Page 46 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 2. The diversity of methane cycling microorganisms
Such a potential switch can therewith change the contribution of S-AOM organisms to the CH4
budget of an ecosystem.
Nitrite- and nitrate-dependent anaerobic oxidation of methane
After the discovery of S-AOM in marine sediments, the hunt for nitrate- and nitrite-dependent CH4 oxidation (N-AOM) intensified. Based on redox calculations, both nitrate (NO3-) and nitrite (NO2-) are suitable electron acceptors for CH4 oxidation and, compared to SO42-, have much higher energy yields of up to -503 kJ per mole of CH4. In 2006, Raghoebarsing et al. reported the first enrichment culture coupling AOM to denitrification. The enrichment culture contained archaea (10–20% of the community) distantly related to ANME-2, and an NC10 phylum bacterium named “Candidatus Methylomirabilis oxyfera” (70–80% of the community). Interestingly, “Ca. Methylomirabilis oxyfera” possesses an aerobic pathway that is utilized under anoxic conditions (Ettwig et al. 2012). The proposed intra-aerobic pathway for coupling of AOM to NO2- reduction produces oxygen and dinitrogen gas from two molecules of nitric oxide (NO) (Ettwig et al. 2010, 2012). The production of oxygen as a metabolic intermediate has also been observed in the dismutation of chlorite (ClO2-), a toxic intermediate produced by chlorate-reducing bacteria (van Ginkel et al. 1996). An implication of this proposed mechanism is that aerobic pathways might have been present independently from, and potentially even before the evolution of oxygenic photosynthesis. Surveys of both 16S rRNA and pmoA genes (which encode the beta subunit of particulate methane monooxygenase) revealed a wide environmental distribution of N-AOM from wetlands to marine sediments and mud volcanos (Welte et al. 2016; Zhu et al. 2017).
The role of the ANME-2 archaea was resolved much later. In a bioreactor fed with NO3-, CH4 and ammonium, a stable co-culture of anaerobic ammonium-oxidizing (anammox) bacteria (“Candidatus Kuenenia stuttgartiensis”) and ANME-2d archaea was established (Haroon et al. 2013). These archaea were identified as “Candidatus Methanoperedens nitroreducens” (70%–80% of the community), which are capable of coupling NO3- reduction to CH4 oxidation (Haroon et al. 2013). Analyses of several genomes of “Candidatus Methanoperedens” have revealed that all genes of the (reverse) methanogenic pathway are present (Haroon et al. 2013; Arshad et al. 2015; Berger et al. 2017; Narrowe et al. 2017; Vaksmaa et al. 2017a; Leu et al. 2020). An environmental primer-based study based on 16S
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