Page 160 - 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
reactions were performed on a C1000 Touch thermal cycler equipped with a CFX96 TouchTM Real-Time PCR Detection System (Bio-Rad Laboratories B.V., Veenendaal, the Netherlands); 0.5 ng DNA template was used per reaction. Negative controls were added to each run by replacing the template with sterile Milli-Q water. Standard curves were constructed with a 10- fold serial dilution of a quantified copy number of pGEM®-T Easy plasmids with inserted Illumina PCR fragments of archaeal and bacterial 16S rRNA gene (Promega, Madison WI, USA). All qPCR data was analyzed using the Bio-Rad CFX Manager version 3.0 (Bio-Rad Laboratories B.V., Veenendaal, the Netherlands).
Results
Biogeochemistry of the thermokarst lake sediments
Sediment cores were taken in duplicates from four different sites of Lake Emaiksoun and three sites from Unnamed Lake in the vicinity of Utqiaġvik, Alaska, USA, during a winter field campaign in November 2015 (Supplemental Table S1, Supplemental Figure S1). The top 30- 40 cm of all cores was made up by clay-dominated silty, organic-rich (30 wt% total organic carbon), brown mud that was abruptly truncated at the bottom by a sharp boundary to the underlying sediment. These lower halves of the cores consisted of old coastal sand, pebble and silt deposits, which gradually turned into clayey marine mud towards the bottom. This sequence of marine layers documents the regression of the paleo shoreline. The associated marine and coastal deposits can be attributed to the Barrow Unit of the Gubik formation, which is a well- documented Quaternary deposit in near-coastal landscapes of northernmost Alaska (Black 1964). Porewater sample analysis showed that total nitrogen was 8.1 mg L-1, predominantly present in the form of ammonium (8 mg L-1) and only trace amounts of nitrate (0.1 mg L-1) (Supplemental Tables S2 and S3). Total phosphate and sulfate were 0.8 and 1.8 mg L-1 respectively. The total salinity of the pore water was around 500 ppm, indicating that these thermokarst lakes are freshwater systems.
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