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Marine Ecology Progress Series

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MEPS 672:15-32 (2021)  -  DOI: https://doi.org/10.3354/meps13782

Role of mixotrophic nanoflagellates in the Eastern Mediterranean microbial food web

Eleni Livanou1,2,*, Andreas Oikonomou2, Stella Psarra2, Konstadia Lika1,3

1Department of Biology, University of Crete, Voutes University Campus, 70013 Heraklion, Greece
2Institute of Oceanography, Hellenic Centre for Marine Research, 71003 Heraklion, Greece
3Institute of Computational Mathematics, Foundation of Research and Technology Hellas, 70013 Heraklion, Greece
*Corresponding author:

ABSTRACT: 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.


KEY WORDS: Eastern Mediterranean Sea · Mixotrophic nanoflagellates · Phosphorus limitation · Microbial food web model · DEB theory


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Cite this article as: Livanou E, Oikonomou A, Psarra S, Lika K (2021) Role of mixotrophic nanoflagellates in the Eastern Mediterranean microbial food web. Mar Ecol Prog Ser 672:15-32. https://doi.org/10.3354/meps13782

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