Influence of microbial and molecular diversity on the storage of marine dissolved organic matter: Insights from a modelling perspective

Heterotrophic microorganisms play a central role in the global carbon cycle by oxidizing dissolved organic carbon (DOC) to CO₂. Marine DOC is one of the largest active organic carbon pools on Earth, thus, small changes in its size can affect the global carbon cycle and the climate system. A major fraction of DOC in the ocean is stable for time scales around 4000-6000 years, but the reasons for this long-term stability are still uncertain. A better knowledge about main factors determining the dynamic of this large carbon reservoir is vital for understanding biological feedbacks in the ocean and climate system. Here we present a novel model framework addressing the hypothesis of molecular diversity as a reason for its recalcitrance. In a box model approach, we show that the formation of a recalcitrant DOC pool is not necessarily a function of its structural lability, but rather a function of the diversity of the network between molecules and microbes. We scale this simple network-model up to the global ocean using an Earth System Model of Intermediate complexity and discuss emerging spatial patterns in relation to present-day observations. Our results highlight the importance of accounting for the ecology of heterotrophic picoplankton to understand the dynamics of this major carbon reservoir.