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Marine Ecology Progress Series

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MEPS 418:73-85 (2010)  -  DOI:

Disentangling the effect of viruses and nanoflagellates on prokaryotes in bathypelagic waters of the Mediterranean Sea

Serena Fonda Umani1,*, Elisa Malisana1, Francesca Focaracci1, Mirko Magagnini2, Cinzia Corinaldesi2, Roberto Danovaro2

1Department of Life Sciences, University of Trieste, v. Valerio 28/1, 34127 Trieste, Italy
2Department of Marine Science, Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy

ABSTRACT: Bathypelagic ecosystems depend on prokaryotic heterotrophic biomass fuelled by vertical particulate organic matter (POM) fluxes, but very little information is available on the interactions among viruses, prokaryotes and nanoflagellates in deep waters. We simultaneously investigated the relative importance of the viral and heterotrophic nanoflagellate (HNF) grazing-induced prokaryotic mortality in bathypelagic waters by means of dilution experiments performed on samples collected at 1500 m depth from the Atlantic Ocean to the Eastern Mediterranean Sea. Prokaryotic abundance (range: 1.4 to 8.9 × 104 cells ml–1), although different from one station to another, was on average not significantly different among biogeographic regions. The potential predators of prokaryotes (viruses, HNF and microzooplankton) followed a similar spatial pattern. Viruses were responsible for an important fraction of prokaryotic mortality (on average 13.4% d–1). Dilution experiments carried out to estimate the potential predation of HNF suggested a high effect on prokaryotic abundance. However, since the latter experiments also include the effect of viruses on prokaryotes, when this factor was disentangled from the overall mortality, the potential rates of HNF predation on prokaryotes (on average 49.5%) were ca. 4 times higher than the effect due to viral infections. Conversely to patterns of distribution, the relative importance of virus-mediated mortality vs. HNF predation changed significantly among different regions. Results should be treated with caution due to the intrinsic difficulty in reproducing experimentally natural deep-sea conditions, but they permit disentangling of the relative effect of viruses and HNF on prokaryotes and compare the potential predatory control in different biogeographic regions.

KEY WORDS: Virioplankton · Prokaryotes · Heterotrophic · Nanoplankton · Microzooplankton · Bathypelagic waters · Mediterranean Sea

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Cite this article as: Fonda Umani S, Malisana E, Focaracci F, Magagnini M, Corinaldesi C, Danovaro R (2010) Disentangling the effect of viruses and nanoflagellates on prokaryotes in bathypelagic waters of the Mediterranean Sea. Mar Ecol Prog Ser 418:73-85.

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