West Antarctic Ice Sheet – Microbial Oxidation as Methane Sink Beneath WAIS

Australia: The Land Where Time Began

West Antarctic Ice Sheet – Microbial Oxidation as Methane Sink Beneath WAIS

Beneath the ice masses the aquatic habitats contain active microbial ecosystems that are capable of cycling important greenhouse gases, such as methane (CH₄). It is thought that beneath the West Antarctic Ice Sheet there is a large methane reservoir, though the quantity, source and ultimate fate of this source is not well understood. E.g., oxygen that has been supplied by melting at the base of the ice sheet should provide favourable conditions for aerobic oxidation of methane. In this paper Michaud et al. present the results of their study that used concentrations of methane and compositions of stable isotopes, as well as genomic analyses to assess the sources and cycling of methane in Subglacial Lake Whillans (SLW) in West Antarctica. It was shown that the methane beneath the ice sheet methane is produced by the biological reduction of CO₂ using H₂. Subsequently, this pool of methane is consumed by aerobic bacterial oxidation of methane at the interface of the sediment and the water. More than 99 % of the methane is consumed by bacterial oxidation, which represents a significant methane sink, as well as a source of biomass carbon and metabolic energy to the surficial SLW sediments. It was concluded by Michaud et al. that the release of methane to the atmosphere when subglacial drainage of water to the margins of the ice sheet occurs, and at periods when deglaciation is occurring, may be mitigated by the aerobic methanotrophy.

Methane (CH₄), an important greenhouse gas, affects atmospheric chemistry as well as the radiative balance of the Earth. Understanding the global sources, sinks and feedbacks within the climate system of methane is, therefore, of considerable importance (Kirschke et al., 2013). Anaerobic fermentation of simple organic compounds by certain archaea (acetoclastic or methylotrophic methanogenesis (Thauer et al., 2008), is the primary pathway by which biological methane is produced in carbon-rich habitats, such as bogs and wetlands. The reduction of CO₂ coupled to the oxidation of H₂ (hydrognotrophic methanogenesis) is a common alternative pathway to the production of CH₄, which is common in environments that are anoxic and are low in sulphate such as in the methanogenic zone in marine sediments (Thauer et al., 2008). An important pathway that releases CH₄ to the atmosphere is, conversely, oxidation of CH₄ by bacteria (aerobic) and archaea (anaerobic) (Conrad, 2009).

In sediments beneath the Antarctic ice sheet anoxic habitats may be important as biological CH₄ production that could potentially add significant CH₄ to the atmosphere when subglacial water drains to the margins of the ice sheet or deglaciation occurs (Wadham et al., 2012; Dieser et al., 2012; Wadham et al., 2013). Though, as a result of the release of oxygen into the subglacial environment from the overlying ice sheet, by way of melting induced by geothermal heating (Christner et al., 2014; Skidmore, 2011; Fisher et al., 2015), CH₄ release to the atmosphere by aerobic methanotrophic activity can ultimately be mitigated. In this paper Michaud et al. present data on the concentration of CH₄ and the stable isotope composition, as well as genomic data that were collected from Subglacial Lake Whillans (SLW), which is located ⁓800 m beneath the West Antarctic Ice Sheet (WAIS). These data, collectively, reveal that there is an ecosystem that is supported, in part, by active microbial transformations of CH₄.

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