Award: OCE-1431598

Marine sediments worldwide contain a microbial population large enough to rival that of Earth's oceans, but many basic questions about this vast community of uncultured archaea and bacteria persist. Innovations in total cell counting methods have refined our estimates of total cell concentrations, but quantifications of the viable populations of specific taxa still remain elusive. Isotopic evidence indicates that the majority of subsurface microorganisms subsist by degrading organic matter left over from the burial of dead biomass. Yet links between specific microbial taxa and their organic matter substrates are as yet untested. We examined deep subsurface sediments from the IODP Leg 347: Baltic Sea Paleoenvironment expedition to answer these questions in deep subsurface sediments. We used single cell genomics, metabolomics, metatranscriptomics, and enzyme assays to link microbial taxa to their organic matter substrates. We found that bacteria were more numerous in these sediments than archaea, although predicted improvements for CARD-FISH and qPCR were not effective at increasing the ability of these methods to measure absolute abundances. with most of the activity, as shown by mRNA transcript recruitment, coming from uncultured Atribacteria, Aerophobetes, and Actinobacteria OPB41. We discovered that these organisms have different catabolic preferences, which may explain how they can co-exist in this low-energy ecosystem, rather than outcompeting each other for limited resources. We found that these genomes from intact and active microbes have adaptations that likely improve their ability to persist burial in marine sediment over 44,000 years. Last Modified: 09/15/2017 Submitted by: Karen Lloyd