Viral density and virus-to-bacterium ratio in deep-sea sediments of the Eastern Mediterranean

Viruses are now recognized as a key component in pelagic systems, but their role in marine sediment has yet to be assessed. In this study bacterial an d viral densities were determined at nine deep-sea stations selected from t hree main sites (i.e., the Sporades Basin, the Cretan Sea, and the Ierapetr a Trench at depths of 1,232, 1,840, and 4,235 m, respectively) of the Easte rn Mediterranean. The three areas were characterized by different phytopigm ent and biopolymeric carbon concentrations and by changes in the protein an d carbohydrate pools. A gradient of increasing trophic conditions was obser ved from the Sporades Basin (North Aegean) to the Ierapetra Trench (South A egean). Viral densities (ranging from 1 x 10(9) to 2 x 10(9) viruses mi of sediment(-1)) were significantly correlated to bacterial densities (n = 9, r(2) = 0.647) and reached values up to 3 orders of magnitude higher than th ose generally reported for the water column. However, the virus-to-bacteriu m density ratio in deep-sea sediments was about 1 order of magnitude lower (range of 2 to 5, with a modal value of 2.6) than in pelagic environments. Virus density decreased vertically with depth in sediment cores at all stat ions and was below detection limits at the 10-cm depth of the abyssal sedim ents of the Ierapetra Trench. Virus density in the sediment apparently refl ected a gradient of particle fluxes and trophic conditions, displaying the highest values in the Sporades Basin. The low virus-to-bacterium ratios and their inverse relationship with station depth suggest that the role played by viruses in controlling deep-sea benthic bacterial assemblages and bioge ochemical cycles is less relevant than in pelagic systems.