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.
