Non-monotonic Export of Bacterially-derived Sinking Particulate Organic Matter to the Deep Ocean

Marine particles sinking through the ocean are known to undergo diverse bacterial transformations, but the magnitude and temporal pattern of bacterial contribution to deep-sea carbon and nitrogen export are poorly constrained. We measured bacterial biomarkers (D-amino acids) in a sediment trap time series recovered at 1200 m on the California margin. D-amino acids were consistently detected, indicating widespread bacterial contribution to sinking particles. Estimates based on D-amino acid yields in bacterial cultures and field organic matter samples showed that 5-40% of the sinking particulate organic carbon (POC) and 10-70% of particulate nitrogen being exported to the deep ocean were directly derived from bacterial biosynthesis. However, mass flux of bacterial organic matter did not vary linearly with total POC flux, as might be expected, but instead displayed a reproducible and non‑monotonic temporal pattern that peaked during moderate POC flux states. This export pattern was observed here for the first time, and could be explained by the inverse changes in bacterial contribution and total POC flux associated with temporal variation in overlying planktonic community structure. Our observations suggest pervasive yet widely variable degree of bacterial processing on sinking particles, and imply a more detailed mechanistic understanding of particle-attached bacteria in assisting microbial carbon sequestration in the dark ocean.