Molecular properties are a primary control on the microbial utilization of dissolved organic matter in the ocean

Abstract The global ocean sequesters a large amount of reduced carbon in dissolved organic molecules that can persist for centuries to millennia. The persistence of dissolved organic carbon (DOC) in the deep ocean has been attributed to inherently refractory molecules and to low concentrations of molecules, but the relative roles of molecular properties and molecular concentrations remain uncertain. We investigate both of these possibilities using bioassay experiments with unfiltered seawater collected from five depths (50–1500 m) at the Bermuda Atlantic Time‐Series Study site. The microbial utilization of compositionally distinct forms of seawater DOC at in situ and elevated concentrations was determined. Microbial utilization of in situ organic carbon ranged from 6% to 7% in surface waters to 0% in deep water after 180 d. Additions of surface plankton‐derived DOC (~18 μmol L −1 ), which was enriched in amino acids and carbohydrates, revealed substantial (50–75%) removal of the added DOC at all depths within 7 d. In sharp contrast, additions of C‐18 isolated deep‐sea DOC (~20 μmol L −1 ) showed insignificant or minimal utilization at all depths after 7 or 180 d, even when primed with labile substrates. These experiments demonstrate microbial communities from varying depths and environments in the ocean could rapidly utilize elevated concentrations of plankton‐derived DOC, whereas these same microbes failed to utilize elevated concentrations of C‐18 DOC. These results indicate molecular properties are the primary control on the microbial utilization of DOC in the ocean. Our findings imply a dynamic DOC reservoir with a flexible capacity for carbon sequestration in the global ocean.