The development of permafrost bacterial communities under submarine conditions

Abstract

Submarine permafrost is more vulnerable to thawing than permafrost on land. Besides increased heat transfer from the ocean water, the penetration of salt lowers the freezing temperature and accelerates permafrost degradation. Microbial communities in thawing permafrost are expected to be stimulated by warming, but how they develop under submarine conditions is completely unknown. We used the unique records of two submarine permafrost cores from the Laptev Sea on the East Siberian Arctic Shelf, inundated about 540 and 2500 years ago, to trace how bacterial communities develop depending on duration of the marine influence and pore water chemistry. Combined with geochemical analysis, we quantified total cell numbers and bacterial gene copies and determined the community structure of bacteria using deep sequencing of the bacterial 16S rRNA gene. We show that submarine permafrost is an extreme habitat for microbial life deep below the seafloor with changing thermal and chemical conditions. Pore water chemistry revealed different pore water units reflecting the degree of marine influence and stages of permafrost thaw. Millennia after inundation by seawater, bacteria stratify into communities in permafrost, marine‐affected permafrost, and seabed sediments. In contrast to pore water chemistry, the development of bacterial community structure, diversity, and abundance in submarine permafrost appears site specific, showing that both sedimentation and permafrost thaw histories strongly affect bacteria. Finally, highest microbial abundance was observed in the ice‐bonded seawater unaffected but warmed permafrost of the longer inundated core, suggesting that permafrost bacterial communities exposed to submarine conditions start to proliferate millennia after warming.

Microbial community composition and abundance after millennia of submarine permafrost warming

“…2a) that were retrieved along an onshore-offshore transect in the coastal region of Cape Mamontov Klyk in 2005 (Overduin, 2007;Rachold et al, 2007). Cores were named after the order of drilling and we kept this order (C1, C4, C3, C2) for better comparability with previous studies (Koch et al, 2009;Mitzscherling et al, 2017;Overduin et al, 2008;. From onshore to offshore all cores were characterized by an increase in water depth, in depth to the ice-bonded permafrost table (Fig.…”

“…In a recent study, we compared submarine sediment cores from two locations on the Siberian Arctic Shelf and looked at the combined effect of permafrost inundation time and seawater intrusion on microbial communities. We showed that flooding by seawater reduced permafrost bacterial abundance and changed bacterial community composition due to the penetration of seawater into a former freshwater habitat (Mitzscherling et al, 2017). It was suggested that in addition to the effect of seawater infiltration, the sediment warming taking place over millennia could lead to proliferation.…”

“…Both sandy and organicrich deposits were represented by three replicates in C1, C4, and C3 and four replicates in C2 (Table S4). Furthermore, to check for reproducibility we included samples from C2 retrieved in a previous study (Mitzscherling et al, 2017) (sample names CK12xx). In order to prevent contamination caused by the drilling equipment, we took the subsamples from the centre of the core.…”

“…Core subsamples were homogenized in liquid nitrogen and DNA was extracted from ∼ 5 g of sediment using a modified protocol of Zhou et al (1996). The method was described before (Mitzscherling et al, 2017), and in the following we refer to these samples as molecular samples. The quality of the extracted genomic DNA was assessed via gel electrophoresis ( Fig.…”

“…Preparation and quantification of the total cell abundance per gram of sediment were performed after Llobet-Brossa et al (1998). The modified protocol was described before by Mitzscherling et al (2017). Briefly, cells were fixed with 4 % paraformaldehyde in phosphate-buffered saline (PBS).…”