Microbial  controls on dissolved organic nitrogen cycling during long-term degradation  experiments

Dissolved organic nitrogen (DON) is critical for marine microbial growth by providing carbon and nitrogen. Although DON is rapidly cycled, some compounds within this pool can persist for long-periods of time. To better understand how DON is cycled by marine microorganisms, we conducted 548-day incubations using surface-derived dissolved organic matter along with microbial communities from the surface, mesopelagic and bathypelagic regions of the Labrador Sea. Across all depths, ~2 µmol L-1 DON was produced and persisted for several months (i.e., semilabile DON), with ~20% attributed to cell growth-and-death cycles and ~80% to direct exudation by microbial communities. This newly synthesized DON was subsequently transformed by microbial communities, indicated by increased protein-like fluorescence and decreased amino acids contribution to DON. These findings suggest that microbial communities can produce transiently persistent DON with potential implications for nitrogen storage and recycling in the ocean's interior.