Page 169 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Methanosaetaceae had a highest relative abundance followed by Methanosarcinaceae, which is expected based on their substrate spectrum (Garrity, Bell and Lilburn 2004). These CH4 production rates were higher than those found by Allan et al. (2014), who observed an average CH4 flux of 1.8 nmol (g wet weight)−1 d−1 at 4°C and 0.9 nmol (g wet weight)−1 d−1 at 22°C in Siberian permafrost microcosms amended with 30 mM acetate during 40-50 days. Although the incubation period in this study was longer, higher rates were also observed after 40 days (0.1 ± 0.02 and 0.2 ± 0.02 μmol CH4 gdw-1 d−1 at 4°C and 10°C). The difference in rates is probably mainly caused by the high organic matter content of the Alaskan lake sediments used in this study (25-35wt%) compared to the Siberian permafrost land soils studied by Allan et al. (2014) (4-6 wt%). In the active methanogenic incubations, there was a high temperature sensitivity for TMA and acetate after 98 days (Q10 value of 9.8 and 4.2 respectively). High Q10 values for methane production in lake sediments were observed before for short incubations (<50 days) (Duc, Crill and Bastviken 2010; Sepulveda-Jauregui et al. 2018). However, the Q10 values in our incubations decreased after 279 days to 0.9 and 1.1 for TMA and acetate respectively. At this point, warming did no longer cause significant differences (two-sided t- test; p>0.05) to the total CH4 produced with the two different substrates, indicating that over a longer time period (>150 days) temperature has no strong effect on the activity of methylotrophic methanogenesis in the incubation bottles. Hydrogenotrophic methanogens were most strongly influenced by the temperature increase (total CH4 production increased by 66% at 10°C). However, the stoichiometry of the reaction did not match very well as only 3% (for 4°C) and 5% (for 10°C) of the substrates was converted to CH4. This is an indication that other microbial processes than methanogenesis may be occurring. The total methane production was more pronounced in the presence of acetate and TMA, corroborated by the shifts from hydrogenotrophic to acetoclastic methanogens abundance at elevated temperatures in other studies (Allan et al. 2014; McCalley et al. 2014; Coolen and Orsi 2015). The increase in acetate concentration in a natural system often occurs from the reduction of CO2 with H2 to acetate by acetogens, which is followed by acetoclastic methanogenesis (Angel, Claus and Conrad 2012).
Our results are in contrast with studies that found higher abundances of hydrogenotrophic methanogens in the active layer of permafrost (Barbier et al. 2012; Lansing et al. 2015). In the H2/CO2 incubations, no clear shift in community composition was observed. The relative abundance of the hydrogenotrophic orders Methanomicrobiales and Methanocellales were
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