Geochemical evidence for iron‐mediated anaerobic oxidation of methane

Abstract

Anaerobic oxidation of methane (AOM) by sulfate has been recognized as a critical process to maintain this greenhouse gas stability by limiting methane flux to the atmosphere. We show geochemical evidence for AOM in deep lake sediments and demonstrate that AOM is likely driven by iron (Fe) reduction. Pore-water profiles from Lake Kinneret (Sea of Galilee, Israel) show that this sink for methane is located below the 20-cm depth in the sediment, which is well below the depths at which nitrate and sulfate are completely exhausted, as well as below the zone of methanogenesis. Iron-dependant AOM was verified by Fe(III)-amended mesocosm studies using intact sediment cores, and native iron oxides were detectable throughout the sediments. Because anaerobic Fe(III) respiration is thermodynamically more favorable than both sulfate-dependent methanotrophy and methanogenesis, its occurrence below the zone of methane production supports the idea that reduction of sedimentary iron oxides is kinetically or biologically limited. Similar conditions are likely to prevail in other incompletely pyritized aquatic sediments, indicating that AOM with Fe(III) is an important global sink for methane.

Quantifying rates of methanogenesis and methanotrophy in Lake Kinneret sediments (Israel) using pore‐water profiles

“…2a). Profiles of sulfide, the product of BSR, complement the sulfate concentration profiles with maxima at the sediment-water interface and depletion above the 10-cm sediment depth (Sivan et al 2011).…”

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

“…However, results from freshwaters are inconsistent: some evidence of extreme depletion of 13 C in microbial lipids 14 has been noted while others report little to no depletion in lipid isotopic signatures 11,16 . Despite the clear importance of AOM in these wetlands, as established by the d 13 C profiles of DIC and methane and the measured rates of AOM (Fig.…”

“…In contrast, a dominant electron acceptor for freshwater AOM has not been identified. Experimental evidence suggest sulphate is not the sole electron acceptor for AOM in freshwater sediments and peat 2,9,15,20 , and AOM in low-sulphate environments has been linked to the reduction of nitrate, nitrite, iron and manganese 4,5,12,13,16,33 . The geochemical and rate profiles in the wetlands studied here support a linkage between SR and AOM.…”

“…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.…”

“…Yet, despite low sulphate concentrations, FWW support a dynamic sulphur cycle and high rates of sulphate reduction 9 (SR) are maintained by rapid recycling of reduced sulphur species 10 . Moreover, studies of freshwater AOM have indicated several electron acceptors for methane oxidation, including sulphate 11 , nitrate/nitrite 5,[12][13][14][15] , iron 16 and manganese 17 . Geochemical profiles 4,18,19 , stable isotope geochemistry 16,18,19 , potential methane oxidation assays 2,4,5,20 and long-term enrichments 4 have provided tantalizing evidence of AOM in FWW, but environmentally relevant, in vitro, rate measurements are rare 9 .…”