Dissolved organic matter release by phytoplankton in the context of the Dynamic Energy Budget theory

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. In this paper, a dynamic energy budget model is proposed for describing DOM release by phytoplankton under nitrogen and phosphorus limiting conditions. The model allows for the distinction of the two major mechanisms of DOM release; passive diffusion related to growth and lysis of the cells and active exudation related to rejection of unprocessed substrates due to stoichiometric constraints. Model results suggest that phosphorus deficiency has less severe effect on phytoplankton growth and primary production (PP) rate than nitrogen deficiency, while co-limitation by both nutrients has the most severe effect. The dependence of dissolved organic carbon (DOC) release rate on the cellular carbohydrates concentration is also highlighted by the model. Furthermore, model predictions resolve the relationship between PP and DOC release under different nutrient availability scenarios, providing a possible explanation for the deviations from 1 : 1 linear relationship between PP and DOC release, often observed in oligotrophic systems. This deviation is a result of the prevalence of the active exudation mechanism and the reduction of the PP rate due to nutrient limitation. Conversely, passive diffusion is more important under nutrient-replete conditions. The different relative contributions of the two mechanisms result in different qualities of DOM produced by phytoplankton in terms of elemental and molecular composition and size fractions, with potential implications for the bioavailability of the produced DOM for bacteria and the coupling of phytoplankton-bacteria dynamics.