Role of mixotrophic nanoflagellates in the Eastern Mediterranean microbial food web

In the oligotrophic, phosphorus (P)-limited Eastern Mediterranean Sea (EMS), grazing on heterotrophic bacteria (HB) by pigmented nanoflagellates (PNF) through mixotrophy is a significant source of bacterial mortality and is P-dependent. Heterotrophic nanoflagellates (HNF) are also important consumers of HB. However, there is still no conceptual framework describing the flows of carbon (C) and P through the EMS microbial food web that takes into account the mixotrophic behavior of PNF. In the present modelling study, we explore qualitatively the pathways of C- and P-flow through the HB-mixotrophic PNF-HNF link under varying availability of inorganic P. At low P availability, given the assumption that PNF can fulfill most of their P requirements through HB consumption when P-limited, model simulations show that PNF support bacterial production and P uptake by supplying HB with dissolved organic carbon (DOC), leading to a win-win stable scenario, with PNF holding a key role in P-transfer through the microbial food web. Conversely, increased P availability results in lower PNF grazing upon HB, amplifying the relative contribution of HNF to bacterial grazing. Based on our modelling study we propose a novel conceptual framework for bacteria-nanoflagellate interactions in the EMS, where PNF have the functional role of DOC primary producers and also share with HNF the functional role of bacterial consumers via mixotrophy. This alternative microbial loop operates in a Pdependent way. Our modelling study pinpoints the significance of the adaptive physiology of PNF for the functioning of the oligotrophic ecosystem of the EMS.