Page 193 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Increased acetate and TMA availability can boost methane production from thermokarst lake sediments
Overall, we observed a strong enrichment in methanogenic archaea upon acetate and TMA amendment of thermokarst lake sediments. An acetate conversion efficiency of 50% to CH4 together with an increase in acetoclastic Methanosaetaceae/Methanotrichaceae indicates the establishment of a stable methanogenic community (de Jong et al. 2018). Our initial 16S rRNA gene-based study was, however, not detailed enough to unravel potential species-specific responses. Upon metagenomic sequencing we found that several bacterial species could support the methanogenic community under increased acetate availability. These results highlight that an increase in acetate availability could likely result in elevated CH4 production on the long term. This is highly relevant in the context of a warming Arctic (Mack et al. 2004; Herndon et al. 2015).
The strong increase of Methanosarcinaceae on TMA, together with an efficient substrate conversion efficiency of 73%, highlights their success upon an increased availability of methylated compounds in thermokarst lake sediments. Interestingly, the different responses of
the MAGs to the temperature scenarios stressed that unique methanogenic species are responsible for methanogenesis at different temperatures. This change could only be unraveled
with our metagenomics sequencing approach. 8
Methanogens are part of a complex microbial community. Due to intricate species interactions, the methanogenesis potential is partially controlled by the activity of fermentative microorganisms. In turn, the fermentative processes are controlled by several ecosystem characteristics, including pH, temperature and organic matter types (Bastviken et al. 2004; Ye et al. 2012; Roy Chowdhury et al. 2015). Here we provided a link between the complex bacterial community and methanogenic archaea in thermokarst lake sediments using a controlled set-up. Future research on the link between temperature, substrate availability and species dynamics in situ is highly needed, since this ultimately determines the ecosystem CH4 fluxes. Understanding these dynamics is important to better comprehend the mechanisms that underlie GHG production from Arctic lakes in a warming world.
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