Dissolved organic carbon (DOC) is the main energy source for marine heterotrophic microorganisms, but a small fraction of DOC resists microbial degradation and accumulates in the ocean. The reason behind this recalcitrance is unknown. We test whether the long-term stability of DOC requires the existence of structurally refractory molecules, using a mechanistic model comprising a diverse network of microbe-substrate interactions. Model experiments reproduce three salient observations, even when all DOC compounds are equally degradable:(i)> 15% of an initial DOC pulse resists degradation, but is consumed by microbes if concentrated,(ii) the modelled deep-sea DOC reaches stable concentrations of 30–40 mmolC/m 3, and (iii) the mean age of deep-sea DOC is several times the age of deep water with a wide range from< 100 to> 10,000 years. We conclude that while structurally-recalcitrant molecules exist they are not required in the model to explain either the amount or longevity of DOC.