Abstract
Methanotrophs are the only biofilters for reducing the flux of global methane (CH4) emissions in water-logged wetlands. However, adaptation of aerobic methanotrophs to low concentrations of oxygen and nitrogen in typical swamps, such as that of the Qinghai–Tibetan Plateau, is poorly understood. In this study, we show that Methylobacter-like methanotrophs dominate methane oxidation and nitrogen fixation under suboxic conditions in alpine swamp soils. Following incubation with 13C-CH4 and 15N-N2 for 90 days under suboxic conditions with repeated flushing using an inert gas (i.e., argon), microbial carbon and nitrogen turnover was measured in swamp soils at different depths: 0–20 cm (top), 40–60 cm (intermediate), and 60–80 cm (deep). Results show detectable methane oxidation and nitrogen fixation in all three soil depths. In particular, labeled carbon was found in CO2 enrichment (13C-CO2), and soil organic carbon (13C-SOC), whereas labeled nitrogen (15N) was detected in soil organic nitrogen (SON). The highest values of labeled isotopes were found at intermediate soil depths. High-throughput amplicon sequencing and Sanger sequencing indicated the dominance of Methylobacter-like methanotrophs in swamp soils, which comprised 21.3–24.0% of the total bacterial sequences, as measured by 13C-DNA at day 90. These results demonstrate that aerobic methanotroph Methylobacter is the key player in suboxic methane oxidation and likely catalyzes nitrogen fixation in swamp wetland soils in the Qinghai–Tibetan Plateau.
Highlights
Methane-oxidizing bacteria are the only biofilter of atmospheric methane, a potent greenhouse gas emitted from anoxic environments
Methane oxidation potential was assessed as the sum of 13CO2 and 13C-soil organic carbon (SOC)
Stable isotope tracing with 13C-CH4 and 15N-N2 indicates significant methane oxidation and nitrogen fixation potentials in all three soil depths in the Qinghai–Tibetan Plateau swamp
Summary
Methane-oxidizing bacteria are the only biofilter of atmospheric methane, a potent greenhouse gas emitted from anoxic environments. It is estimated that up to 90% of anaerobically produced methane could be consumed by methanotrophs before it escapes to the atmosphere from waterlogged suboxic or even anaerobic environments, such as natural wetlands, marine sediment, paddy soils, and rivers (Conrad, 2009, 2020). Anaerobic methane oxidation plays a pivotal role in marine ecosystems (Knittel and Boetius, 2009), oxygen functions as the primary electron acceptor to facilitate methane oxidation by aerobic methanotrophs in terrestrial environments (Ho et al, 2013, 2017; Serrano et al, 2014). In the absence of oxygen, anaerobic methane-oxidizing archaea (ANME) use alternative electron acceptors to perform anaerobic oxidation of methane (AOM). These ANMEs include the euryarchaeal ANME-1, ANME-2, and ANME-3 groups. AOM, according to their respective electron acceptor (i.e., sulfate, nitrate, nitrite, and metals, respectively)
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