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MEPS prepress abstract   -  DOI:

Role of mixotrophic nanoflagellates in the Eastern Mediterranean microbial food web

Eleni Livanou*, Andreas Oikonomou, Stella Psarra, Konstadia Lika

*Corresponding author:

ABSTRACT: In the oligotrophic, phosphorus (P) limited Eastern Mediterranean Sea (EMS), pigmented nanoflagellates (PNF) grazing on heterotrophic bacteria (HB) through mixotrophy is a significant source of bacterial mortality and it is P-dependent. Heterotrophic nanoflagellates (HNF) are also important consumers of HB. However, a conceptual framework describing the flows of carbon and phosphorus through the EMS microbial food web, taking into account the mixotrophic behavior of PNF, is still missing. In the present modelling study, we explore qualitatively the pathways of carbon and phosphorus flow through the heterotrophic bacteria-mixotrophic pigmented nanoflagellates-heterotrophic nanoflagellates link under varying availability of inorganic phosphorus. 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 the bacteria-nanoflagellates interactions in the EMS, where PNF have the functional role of DOC primary producers, while they share with HNF the functional role of bacterial consumers via mixotrophy. This alternative microbial loop operates in a phosphorus-dependent way. Our modelling study pinpoints the significance of the adaptive physiology of PNF for the functioning of the oligotrophic ecosystem of the EMS.