Detritus‐hosted methanogenesis sustains the methane paradox in an alpine lake

Abstract

Accumulation of methane in oxic waters of lakes and the ocean has been widely reported. Despite the importance for the greenhouse gas budget, mechanistic controls of such "methane paradox" remain elusive. Here, we use a combination of CH 4 concentration and isotopic (δ 13 C CH4 , δD H2O and δ 18 O H2O ) measurements, plankton incubations and microbial community assessments to demonstrate the existence of the methane paradox in oxygenated waters of a meromictic lake (Lake Cadagno, Switzerland). Following mass dynamics using water isotopes, we exclude the possibility that the accumulation of CH 4 at the thermocline results solely from lateral transport. Interannual variability in the magnitude of the methane paradox (between 0.5 and 5 μmol L À1 ) is associated to stratification patterns, changes in zooplankton biomass and planktonic detritus accumulation along density gradients, as well as fluctuating microbial cell numbers. The links between hydrodynamic conditions, aggregation of planktonic detritus and its microbiome, as well as the accumulation of CH 4 in the water column are further supported by high-resolution echosounder revealing backscatter maxima at the top of the thermocline, where detritus is effectively trapped, and by oxic incubations showing that CH 4 is produced in zooplankton detritus (0.046 nmol L À1 to 0.095 CH 4 mg dry mass L À1 d À1 ). Our results also show that detritushosted methanogenesis is stimulated through the addition of methylphosphonate, suggesting that zooplankton-associated microbiomes exploit organic phosphorus compounds to release CH 4 . Understanding the variability of the methane paradox in relation to changing hydrodynamics and plankton communities will be crucial to predict the future role of lakes in the global methane budget.

Oxic methane production from methylphosphonate in a large oligotrophic lake: limitation by substrate and organic carbon supply

“…Given that dissolved organic carbon in Flathead Lake is ~100 µM, similar to concentrations in the open ocean ( 76 ) where MPn is thought to contribute significantly to methane production, the type and lability of the carbon available may be important in how organisms respond to MPn. MPn cleavage may, therefore, occur at labile carbon hotspots, for example, associated with zooplankton detritus ( 77 ) or in the phycosphere, and may be a more prominent source of OMP in eutrophic lakes with excess C and N. Future work should consider the importance of the types of carbon in how and which organisms respond to MPn.…”

Linking Biogeochemical and Hydrodynamic Processes to Model Methane Fluxes in Shallow, Tropical Floodplain Lakes

“…The production of CH 4 via the CO 2 reduction pathway or acetoclastic methanogenesis and associated differences in isotopic fractionation, and the composition of microbial communities active in methane processes are not represented. Methane production in oxic waters has been detected (e.g., Grossart et al., 2011), with several mechanisms supporting the process suggested (Bartosiewicz et al., 2022). However, as is evident in the exchange between Günthel et al.…”

Hydrogen–independent CO2 reduction dominates methanogenesis in five temperate lakes that differ in trophic states

“…More generally, while microniches can explain methanogenesis in environments where methanogenesis is a background process, e.g. oxic water columns or sulfate-reducing marine surface sediments [ 85 , 86 ], it is difficult to envision how they would be the main sites of CH 4 production in sediments where methanogenesis is the dominant respiration reaction [ 6 ].…”