16S rRNA (gene) amplicon sequencing of collapsing palsas, thawing permafrost soils, at Stordalen Mire, Abisko, Sweden in July 2019.

Reactive iron (Fe) minerals can preserve organic carbon (OC) in intact permafrost soils. With permafrost thaw, reductive dissolution of iron minerals releases Fe and OC into the porewater, potentially increasing the bioavailability of OC for microbial decomposition. However, the stability of this so-called rusty carbon sink in intact permafrost soils, the microbial community driving mineral dissolution, the identity of the iron-associated carbon and the resulting impact on greenhouse gas emissions are unknown. We examined microbial community relative abundance and potential activity (with DNA- and RNA-based 16S rRNA (gene) amplicon sequencing) of collapsing palsas, the first thaw stage along a thaw gradient in a permafrost peatland in Abisko (Sweden). Sampling took place at Stordalen Mire (68.357592 N 19.050531 E) on 4th of July 2019. A Humax corer of 50 cm length and 3-cm diameter with inner liners was used. The cores for microbial community analysis were split directly in the field, immediately frozen with liquid nitrogen and stored at -80C until further processing. The cores were split into three soil layers based on texture and color changes: (1) A peat or organic horizon, followed by (2) a transition zone between the organic-rich and mineral-rich layer and (3) a mineral horizon. Total RNA and DNA was extracted using the PowerSoil RNA and DNA isolation kit (MO BIO Laboratories, Carlsbad, CA, USA) with 2-3 g soil; 10 min bead-beating; centrifugation steps at maximal speed (7000g) at 4C; and longer incubation times at -20C (1.5 h). To investigate RNA, subsequent DNA digestion and reverse transcription reactions were performed using a Reverse Transcriptase (Invitrogen, Life Technologies). Subsequent library preparation steps (Nextera, Illumina) and sequencing were performed by Microsynth AG (Switzerland) using the 2x250 bp MiSeq Reagent Kit v2 on an Illumina MiSeq sequencing system (Illumina, San Diego, CA, USA). From 10,112 to 396,483 (average 113,374) read pairs were generated per sample in three separate sequencing runs on the same MiSeq machine, resulting in total in 8.6 million read pairs.