Page 166 - 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
population included potentially methylotrophic Methylophilales with a relative abundance of 5% to 6%. Most surprisingly, Flavobacteriales showed highest relative abundance in both methanotrophic incubations (49% at 4°C, 40% at 10°C). The community composition of the controls showed a relative dominance of the Nitrosomonadales, which are ammonia oxidizing bacteria that could grow on the ammonium (1 mM) provided as the sole nitrogen source. qPCR showed a ratio of 14:1 of bacterial to archaeal 16S rRNA gene copy number for CH4 amended cultures (Table 3). The archaeal abundance decreased substantially, but the community composition did not change much compared to the control and the original sediments (Fig. 1; Supplemental Figure S5).
400 A 300 200 100
00
4oC 10oC 4oC 10oC 4oC 10oC
CH /O 4 2
CH /O + Chl 4 2
Control
3 B 2
1
0 4oC 10oC
μmol CH4 gdw-1
μmol CH4 gdw-1
10
20
30
40
50
60
Figure 4. A) Oxidation of CH4 by aerobic methanotrophs in batch incubations at 4°C and 10°C. Each data point represents the average from triplicate incubations with CH4 & O2, biotic control with only O2 & abiotic control with chloroform. Error bars indicate standard deviation of the mean. When there was no activity observed, horizontal lines, oxygen was depleted. B) Cumulative CH4 oxidation at 4°C and 10°C.
Table 3. Results of methanotrophic qPCR for archaeal and bacterial 16S rRNA gene copy number per g dry weight after 62 days.
Electron acceptors
O2 Control
Archaeal 16S rRNA gene copies per gdw
Bacterial 16S rRNA gene copies per gdw
Archaeal reads to control 4°C 10°C
Bacterial reads to control
4°C 1.3×107 ± 1.6×105 4.3×106 ± 4.7×104
10°C 1.4×107 ± 4.3×105 5.0×106 ± 4.0×104
4°C 1.8×108 ± 3.6×106 2.8×107 ± 4.2×105
10°C 2.1×108 ± 6.8×105 2.9×107 ± 7.0×105
4°C 10°C 3367
Anaerobic nitrite- and nitrate-dependent methane oxidation
The potential for AOM was determined by the amendment of nitrite (NO2-) or nitrate (NO3-) under anoxic conditions. Although substrates were replenished when depleted, no stimulation of anaerobic methanotrophic activity was observed, since no increase in 13CO2 was measured in the headspace over 450 days.
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