The deep subseafloor sediment biosphere contains a vast reservoir of physiologically diverse yet viable microbial cells with exceedingly low rates of metabolic activity, subsisting mainly on buried detrital organic material that has been decaying for millions of years. Thus, this life persists in the face of both extreme energy and nutrient deficiency. Growth limitation by low energy (i.e., organic carbon; OC) flux is well-studied and widely accepted, but little information exists on potential nutrient limitation of anabolism in this dark realm. We hypothesize that biomass synthesis of sedimentary communities beneath the North Atlantic subtropical gyre may be more directly limited by phosphorus (P) availability than by that of OC, and we expect that these microbes maintain efficient cellular machinery for organic P acquisition despite the low energy flux available to them. We will address these hypotheses using sediments collected from up to 27 m beneath the seafloor during the 2014 KN-223 research cruise. Our study will combine enzyme activity assays, radioisotope-based nutrient stimulation bioassays of DNA and protein synthesis, and molecular investigations of P-acquisition gene expression of microbial isolates. This work will provide novel experimental data on the functioning of one of the largest and most P-deficient ecosystems on Earth.