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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 sea
water, 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.
