Page 172 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 7. Methane cycling in Arctic thermokarst lake sediments
is needed to understand potential cross-feeding processes between methanotrophs, methylotrophs and heterotrophs.
Besides the aerobic oxidation of CH4 by methanotrophic bacteria, the nitrite- and nitrate- dependent AOM by methanotrophic archaea was studied. These processes are carried out by “Candidatus Methylomirabilis oxyfera” (Raghoebarsing et al. 2006) and “Candidatus Methanoperedens nitroreducens” (Haroon et al. 2013). “Candidatus Methanoperedens” archaea were present in the original sediment (0.3-2.1% of archaeal reads), indicating that AOM might be possible. However, no AOM activity was observed in our incubations. In the sampled environment, the NO3- concentration in the sediments was low, ranging between 0.8 and 2.4 μM. In submarine permafrost, where ANME were found, higher NO3- concentrations were detected (Winkel et al. 2018). This indicates that NO3- does not play an important role in this ecosystem, and when nitrite and nitrate are present, they are probably consumed by heterotrophic denitrifiers rather than by ANME archaea. Other substrates involved in anaerobic methanotrophy should be tested in future studies to determine if AOM is occurring in the sediments of these lakes.
Altogether our results indicate that with the availability of various substrates and increasing temperature both the CH4 production and consumption rates increase. The microbial community structure was strongly affected by the provided substrate. At higher temperature, there is a rapid response of the methanotrophic community to counteract the increase of the CH4 emission from methanogens. These laboratory slurry incubations showed that the microbial community was able to adapt to different environmental conditions after nearly a year of incubation. It is essential to better understand the interaction of methanogens and methanotrophs in this ecosystem under increasing substrate availability and elevated temperature scenarios before predictions can be made about in situ situations.
Acknowledgements
We thank Jeroen Frank for the great discussions about sequencing analysis, Sebastian Krosse for pore water data analysis, and Han Dolman for fruitful discussions and input. This research was supported by the Netherlands Organization for Scientific Research through the Netherlands
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