Page 47 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

rRNA and methyl-coenzyme M reductase subunit alpha (mcrA) gene sequences showed that “Ca. Methanoperedens nitroreducens” is abundantly present in paddy fields (9% relative abundance of the archaeal community), river sediments, and even marine sediments (Vaksmaa et al. 2016, 2017b).
Terrestrial agriculture-affected ecosystems that receive high concentrations of nitrogen compounds are also facilitating environments for N-AOM. However, little is known about the relevance of N-AOM in terrestrial ecosystems, particularly those with prolonged anoxic 2 conditions, such as natural or restored wetlands.
Metal-dependent anaerobic oxidation of methane
In addition to NO3- and NO2-, oxidized iron [Fe(III)] and oxidized manganese [Mn(IV)] should be suitable electron acceptors for AOM based on Gibbs free energy calculations. Iron is the most abundant metal in the Earth's crust and can serve as both an electron donor and acceptor in microbial metabolism. Iron forms stable minerals in both the divalent and trivalent states depending on geochemical conditions. Fe(III) is most stable under oxic conditions (Raiswell and Canfield 2012). The reduction-oxidation cycle is coupled to other elements, including carbon, nitrogen, oxygen, and sulfur. Conversion in the iron cycle can be abiotic or mediated by microorganisms (Weber et al. 2006; Melton et al. 2014). Iron bioavailability is generally low due to the poor solubility of iron minerals at neutral pH, but microorganisms have developed strategies to mediate electron exchange with insoluble iron forms (Weber, Achenbach and Coates 2006). Although a wide variety of organisms are known to reduce iron, the microorganisms responsible for the reduction of metal-oxides coupled to AOM (here abbreviated as Fe-AOM) have remained elusive till recently.
Geochemical profiling and stable isotope tracer studies have demonstrated the occurrence of Fe-AOM in lake sediments (Sivan et al. 2011; Norði, Thamdrup and Schubert 2013; Torres et al. 2014), marine sediments (Beal, House and Orphan 2009; Wankel et al. 2012; Riedinger et al. 2014; Egger et al. 2015), paddy field sediments (Miura et al. 1992; Murase and Kimura 1994), lake water (Crowe et al. 2011), a terrestrial mud volcano (Chang et al. 2012), and in a crude oil-contaminated aquifer (Amos et al. 2012).
Several studies have identified potential microorganisms involved in Fe-AOM. Ettwig et al. (2016) reported on a freshwater enrichment culture of Methanosarcinales related to
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