play a pivotal role. For instance, in the sediments of a shallow seep near Elba, Italy, the presence of MMO
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