Résumé
Viruses are the most abundant biological entities in aquatic ecosystems and heterotrophic bacteria form the major host reservoir for these viruses. Losses of bacteria, which constitute a key process in pelagic ecosystems, are mainly due to viral lysis and protistan grazing, both of which being comparable at times. Also, both viruses and protozoan are known as driving forces shaping bacterial diversity. A significant correlation between viruses and protistan grazers has been reported in different ecosystems. Some studies have also shown that protists can directly feed on viruses although the transfer of carbon to the higher trophic levels via this activity seems to be of minor importance. There are thus many routes of interaction between grazers, bacteria and viruses in pelagic systems but there are only few studies that have been made to characterise these interactions. Very interestingly, experimental studies have recently shown the existence of a direct or indirect relationship between viral lysis and protozoan predation on bacteria. In most studies, viral-induced bacterial mortality indeed has been shown to be positively influenced by the presence of small predators. This talk will try to highlight 1) this synergy between viruses, bacteria and their predators, and 2) the role of viruses and predators in nutrient regeneration through experiments conducted in some lakes and marine ecosystems of various trophic status.