Higher abundance of bacteria than viruses in deep Mediterranean sediments

In this study we examined interactions between virus abundance and bacterial density, biomass and production, in relation to the different trophic conditions observed along a longitudinal transect of nine deep-sea stations crossing the entire Mediterranean basin. Viral numbers were very low, displayed a decreasing gradient from western to eastern Basin (on average 8.19 and 5.84 x 107 viruses g-1, in the western and eastern basin, respectively) and were not correlated with benthic bacterial densities. The virus-to-bacterium ratio ranged from 0.24 in the western to 0.13 in the eastern stations, indicating that the deep-sea sediments of the Mediterranean Sea are the first example of marine ecosystem not numerically dominated by viruses. Virus abundance displayed highest values in the western basin, where highest concentrations of labile organic compounds were found. Lowest virus numbers were observed in eastern Mediterranean sediments, where the lowest bacterial metabolism, turnover rates and the largest cell size were observed. This fact allows hypothesizing that benthic bacterial doubling time might play an important role on virus development, possibly influencing benthic virus life- strategies.