Page 168 - 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 Discussion
In this study, organic-rich sediment cores were taken from two freshwater thermokarst lakes in Utqiaġvik, Alaska. The respective microbial communities were investigated by 16S rRNA gene amplicon sequencing. The bacterial and archaeal community was very diverse. The sediment cores had a relatively rare bacterial biosphere where 50% of the reads was affiliated to groups with an individual abundance of less than 2%. This high bacterial diversity was not observed in previously described thaw ponds/freshwater sediments (Liebner, Harder and Wagner 2008; Rossi, Laurion and Lovejoy 2013; Crevecoeur et al. 2015; Wang et al. 2016; Wagner et al. 2017). The dominant members of the bacterial community were Bacteriodetes (16%-19%) similar to a study of polygonal tundra in the Lena Delta, Siberia, where Bacteroidetes constituted almost 50% of the soil microbial community (Liebner, Harder and Wagner 2008).
Most studies of permafrost thaw ponds that used different DNA extraction and amplification methods, however, have found Proteobacteria to be the most dominant Bacteria (Rossi, Laurion and Lovejoy 2013; Crevecoeur et al. 2015; Wang et al. 2016; Wagner et al. 2017), a pattern that was also shown by Wagner et al. (2017) who used the same extraction method and 16S rRNA gene amplification as our study. The most abundant Archaea belonged to Rice Cluster II, Methanosaetaceae, and Bathyarchaeota, which are commonly found in permafrost sediments and soils (Mondav et al. 2017; Winkel et al. 2018).
With thaw, previously frozen organic matter will become available for degradation; however, it is impossible to predict what will become available when and in which concentrations. To investigate the microbial response to warming and nutrient availability on the CH4 emissions, the activity of methanogens and methanotrophs were studied in batch incubations. Here we provided substrate concentrations that were lower than previous studies have used (Allan et al., 2014: 30 mM acetate; Wagner et al., 2017: 20 mM trimethylamine [TMA]), and are therefore closer to environmentally relevant conditions. Highest methanogenic activity was observed with TMA (6.6 and 8.5 μmol CH4 gdw-1 d-1 at 4°C and 10°C respectively). 50% of the TMA was converted to CH4. The methanogenic community was clearly enriched in Methanosarcinaceae, which is expected based on their substrate spectrum (Garrity, Bell and Lilburn 2004). Acetoclastic methanogenesis was the second most active process (3.4 and 4.7 μmol CH4 gdw-1 d−1 at 4°C and 10°C respectively). 73% of the acetate was converted to CH4. Here,
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