A DEB-based approach of modeling dissolved organic matter release by phytoplankton

Abstract Extracellular release of dissolved organic matter (DOM) by phytoplankton is a significant process that drives the microbial loop, providing energy and nutrients to bacteria. A model based on Dynamic Energy Budget (DEB) theory is proposed for describing DOM release by phytoplankton under nitrogen-replete and nitrogen-limiting conditions. The model allows for variable biomass stoichiometry by partitioning biomass into structural mass and a number of reserves masses (C-reserves, N-reserves and generalized reserves), with each pool having a constant chemical composition. In the context of DEB theory, two alternative pathways of DOM release emerge from the theory, without additional assumptions; one relates to growth and lysis of the cells and one to rejection of unprocessed substrates due to stoichiometric constraints. We assume that these pathways correspond to the two conceptual mechanisms of DOM release, i.e., passive diffusion and active exudation respectively. Model parameters were estimated by fitting the model to literature data for the diatom Thalassiosira pseudonana . Model results suggest higher mass-specific growth, dissolved organic carbon (DOC) and nitrogen (DON) release rates during the nitrogen-replete phase of growth, while the percentage extracellular release (DOC release rate over primary production rate) is higher during the nitrogen-limited phase of growth. Moreover, nutrient limitation induces an increase in biomass C:N ratio which is followed by an increase in C:N ratio of the produced DOM. These results are in accordance with a wide range of experimental evidence. The relative contribution of the two release mechanisms affects the quality of DOM produced in terms of elemental and molecular composition and size fractionation, which, in turn, may have implications for the bioavailability of the produced DOM to bacteria. Moreover, the model predicts a close to linear relationship between (log-transformed) primary production and DOC release under nitrogen-replete conditions. Deviations from linearity are related to the prevalence of the active exudation mechanism and the reduction of the primary production rate under nitrogen-limiting conditions.