Abstract
Freshwater ecosystems are responsible for an important part of the methane (CH4) emissions which are likely to change with global warming. This study aims to evaluate temperature-induced (from 5 to 20 °C) changes on microbial community structure and methanogenic pathways in five sub-Antarctic lake sediments from Magallanes strait to Cape Horn, Chile. We combined in situ CH4 flux measurements, CH4 production rates (MPRs), gene abundance quantification and microbial community structure analysis (metabarcoding of the 16S rRNA gene). Under unamended conditions, a temperature increase of 5 °C doubled MPR while microbial community structure was not affected. Stimulation of methanogenesis by methanogenic precursors as acetate and H2/CO2, resulted in an increase of MPRs up to 127-fold and 19-fold, respectively, as well as an enrichment of mcrA-carriers strikingly stronger under acetate amendment. At low temperatures, H2/CO2-derived MPRs were considerably lower (down to 160-fold lower) than the acetate-derived MPRs, but the contribution of hydrogenotrophic methanogenesis increased with temperature. Temperature dependence of MPRs was significantly higher in incubations spiked with H2/CO2 (c. 1.9 eV) compared to incubations spiked with acetate or unamended (c. 0.8 eV). Temperature was not found to shape the total microbial community structure, that rather exhibited a site-specific variability among the studied lakes. However, the methanogenic archaeal community structure was driven by amended methanogenic precursors with a dominance of Methanobacterium in H2/CO2-based incubations and Methanosarcina in acetate-based incubations. We also suggested the importance of acetogenic H2-production outcompeting hydrogenotrohic methanogenesis especially at low temperatures, further supported by homoacetogen proportion in the microcosm communities. The combination of in situ-, and laboratory-based measurements and molecular approaches indicates that the hydrogenotrophic pathway may become more important with increasing temperatures than the acetoclastic pathway. In a continuously warming environment driven by climate change, such issues are crucial and may receive more attention.
Highlights
Freshwater ecosystems represent 3.7% of the Earth’s land surface (Verpoorter et al, 2014), they are a major natural source of biogenic methane (CH4), emitting up to 159 tons of CH4 per year (Saunois et al, 2020).Biogenic methane is produced through methanogenesis mainly by methanogenic archaea in freshwater sediments, and similar environ ments (Conrad, 2020), characterized by the accumulation of labile organic matter (OM) and anaerobic conditions
Endogenic MPR from unamended microcosms Endogenic MPR measured in lake sediment samples at in situ temperature (5–15 ◦C) ranged from 0.03 to 1.22 μmol CH4 g− 1 DW d-1
They are comparable to MPR measured from some Arctic Alaskan lake sediments but are one to two orders of magnitude higher than values from lake sediments from high-altitude Yunnan plateau (Yang et al, 2017) and Japanese lake Biwa (Koizumi et al, 2004)
Summary
Freshwater ecosystems (i.e., lakes, reservoirs and ponds) represent 3.7% of the Earth’s land surface (Verpoorter et al, 2014), they are a major natural source of biogenic methane (CH4) (i.e., up to 43.1% of the natural CH4 emissions), emitting up to 159 tons of CH4 per year (Saunois et al, 2020). Biogenic methane is produced through methanogenesis mainly by methanogenic archaea in freshwater sediments, and similar environ ments (Conrad, 2020), characterized by the accumulation of labile organic matter (OM) and anaerobic conditions. The main reported methanogenic pre cursors are acetate and H2/CO2 for acetoclastic and hydrogenotrophic methanogenesis, respectively (Conrad, 1999; Kotsyurbenko et al, 2019). CH4 production from trimethylamine has been identified in Arctic thermokarst lake sediments (de Jong et al, 2018)
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