Page 79 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Introduction
Coal is the most important fossil fuel on the planet, comprising 70% of the total fossil fuel stock (Iram, Akhtar and Ghauri 2017). Coal mining has a big environmental impact and is estimated to result in 24-42 Tg of methane emissions per year. Methane in coalbeds is produced biologically or thermogenically and escapes from both active and abandoned coal mines. This amounts to 11% of total yearly anthropogenic methane emissions (Saunois et al. 2016) and thus has a considerable climate impact due to the high global warming potential of methane [34 for 100 years, (Myhre et al. 2013)]. Coalbed methane can be used as a more environmentally sustainable alternative to direct coal burning (Fakoussa and Hofrichter 1999; Al-Jubori et al. 2009; Kinnon et al. 2010) as direct coal burning leads to pollution in the form of heavy metals and sulfur compounds (Querol, Fernández-Turiel and López-Soler 1995). At the same time, the exploitation of coalbed methane could effectively reduce methane leaking into the atmosphere. In many cases, coalbed methane harvesting is not profitable due to unfavorable system properties, like highest rank coal species or unfeasible methane production rates as well as barriers to a selling market (Moore 2012). As a result, many active methane-producing coal systems are neglected and thus remain sources of methane emission.
Biogenic coalbed methane production requires a consortium of anaerobes that convert coal compounds to substrates for methanogenic archaea (Jones et al. 2010). Recently, Mayumi et al. showed that coal-derived methoxylated compounds can be directly used for methanogenesis by Methermicoccus species (Mayumi et al., 2016). Coal biosolubilization, the rate limiting step, is carried out by fungi expressing peroxidases, laccases, hydrolytic esterases and also abiotically by alkaline metabolites and naturally occurring chelators (Fakoussa and Hofrichter 1999; Strąpoć et al. 2008; Papendick et al. 2011). The microbial degradation of intermediates to volatile fatty acids (VFAs) and the subsequent production of acetate and H2/CO2 fuels the methanogens (Robbins et al. 2016a).
In Australia, roughly 5,000 coal seam gas wells were operated in 2014 (Day et al. 2014) which accounted for 18% of the total gas production in 2014-2015 and almost half of the total East Coast gas production (Ball et al. 2016). Several studies focused on enhancing methane production by applying nutrient, trace element and biotic amendment to low-emitting coal seams (Jones et al. 2010; Penner, Foght and Budwill 2010; Ünal et al. 2012). In Australia, coalbed exploitation for biogas production has gained more attention in the last decade and
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