Abstract
The contribution of oxic methane production to greenhouse gas emissions from lakes is globally relevant, yet uncertainties remain about the levels up to which methanogenesis can counterbalance methanotrophy by leading to CH4 oversaturation in productive surface waters. Here, we explored the biogeochemical and microbial community variation patterns in a meromictic soda lake, in the East African Rift Valley (Kenya), showing an extraordinarily high concentration of methane in oxic waters (up to 156 µmol L−1). Vertical profiles of dissolved gases and their isotopic signature indicated a biogenic origin of CH4. A bloom of Oxyphotobacteria co-occurred with abundant hydrogenotrophic and acetoclastic methanogens, mostly found within suspended aggregates promoting the interactions between Bacteria, Cyanobacteria, and Archaea. Moreover, aggregate sedimentation appeared critical in connecting the lake compartments through biomass and organic matter transfer. Our findings provide insights into understanding how hydrogeochemical features of a meromictic soda lake, the origin of carbon sources, and the microbial community profiles, could promote methane oversaturation and production up to exceptionally high rates.
Highlights
The contribution of oxic methane production to greenhouse gas emissions from lakes is globally relevant, yet uncertainties remain about the levels up to which methanogenesis can counterbalance methanotrophy by leading to CH4 oversaturation in productive surface waters
The chemocline was apparent from abrupt changes in electrical conductivity, redox potential, pH, inorganic and organic solutes, and dissolved organic matter (DOM) (Supplementary Fig. 3; Supplementary Tables 2 and 3)
This study reports the accumulation of an unusual high amount of biogenic methane in surface oxic waters of the meromictic soda lake Sonachi, occurring together with a high availability of autochthonous dissolved organic matter and abundant cyanobacteriabacteria-methanogens interacting cells
Summary
The contribution of oxic methane production to greenhouse gas emissions from lakes is globally relevant, yet uncertainties remain about the levels up to which methanogenesis can counterbalance methanotrophy by leading to CH4 oversaturation in productive surface waters. We explored the biogeochemical and microbial community variation patterns in a meromictic soda lake, in the East African Rift Valley (Kenya), showing an extraordinarily high concentration of methane in oxic waters (up to 156 μmol L−1). Our findings provide insights into understanding how hydrogeochemical features of a meromictic soda lake, the origin of carbon sources, and the microbial community profiles, could promote methane oversaturation and production up to exceptionally high rates. Estimates of carbon gas fluxes across the air-water interface showed that freshwater bodies represent a source of methane with a disproportionate contribution to global CH4 emissions, regardless the relatively small surface they cover[5,6]. Soda lakes, characterized by saline alkaline waters in which Na+ and carbonate species are the dominant ions, are common in regions with volcanic bedrock, including the eastern branch of the East African Rift and several lake basins across the globe[20,21]
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