Humic Substances Mediate Anaerobic Methane Oxidation Linked to Nitrous Oxide Reduction in Wetland Sediments

Abstract

Humic substances are redox-active organic molecules, which play pivotal roles in several biogeochemical cycles due to their electron-transferring capacity involving multiple abiotic and microbial transformations. Based on the redox properties of humic substances, and the metabolic capabilities of microorganisms to reduce and oxidize them, we hypothesized that they could mediate the anaerobic oxidation of methane (AOM) coupled to the reduction of nitrous oxide (N 2 O) in wetland sediments. This study provides several lines of evidence indicating the coupling between AOM and the reduction of N 2 O through an extracellular electron transfer mechanism mediated by the redox active functional groups in humic substances (e.g., quinones). We found that the microbiota of a sediment collected from the Sisal wetland (Yucatán Peninsula, southeastern Mexico) was able to reduce N 2 O (4.6 ± 0.5 µmol N 2 O g sed. −1 day −1) when reduced humic substances were provided as electron donor in a close stoichiometric relationship. Furthermore, a microbial enrichment derived from the wetland sediment achieved simultaneous 13 CH 4 oxidation (1.3 ± 0.1 µmol 13 CO 2 g sed. −1 day −1) and N 2 O reduction (25.2 ± 0.5 µmol N 2 O g sed. −1 day −1), which was significantly dependent on the presence of humic substances as an extracellular electron shuttle. Taxonomic characterization based on 16S rRNA gene sequencing revealed Acinetobacter (a G-proteobacterium), the Rice Cluster I from the Methanocellaceae and an uncultured archaeon from the Methanomicrobiaceae family as the microbes potentially involved in AOM linked to N 2 O reduction mediated by humic substances. The findings reported here suggest that humic substances might play an important role to prevent the emission of greenhouse gases (CH 4 and N 2 O) from wetland sediments. Further efforts to evaluate the feasibility of this novel mechanism under the natural conditions prevailing in ecosystems must be considered in future studies.

Anaerobic oxidation of methane in sediments of a nitrate-rich, oligo-mesotrophic boreal lake

“…N 2 O) could promote production/replenishment of organic EAs (e.g. humic substances), and hence indirectly support organic EA reduction - dependent AOM (Valenzuela et al 2020).…”

Methanotrophs: Discoveries, Environmental Relevance, and a Perspective on Current and Future Applications

“…Humic-dAOM has been observed in ANME2a and ANME2c in the presence of humic acids and AQDS as electron acceptors in the presence of chelated ferric iron (Scheller et al, 2016). Furthermore, humic substances were shown to stimulate methane oxidation linked to N 2 O reducing microbes in coastal sediments in microbial communities composed by Acinetobacter and archaea from the Rice Cluster I and an uncultured member of the Methanomicrobiaceae (Valenzuela et al, 2020).…”

Higher Abundance of Sediment Methanogens and Methanotrophs Do Not Predict the Atmospheric Methane and Carbon Dioxide Flows in Eutrophic Tropical Freshwater Reservoirs

“…Furthermore, methanotrophic microorganisms can consume CH 4 , which is oxidized prior to atmospheric release, reducing its flow and global warming impact ( Zigah et al, 2015 ; Martinez-Cruz et al, 2018 ). It is known that CH 4 solubility and a well established methanotrophic community are important factors to control CH 4 emission in aquatic ecosystems ( Wen et al, 2018 ; Valenzuela et al, 2020 ). This methanotrophic community can include aerobic methanotrophic bacteria or anaerobic methanotrophic archaea (ANME), the last one participating of CH 4 oxidation by reversion of the methanogenic pathway ( Timmers et al, 2017 ) and by association with bacteria ( Su et al, 2020 ).…”