Page 188 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 8. Species-specific responses of enriched thermokarst lake sediments
Due to their low substrate threshold Methanosaetaceae/Methanotrichaceae are expected to dominate over Methanosarcinaceae in substrate-limited conditions (Westermann, Ahring and Mah 1989). Minimum acetate threshold concentrations for Methanosaetaceae/Methanothrix sp. are below 0.01 mM, whereas Methanosarcina sp. have a much higher substrate limit of 0.2- 1.2 mM (Westermann, Ahring and Mah 1989; Jetten, Stams and Zehnder 1990, 1992). Upon acetate amendment at low substrate concentrations (2-4 mM) we observed a clear increase of Methanosaetaceae/Methanotrichaceae (1.6-2.1% to 5.3-9.2%) and a lower increase in Methanosarcinaceae (0.2-0.3% to 1.8-3.1%). It is therefore likely that a rapid turnover of acetate, which was supported by CH4 production rates, led to the dominance of Methanosaetaceae/Methanotrichaceae.
Surprisingly, there was no acetoclastic Methanosaetaceae/Methanotrichaceae MAG amongst the dominant assembled genomes. A single Methanosaetaceae/Methanotrichaceae MAG could, however, be recovered with an estimated 65.5% genome completeness. This MAG showed highest read coverages on acetate at 4°C (2.0%) and 10°C (3.5%) and lower on TMA at 4°C (0.13%) and 10°C (0.30%) and unamended controls at 4°C (0.49%) and 10°C (0.61%). Low genome completion and high fragmentation of the MAG, however, hampered further analysis.
TMA strongly induces versatile Methanosarcinaceae with distinct temperature responses Upon TMA amendment the methanogenic community was dominated by versatile Methanosarcinaceae. They can perform acetoclastic, methylotrophic and hydrogenotrophic methanogenesis, and they rapidly respond to increased nutrient availability (Patel and Sprott 1990; Sprenger et al. 2000; Spring et al. 2010). This is in line with the high substrate turnover efficiency of 73% that was measured in our previous study (de Jong et al. 2018). All three MAGs contained a partial or complete nifDHK gene cluster encoding the nitrogenase complex (Supplementary Table S8). This nitrogen fixation potential was not observed in the other seven MAGs, indicating a potential important role of methanogenic archaea in the nitrogen cycle of this ecosystem. The three Methanosarcinaceae MAGs were highly abundant upon TMA amendment covered 53.0% and 48.6% of the assigned reads at 4°C and 10°C, respectively (Table 1).
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