Modelling the nutritional strategies in mixotrophic nanoflagellates

Mixotrophic nanoflagellates (MNF) play a key role in the carbon flux within aquatic ecosystems. They contribute significantly to primary production while, at the same time, they are important consumers of bacteria. MNF comprise a diverse assemblage with a whole range of photosynthetic and feeding potential and with various metabolic responses to environmental drivers, such as light, nutrients and prey availability. However, in most cases MNF are modelled as a single functional group with no reference to the distinct nutritional strategies and constraints of the different types of MNF. Here we develop a mathematical framework for the distinct functional responses of four types of MNF. Our approach is based on the Dynamic Energy Budget (DEB) theory and the concept of Synthesing Unit (SU), which is used to merge phototrophic and phagotrophic nutrition into mixotrophic growth. The resulting models describe explicitly the functional diversity of the MNF group taking into account the dynamic interaction of phototrophy and phagotrophy within the four types of MNF. Our results are in agreement with theoretical expectations and a wide range of experimental observations for the different types of MNF. Our simulations suggest that the growth dynamics of the four types of MNF are affected differently by the availability of resources. Moreover, using our models to compare the rates of organic carbon production and prey consumption by the various MNF types, we show that their net ecosystem role, as producers or consumers, depends on the mixotrophic strategy and it can vary as a function of the prevailing environmental conditions. Overall, our study highlights the importance of mechanistically modelling the distinct metabolic responses of the different types of MNF to environmental drivers in order to better understand their role in the carbon cycle in anticipation of global environmental changes.