In situ growth of anammox bacteria in subseafloor sediments

The deep biosphere buried in marine sediments was estimated to host an equal number of microbes as found in the above oceans 1. It has been debated if these cells are alive and active 2, and their per cell energy availability does not seem to allow for net population growth 3. Here, we report the growth of anammox bacteria in ~80,000 year old subsurface sediments indicated by their four orders of magnitude abundance increase in the nitrate-ammonia transition zone (NATZ). Their growth coincides with a local increase in anammox power supply. The genome of the dominant anammox bacterium from the NATZ was reconstructed and showed an increased index of replication confirming in situ active growth. The genome belongs to a new Scalindua species so far exclusively found in marine environments, which has the genetic capacity of urea and cyanate utilization and is enriched in genes allowing it to cope with external environmental stressors, such as energy limitation. Our results suggest that specific microbial groups are not only able to survive over geological timescales, but also thrive in the deep subsurface when encountering favorable conditions.