Systematic variation in marine dissolved organic matter stoichiometry and remineralization ratios as a function of lability

Remineralization of organic matter by heterotrophic organisms regulates the biological sequestration of carbon, thereby mediating atmospheric CO₂. While surface nutrient supply impacts the elemental ratios of primary production, stoichiometric control by remineralization remains unclear. Here we develop a mechanistic description of remineralization and its stoichiometry in a marine microbial ecosystem model. The model simulates the observed elemental plasticity of phytoplankton and the relatively constant, lower C:N of heterotrophic biomass. In addition, the model captures the observed decreases in DOC:DON and the C:N remineralization ratio with depth for more labile substrates, which are driven by a switch in the dominant source of labile DOM from phytoplankton to heterotrophic biomass. Only a model version with targeted remineralization of N-rich components when DOM concentrations are high is able to simulate the observed profiles of preferential remineralization of DON relative to DOC and the elevated C:N of bulk DOM. Thus the model suggests that consumption of more labile substrates by C-limited heterotrophs does not result in preferential remineralization. Rather, preferential remineralization of DON is associated with growth that is limited by the processing rate of DOM. Critically, our results imply that the specific form of organic carbon produced or excreted by phytoplankton determines the extent to which a proportional increase in carbon production resulting from changes in phytoplankton stoichiometry will increase the efficiency of the biological pump. This emphasizes the importance of understanding the physiology of both phytoplankton and heterotrophs for anticipating changes in biologically driven ocean carbon storage.