Bacterial interactions shape dissolved organic matter molecular composition

Heterotrophic microorganisms in the surface ocean mediate one of the largest global carbon fluxes by rapidly recycling phytoplankton-derived dissolved organic matter (DOM) to CO2, while transforming a small fraction into persistent DOM through molecular-level processes that remain poorly constrained. Here, we used a reductionist, full-factorial design with four marine bacterial isolates to systematically investigate microbe–DOM interactions at the molecular scale. The metabolic capacity of individual microorganisms determined the degree of substrate diversification in pure cultures. In co-cultures of two or more strains, microbial interactions generated DOM with molecular signatures specific to each consortium and produced novel, largely unique, molecular formulas. In this system, DOM persistence emerged primarily as a property of community metabolism, with a minor contribution from intrinsic molecular characteristics. These findings suggest that interactions between heterotrophic microorganisms are a major driver of the molecular diversity, composition, and persistence of marine DOM.