Abstract

Deep marine sediments harbor an abundance of microbial cells that, if active, are likely to exert a strong influence on biogeochemical cycling. Despite decades of study, our understanding of the fraction of cells that are active in situ and the metabolic processes that sustain them remain underexplored. We carried out an integrated set of analyses aimed at unraveling the links between geochemical heterogeneity, cellular viability and synthesis, and metabolism in sediment cores collected during the North Atlantic long coring expedition. Our four study sites covered a large range of sedimentation rates (hence organic carbon supply) and showed large differences in the degree of oxygen penetration. We hypothesized that these geochemical gradients would influence the extent and metabolic characteristics of subseafloor microbial life. Most probable number assays utilizing selective electron donor/acceptor enrichments revealed that populations of viable cells displaying a variety of metabolic capabilities exist within the sediment column to depths of 25 m or more, but their growth in situ appears limited by organic carbon or nutrients. Separate core subsamples were used to measure secondary production (protein and DNA synthesis). In the oxic sediments of the central gyre, microbial production rates were extremely low, but were measurable to 2.75 m depth. Production estimates based on protein synthesis were generally higher than those based on DNA synthesis, suggesting that cellular maintenance continues in the near absence of cell division. DNA sequencing of sediment microbial communities and enrichment cultures was also completed, which will link novel isolates back to in situ populations.