title: Nutrient Limitation in Surface Waters of the Oligotrophic Eastern Mediterranean Sea: an Enrichment Microcosm Experiment.
abstract: The growth rates of planktonic microbes in the pelagic zone of the Eastern Mediterranean Sea are nutrient limited, but the type of limitation is still uncertain. During this study, we investigated the occurrence of N and P limitation among different groups of the prokaryotic and eukaryotic (pico-, nano-, and micro-) plankton using a microcosm experiment during stratified water column conditions in the Cretan Sea (Eastern Mediterranean). Microcosms were enriched with N and P (either solely or simultaneously), and the PO4 turnover time, prokaryotic heterotrophic activity, primary production, and the abundance of the different microbial components were measured. Flow cytometric and molecular fingerprint analyses showed that different heterotrophic prokaryotic groups were limited by different nutrients; total heterotrophic prokaryotic growth was limited by P, but only when both N and P were added, changes in community structure and cell size were detected. Phytoplankton were N and P co-limited, with autotrophic pico-eukaryotes being the exception as they increased even when only P was added after a 2-day time lag. The populations of Synechococcus and Prochlorococcus were highly competitive with each other; Prochlorococcus abundance increased during the first 2 days of P addition but kept increasing only when both N and P were added, whereas Synechococcus exhibited higher pigment content and increased in abundance 3 days after simultaneous N and P additions. Dinoflagellates also showed opportunistic behavior at simultaneous N and P additions, in contrast to diatoms and coccolithophores, which diminished in all incubations. High DNA content viruses, selective grazing, and the exhaustion of N sources probably controlled the populations of diatoms and coccolithophores.
authors.affiliation: Institute of Oceanography, Hellenic Centre for Marine Research (HCMR), Ex American Base Gournes, P.O. Box 2214, 71003, Heraklion, Crete, Greece. atsiola@hcmr.gr.|Biology Department, Marine Ecology Laboratory, University of Crete, Heraklion, Crete, Greece. atsiola@hcmr.gr.
authors.forename: A
authors.initials: A
authors.lastname: Tsiola
authors.affiliation: Institute of Oceanography, Hellenic Centre for Marine Research (HCMR), Ex American Base Gournes, P.O. Box 2214, 71003, Heraklion, Crete, Greece.
authors.forename: P
authors.initials: P
authors.lastname: Pitta
authors.affiliation: Biology Department, Marine Ecology Laboratory, University of Crete, Heraklion, Crete, Greece.|King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia.
authors.forename: S
authors.initials: S
authors.lastname: Fodelianakis
authors.affiliation: Laboratoire d'Ecologie des Systèmes Marins Côtiers (ECOSYM), CNRS-Université Montpellier 2 & 1-Ifremer-IRD, Montpellier, France.
authors.forename: R
authors.initials: R
authors.lastname: Pete
authors.affiliation: Institute of Oceanography, Hellenic Centre for Marine Research (HCMR), Ex American Base Gournes, P.O. Box 2214, 71003, Heraklion, Crete, Greece.|Biology Department, Marine Ecology Laboratory, University of Crete, Heraklion, Crete, Greece.
authors.forename: I
authors.initials: I
authors.lastname: Magiopoulos
authors.affiliation: Chemistry Department, Environmental Chemical Processes Laboratory, University of Crete, Heraklion, Crete, Greece.
authors.forename: P
authors.initials: P
authors.lastname: Mara
authors.affiliation: Institute of Oceanography, Hellenic Centre for Marine Research (HCMR), Ex American Base Gournes, P.O. Box 2214, 71003, Heraklion, Crete, Greece.
authors.forename: S
authors.initials: S
authors.lastname: Psarra
authors.affiliation: INSU-CNRS, Laboratoire d'Océanographie de Villefranche, Villefranche sur Mer cedex, France.|Université Pierre et Marie Curie-Paris 6, Observatoire Océanologie de Villefranche, Villefranche sur Mer cedex, France.
authors.forename: T
authors.initials: T
authors.lastname: Tanaka
authors.affiliation: Laboratoire d'Ecologie des Systèmes Marins Côtiers (ECOSYM), CNRS-Université Montpellier 2 & 1-Ifremer-IRD, Montpellier, France.|Centre d'Ecologie Marine Expérimentale MEDIMEER, Mediterranean Center for Marine Ecosystem Experimental Research, CNRS-Université Montpellier 2, Montpellier, Sète, France.|Marine Biodiversity, Exploitation and Conservation (MARBEC), UMR 9190, CNRS-Université de Montpellier-IFREMER-IRD, Montpellier, France.
authors.forename: B
authors.initials: B
authors.lastname: Mostajir
mesh_terms: D052818:Autotrophic Processes; D001419:Bacteria / Q000145:classification / Q000302:isolation & purification / Q000378:metabolism*; D056890:Eukaryota / Q000145:classification / Q000302:isolation & purification / Q000378:metabolism*; D052836:Heterotrophic Processes; D008522:Mediterranean Sea; D012623:Seawater / Q000737:chemistry / Q000382:microbiology*; D014780:Viruses / Q000145:classification / Q000302:isolation & purification / Q000378:metabolism*
keywords: Eukaryotic phytoplankton; Heterotrophic prokaryotes; Prochlorococcus; Synechococcus; Viruses