High concentrations of dissolved biogenic methane associated with cyanobacterial blooms in East African lake surface water

The contribution of oxic methane production to greenhouse gas emissions from lakes isglobally relevant, yet uncertainties remain about the levels up to which methanogenesis cancounterbalance methanotrophy by leading to CH4 oversaturation in productive surfacewaters. Here, we explored the biogeochemical and microbial community variation patterns ina meromictic soda lake, in the East African Rift Valley (Kenya), showing an extraordinarilyhigh concentration of methane in oxic waters (up to 156 micromol L-1). Vertical profiles ofdissolved gases and their isotopic signature indicated a biogenic origin of CH4. A bloom ofOxyphotobacteria co-occurred with abundant hydrogenotrophic and acetoclastic methano-gens, mostly found within suspended aggregates promoting the interactions between Bacteria, Cyanobacteria, and Archaea. Moreover, aggregate sedimentation appeared critical inconnecting the lake compartments through biomass and organic matter transfer. Our findingsprovide insights into understanding how hydrogeochemical features of a meromictic sodalake, the origin of carbon sources, and the microbial community profiles, could promotemethane oversaturation and production up to exceptionally high rates.