Bacterial interactions shape the molecular composition of dissolved organic matter

Heterotrophic marine microbes drive long-term carbon storage in the oceans by partitioning phytoplankton-derived dissolved organic matter (DOM) into fractions of varying persistence. However, the role of microbial interactions in molecular transformations that channel labile to persistent DOM remains poorly resolved. Here, we investigated the fate of DOM within pure cultures and co-cultures through a full-factorial experimental design with four marine microorganisms that grew on a single carbon source. In pure cultures, we observed a correlation between DOM molecular diversity and metabolic potential of each strain. Yet, organisms grown together revealed DOM molecular signatures containing novel molecular formulas unique to each co-culture, such that the whole differed from the sum of its parts. Co-cultures produced the majority of molecular formulas of which only a small fraction (<1%) corresponded to known metabolites predicted by metabolic network analyses of the four bacterial isolates. These findings suggest that microbial interactions mediated by known and unknown compounds are a major driver of the diversity, composition, and persistence of marine DOM.