Increased microbial and substrate complexity result in higher molecular diversity of the dissolved organic matter pool

Abstract Marine dissolved organic matter (DOM) represents one of the largest reduced carbon pools on Earth. Diverse microbial metabolic activities play important roles in shaping this pool. However, the links between changing DOM substrates and microbial communities and its influence on the microbial produced DOM remain poorly understood. In this study, we investigated how the addition of substrates with different complexity (glucose, laminarin, and Synechococcus ‐derived DOM) impacted the production of new DOM by an individual opportunist Alteromonas strain and a coastal natural microbial community. Our experiments reveal that the microbial complexity was more important than the substrate in shaping the molecular composition of DOM. Furthermore, our results showed that the DOM composition played a larger role in shaping the microbial compositions than the concentrations of DOM and inorganic nutrients. Combined our network analysis revealed that complex substrates resulted in higher molecular diversity of the microbial produced DOM, sustained a higher microbial diversity, and maintained a more diverse association between microbial communities and DOM molecules. Overall, the present study indicate that the microbial diversity contributes to DOM molecular diversity and vice versa, highlighting the importance of microbial–substrate interactions in shaping the marine DOM pool.