Anaerobic oxidation of methane: an underappreciated aspect of methane cycling in peatland ecosystems?

Abstract

Abstract.Despite a large body of literature on microbial anaerobic oxidation of methane (AOM) in marine sediments and saline waters and its importance to the global methane (CH 4 ) cycle, until recently little work has addressed the potential occurrence and importance of AOM in non-marine systems. This is particularly true for peatlands, which represent both a massive sink for atmospheric CO 2 and a significant source of atmospheric CH 4 . Our knowledge of this process in peatlands is inherently limited by the methods used to study CH 4 dynamics in soil and sediment and the assumption that there are no anaerobic sinks for CH 4 in these systems. Studies suggest that AOM is CH 4 -limited and difficult to detect in potential CH 4 production assays against a background of CH 4 production. In situ rates also might be elusive due to background rates of aerobic CH 4 oxidation and the difficulty in separating net and gross process rates. Conclusive evidence for the electron acceptor in this process has not been presented. Nitrate and sulfate are both plausible and favorable electron acceptors, as seen in other systems, but there exist theoretical issues related to the availability of these ions in peatlands and only circumstantial evidence suggests that these pathways are important. Iron cycling is important in many wetland systems, but recent evidence does not support the notion of CH 4 oxidation via dissimilatory Fe(III) reduction or a CH 4 oxidizing archaea in consortium with an Fe(III) reducer. Calculations based on published rates demonstrate that AOM might be a significant and underappreciated constraint on the global CH 4 cycle, although much about the process is unknown, in vitro rates may not relate well to in situ rates, and projections based on those rates are fraught with uncertainty. We suggest electron transfer mechanisms, C flow and pathways, and quantifying in situ peatland AOM rates as the highest priority topics for future research.

High rates of anaerobic methane oxidation in freshwater wetlands reduce potential atmospheric methane emissions

“…Humic substances, which are plentiful in these organic-rich wetlands, are a possible candidate 3 . Humic acids can serve as electron acceptors/shuttles in other anaerobic metabolisms and the addition of humic acids lowered methane emissions from a peatland, possibly through stimulation of AOM 37 .…”

“…In comparison, there is no convergence on the dominant mechanism for AOM in freshwater environments 3,5,9,16,17 . The available empirical evidence is contradictory.…”

“…The recovery rate of labelled methane was 495%. The average activity of the H 14 CO 3 À tracer was 380 kBq. The average activity of the added 2-14 CH 3 COO À was 160 kBq with a specific activity of 1.7-2.2 GBq mmol À 1 .…”

“…Few studies have examined anaerobic oxidation of methane (AOM) in FWW [2][3][4][5] and AOM was largely regarded as inconsequential in these environments until recently. Instead, aerobic methane oxidation was assumed to be the dominant mode of methane consumption in FWW 6 .…”

How does elevated ozone reduce methane emissions from peatlands?

“…This was not clearly supported by the apparent fractionation factors of 1.055 and 1.057 for pore water observed at 20 cm and 30 cm depth, as they are borderline in indicating the dominance of either pathway (Whiticar et al, 1986;Whiticar, 1999;Conrad, 2005;Holmes et al, 2015). These values were, however, probably lowered, indicating more prevalence towards acetoclastic methanogenesis, due to C isotopic fractionation of CH 4 during aerobic, anaerobic or facultative CH 4 oxidation when the CH 4 passed the peat profile to the moss surface (Semrau et al, 2011;Smemo & Yavitt, 2011;Whiticar, 1999). Dominance of the hydrogenotrophic pathway has been observed in several sedge-dominated peatlands (Whiticar et al, 1986;Mörsky et al, 2008;Holmes et al, 2015) and may also explain the lack of increase in CH 4 emission despite the increase in organic acid concentrations (including acetate) in the peat pore water at elevated O 3 in the peatland microcosm study of Mörsky et al (2008).…”