Phosphorus (P) is an essential nutrient that can limit biological activity in some environments. Yet, many components of its cycle remain unclear, including P uptake and cycling in deep-sea sediments. These are critical, since a significant portion of Earth’s prokaryotes thrives in deep marine sediments, which are thought to mainly contain low bioavailable P in mineral phases. This suggests that microorganisms possess mechanisms to utilize recalcitrant P pools. The work performed focuses on identifying possible P sources to the deep biosphere, as well as microbial uptake mechanisms using stable oxygen isotope ratios in phosphate and 31P nuclear magnetic resonance spectroscopy on sediment samples collected during IODP 336 Expedition (North Pond).