Abstract. Warming of the Arctic led to an increase in permafrost
temperatures by about 0.3 ∘C during the last decade. Permafrost
warming is associated with increasing sediment water content, permeability,
and diffusivity and could in the long term alter microbial community
composition and abundance even before permafrost thaws. We studied the
long-term effect (up to 2500 years) of submarine permafrost warming on
microbial communities along an onshore–offshore transect on the Siberian
Arctic Shelf displaying a natural temperature gradient of more than 10 ∘C. We analysed the in situ development of bacterial abundance
and community composition through total cell counts (TCCs), quantitative PCR
of bacterial gene abundance, and amplicon sequencing and correlated the
microbial community data with temperature, pore water chemistry, and sediment
physicochemical parameters. On timescales of centuries, permafrost warming
coincided with an overall decreasing microbial abundance, whereas millennia
after warming microbial abundance was similar to cold onshore permafrost. In
addition, the dissolved organic carbon content of all cores was lowest in
submarine permafrost after millennial-scale warming. Based on correlation
analysis, TCC, unlike bacterial gene abundance, showed a significant rank-based
negative correlation with increasing temperature, while bacterial gene copy
numbers showed a strong negative correlation with salinity. Bacterial
community composition correlated only weakly with temperature but strongly
with the pore water stable isotopes δ18O and δD, as well as with
depth. The bacterial community showed substantial spatial variation and an
overall dominance of Actinobacteria, Chloroflexi, Firmicutes,
Gemmatimonadetes, and Proteobacteria, which are amongst the microbial taxa
that were also found to be active in other frozen permafrost environments.
We suggest that, millennia after permafrost warming by over 10 ∘C,
microbial community composition and abundance show some indications for
proliferation but mainly reflect the sedimentation history and
paleoenvironment and not a direct effect through warming.
Microbial community composition and abundance after millennia of submarine permafrost warming
