Identification of microbial methane sources and sinks in an aquifer-fed continental wetland with a multi-omics approach

Vegetated continental wetlands are responsible for 20% of global methane emissions. Wetlands encompass very diverse ecosystems, from ponds to peatlands. In mid-latitudes, temporary flooded lowlands are very common, due to the seasonal discharge of water from rivers or groundwater but their contribution to the global methane budget is unknown. These sites experience temporal successions of oxic and anoxic conditions and the impact of these dynamic redox conditions on the microbial communities is poorly understood. In particular, methanogenesis typically requires anoxic conditions while aerobic methanotrophy can oxidize large amounts of methane before it reaches the atmosphere. This study is carried out at the Ploemeur-Guidel hydrogeological observatory, where a wetland is created by the seasonal discharge of deep anoxic and iron-rich groundwater. Four one-meter-long cores were recovered along a transect from the inner to the outer side of the wetland and samples were collected every 20 to 30 cm along each core. Batch incubations revealed two potential hotspots of methane production at ca. 40 cm and 100 cm depth, reaching up to 3.5 µmol cm^-3 d^-1, in agreement with highest methane concentrations measured in the porewater collected at these depths. To shed light on the microbial processes involved in the methane cycling at this site, a metagenomic and metatranscriptomic analysis was conducted in combination with the analysis of the carbon isotopic composition of microbial intact polar lipids. Preliminary results of the omics study showed clear taxon stratification with depth. The proportion of metagenomic reads classified to archaeal taxa increased with depth and reached up to 15% of the total analyzed reads at one meter below the surface. Interestingly, the percentage of reads affiliated to known methanogens was highest at the surface (ca. 40% of the archaeal reads) and decreased with depth (ca. 13% at 100 cm depth). Archaeal communities appeared to be dominated by Candidatus Bathyarchaeota since up to 60% of the archaeal reads could be classified to this phylum. Carbon isotopic composition of the phytane and biphytanes (BP) derived from the major archaeal membrane lipids (archaeols and glycerol dibiphytanyl glycerol tetraethers, GDGTs) displayed a predominant heterotrophic signature. However, BP-2, derived from GDGTs with multiple rings was systematically depleted in ¹³C, suggesting autotrophic and/or methanogenic metabolism. Further statistical treatments will be performed to refine the taxonomic links between the detected membrane lipids and the archaeal populations along the cores.