Page 155 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Materials and methods
Site description and sediment sampling
Sediment cores were collected from two nearby thermokarst lakes (Lake Emaiksoun and Unnamed Lake) during a winter field campaign carried out by the Vrije Universiteit Amsterdam in November 2015 to the northernmost US settlement of Utqiaġvik in the North Slope of Alaska, the United States. The two sampled lakes had maximum water depths of about 2.6 m, which made them relatively deep in comparison to the surrounding lakes (Hinkel et al. 2012). Despite the low depth, the lake bathymetry fell off steeply within a few meters from the shorelines. Both sampled lakes were located within wider drained lake basins with old remnant shorelines suggesting lake drainage at some point in the past. The sediment cores were collected in duplicates from seven different sites of the frozen lakes surfaces with a hand-operated Uwitec gravity coring system (Supplemental Table S1, Supplemental Figure S1). The cores had a diameter of 9 cm and lengths ranging from 40 to 86 cm. On the day of coring, pore-water samples were obtained from seven cores (one core at each site) at a 10 cm resolution with ceramic rhizons (pore size 0.12-0.18 μm) from Rhizosphere (Wageningen, the Netherlands).
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Porewater samples were treated with potassium iodide (KI) to prevent microbial activity. The cores were stored horizontally in the dark at 4°C at the Vrije Universiteit Amsterdam, the Netherlands. Porewater data was analyzed by inductive coupled plasma-optical emission spectrometry (ICP-OES) for Al, Ca, Fe, K, Mg, Mn, Na, P, S, Si, Zn (iCap 6300, Thermo Scientific, Waltham MA, USA) and continuous flow analysis (CFA) for NO3, NH4, PO4, Na, K and Cl (Supplemental Table S2 and S3) (Bran+Luebbe Auto Analyzer, SPX Flow, Norderstedt, Germany; Seal Analytical AutoAnalyzer 3, Seal Analytical, Southampton, UK).
Incubation experiments
The incubation experiments were started within half a year of sampling. The elemental data from pore water data analysis was used for medium design. The active top layer sediments from the seven cores (128 g) were mixed 1:10 weight/volume with anaerobic freshwater medium: KH2PO2 0.20 g L-1, NH4Cl 0.25 g L-1, NaCl 1.00 g L-1, MgCl2·6H2O 0.40 g L-1, KCl 0.50 g L-1 and CaCl2·2H2O 0.15 g L-1 in grade 3 demineralized water, 1 mL L-1 trace element solution SL-10 (DSMZ) with CeCl3·7H2O 24 mg L-1, Na2SeO3·5H2O 30 mg L-1 and Na2WO4·2H2O 40 mg L-1, and 1 mL L-1 vitamin solution (DSMZ): biotin 20 mg L-1, folic acid 20 mg L-1,
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