Page 227 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Introduction
Wetlands are the biggest natural methane source and contribute 30% to global methane emissions (167 Tg CH4 yr-1) (Saunois et al. 2016). Methane is an important greenhouse gas (GHG) with a 34-fold higher warming potential than CO2 (Myhre et al. 2013). Eighteen percent of the total global greenhouse effect is currently attributed to methane (Prather, Holmes and Hsu 2012; Myhre et al. 2013). In wetlands, methanogenic archaea carry out the final reaction in the anaerobic degradation of organic matter resulting in methane production. Wetland methane emissions are mitigated by the activity of both anaerobic methanotrophic bacteria and archaea (Raghoebarsing et al. 2006; Ettwig et al. 2010; Haroon et al. 2013) and aerobic methanotrophic bacteria (reviewed by Hanson and Hanson 1996). Aerobic methanotrophy has been estimated to be the most significant methane oxidation pathway in cold ecosystems (Mackelprang et al. 2011; Barbier et al. 2012; Knoblauch et al. 2013), although anaerobic methane oxidizers have also been detected in cold freshwater and peatland ecosystems (Smemo and Yavitt 2011; Gupta et al. 2013; Kao-Kniffin et al. 2015). Early studies on lake and peatland systems indicated that aerobic methanotrophs have the potential to oxidize up to 95% of the methane that is produced (Yavitt, Lang and Downey 1988; Frenzel, Thebrath and Conrad 1990). Spatial coexistence has been observed in, for example, cooperation of nitrogen cycle microorganisms (Sliekers et al. 2002; Yang et al. 2012). Several studies implied that this coexistence in seemingly anoxic environments is probably enabled due to high oxygen consumption rates (Oswald et al. 2016; Martinez-Cruz et al. 2017). In addition, aerobic methanotrophs are tolerant to long periods of anoxic conditions (Roslev and King 1994).
The interactions between methanogens and aerobic methanotrophs that may strongly control the GHG fluxes of cold wetland ecosystems remain poorly understood (Bridgham et al. 2013). Only few studies on methane fluxes in oxic-anoxic systems have been done so far (Gerritse and Gottschal 1993; Shen, Miguez and Bourque 1996; Miguez et al. 1999). Shen et al. designed a bioreactor with an aerobic-anaerobic interface using a granular sludge bed that allowed for sufficient methanogenic activity to support growth of the aerobic methanotroph Methylosinus sporium (Shen, Miguez and Bourque 1996). However, this system did not employ axenic cultures and observations showed gradual reduction of M. sporium, indicating competition for oxygen with facultative anaerobic bacteria. Similarly, Miguez et al. used an upflow anaerobic
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