No other environment hosts as many microbial cells as the marine sedimentary biosphere. While the majority of these cells are expected to be alive, they are speculated to be persisting in a state of maintenance without net growth due to extreme starvation. Here, we report evidence for in situ growth of anaerobic ammonium-oxidizing (anammox) bacteria in ∼80,000-y-old subsurface sediments from the Arctic Mid-Ocean Ridge. The growth is confined to the nitrate–ammonium transition zone (NATZ), a widespread geochemical transition zone where most of the upward ammonium flux from deep anoxic sediments is being consumed. In this zone the anammox bacteria abundances, assessed by quantification of marker genes, consistently displayed a four order of magnitude increase relative to adjacent layers in four cores. This subsurface cell increase coincides with a markedly higher power supply driven mainly by intensified anammox reaction rates, thereby providing a quantitative link between microbial proliferation and energy availability. The reconstructed draft genome of the dominant anammox bacterium showed an index of replication (iRep) of 1.32, suggesting that 32% of this population was actively replicating. The genome belongs to aScalinduaspecies which we nameCandidatus Scalindua sediminis, so far exclusively found in marine sediments. It has the capacity to utilize urea and cyanate and a mixotrophic lifestyle. Our results demonstrate that specific microbial groups are not only able to survive unfavorable conditions over geological timescales, but can proliferate in situ when encountering ideal conditions with significant consequences for biogeochemical nitrogen cycling.

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The deep biosphere buried in marine sediments was estimated to host an equal number 18 of microbes as found in the above oceans 1 . It has been debated if these cells are alive 19 and active 2 , and their per cell energy availability does not seem to allow for net 20 population growth 3 . Here, we report the growth of anammox bacteria in ~80,000 year 21 old subsurface sediments indicated by their four orders of magnitude abundance 22 increase in the nitrate-ammonia transition zone (NATZ). Their growth coincides with a 23 local increase in anammox power supply. The genome of the dominant anammox 24 bacterium from the NATZ was reconstructed and showed an increased index of 25 replication confirming in situ active growth. The genome belongs to a new Scalindua 26 species so far exclusively found in marine environments, which has the genetic capacity 27 of urea and cyanate utilization and is enriched in genes allowing it to cope with external 28 environmental stressors, such as energy limitation. Our results suggest that specific 29 microbial groups are not only able to survive over geological timescales, but also thrive 30 in the deep subsurface when encountering favorable conditions. 31 32 Main text 33 The global cell numbers of microbes in marine sediments is estimated to be on the order of 34 2.9-5.4×10 29 equaling up to 1/3 rd of the total prokaryotic biomass on Earth 1,4 . A considerable 35 portion of these cells reside beyond the bioturbation zone and constitute the marine deep 36 biosphere 5 . Microbial cells in the subseafloor sediments are sealed off from recruitment of 37 new cells and fresh substrates from the surface, and therefore are thought to suffer severe 38 energy limitations 3 . Despite this, several lines of circumstantial evidence indicate that the 39 deep microbial biosphere is alive 6,7 , but with extremely slow metabolic rates 8,9 and long 40 turnover times of hundreds to thousands of years 2,10 . Although microbial growth (net biomass 41 production) was frequently assumed 10,11 and recently observed in laboratory incubations 12 , 42 concrete evidence of in situ microbial growth in the marine deep biosphere is lacking. 43Energy availability is considered one of the most fundamental factors limiting life, but 44 has not been explicitly demonstrated to control the changes of microbial communities in the 45 deep biosphere 13 . Whereas the deep sedimentary realm is a stable environment with low 46 energy availability, geochemical transition zones such as the sulfate-methane transition 47 growth associated with increased power availabilities in ~ 80,000 year old subsurface 55 sediments. 56 We retrieved four sediment cores (2.0-3.6 meters long) from the seabed of the Arctic 57 Mid-Ocean Ridge (AMOR) at water depths of 1653 -3007 m ( Fig. 1a and Table S1), to 58 perform high vertical resolution geochemical measurements and microbiological analyses. All 59 four cores exhibited similar geochemical profiles ( Fig. 2a-c), summarized as follows: 1) O 2 60 monotonically decreased and was deplet...

The Arctic warms faster than any other region of our planet. Besides melting glaciers, thawing permafrost and decreasing sea-ice, this amplified response affects earth surface processes. This geomorphological expression of climate change may alter landscapes and increase the frequency and magnitude of geohazards like floods or mass-movements. Beyond the short span of sparse monitoring time series, geological archives provide a valuable longterm context for future risk assessment. Lake sediment sequences are particularly promising in this respect as continuous recorders of surface process change. Over the past decade, the emergence of new techniques that characterize depositional signatures in more detail has enhanced this potential. Here, we present a well-dated Holocene-length lake sediment sequence from Ammassalik Island on southeast Greenland. This area is particularly sensitive to regional shifts in the Arctic climate system due to its location near the sea-ice limit, the Greenland Ice Sheet and the convergence of polar and Atlantic waters. The expression of Holocene change is fingerprinted using physical (grain size, organic content, density), visual (3-D Computed Tomography) and geochemical (X-Ray Fluorescence, X-Ray Diffraction) evidence. We show that three sharp transitions characterize the Holocene evolution of Ymer Lake. Between 10-9.5 cal. ka BP, rapid local glacier loss from the lake catchment culminated in an outburst flood. Following a quiescent Holocene climatic optimum, Neoglacial cooling, lengthening lake ice cover and shifting wind patterns prompted in-lake avalanching of sediments from 4.2 cal. ka BP onwards. Finally, glaciers reformed in the catchment after 1.2 cal. ka BP. The timing of these shifts is consistent with the regional expression of deglaciation, Neoglacial cooling and Little Ice Age glacier growth, respectively. The novel multi-proxy approach applied in this study rigorously links depositional sediment signatures to surface processes and thereby provides a key step towards a process-based understanding of climate responses.

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