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.

Automated summary

Photosynthesis in the surface ocean converts atmospheric CO2 into organic particles, which sink to depth as part of the ocean's biological pump. Previous studies indicated minimal replacement of plankton-derived particles by bacterial material during sinking. However, our research using bacteria-specific biomarkers reveals widespread accumulation of bacterial material in sinking particles. Bacterial detritus accounts for a significant proportion of the particulate organic carbon and nitrogen, exceeding cell count-based estimates. The contribution of bacteria to the biological pump increases with decreasing export production and is expected to rise with ocean warming.