Important Contribution of Bacterial Carbon and Nitrogen to Sinking Particle Export

Photosynthesis in the surface ocean converts atmospheric CO2 into organic particles, with the fraction sinking to depth representing a major part of the ocean's biological pump. Although sinking particles are known to be altered by attached-bacteria during transit, most prior organic geochemical data indicated only minor replacement of plankton-derived particles by bacterial material. We exploit bacteria-specific biomarkers (d-amino acids) in a multi-year sediment trap in the Pacific Ocean (1,200 m) and suggest a different view. Major d-amino acids were consistently measured at abundance demonstrating widespread accumulation of bacterial material in sinking particles. Bacterial detritus was estimated to account for up to 19% of particulate organic carbon and up to 36% of particulate nitrogen, much higher than cell count-based values. The bacterial relative contribution increased with decreasing export production. Our results indicate that bacterial material constitutes an underappreciated component of the biological pump, a role expected to rise as the ocean warms.