Page 217 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

organic matter fractions fuel the system. The slight increase of CH4 production over time, especially in TL and PF, may indicate a shift towards the degradation of this less labile organic matter pool. Evidence for this is provided by the functional microbial shifts using on end-point metagenome analyses. Due to the higher structural complexity, the degradation of stabile organic molecules requires more energy, which results in higher temperature sensitivities compared to labile compounds (Fierer et al. 2005). The low GHG fluxes on the long term indicate that warming to 4°C is still limiting the turnover of more stable organic matter. While the degradation of the less labile organic matter fraction is much slower, it could still fuel a relatively stable methanogenic community.
In combination with the decreasing GHG production rates and a shift towards more methanogenic conditions, we observed substantial changes in microbial community structure
and functions during the long-term warming scenario in all layers. The observed divergent
shifts in community structure in the three different layers indicates that permafrost thaw and long-term warming does not necessarily lead to the establishment of microbial communities
more similar in taxonomic and functional composition. This is in contrast with previous observations by Mackelprang et al. (2011) who studied permafrost metagenomes of active layer
and permafrost and observed a convergence of functional gene composition on the short-term
of seven days. The difference is potentially linked to the differences in incubation time, but also
batch effects, due to incubations in closed bottles, which prevent natural mixing with the 9 surrounding soil matrix.
Clostridia and Bacteroidia showed a positive response to thaw based on their relative abundances. These bacterial groups include many fermenting species that, for instance, play important roles in anaerobic fermentation of organic matter in anaerobic digestors (Goud et al. 2012). Similar to our observations, Bacteroidetes were also detected within the transition layer on a permafrost soil at Svalbard, Norway (Müller et al. 2018). High occurrences of Bacteroidetes have been linked with their metabolic flexibility, as well as rapid growth on easily accessible substrates (Fierer, Bradford and Jackson 2007). Rapid growth on labile compounds could support their success upon thaw, whereas their metabolic flexibility can support growth in the long term. Clostridia are fermentative bacteria that are generally well-adapted to extreme
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