Les virus de l'estuaire de la Charente el du bassin de Marennes-Oleron. Dynamiques spatio-temporelles et interactions avec les communautés bactériennes

This study is divided in two parts; one aim is to evaluate the in situ spalio-temporal dynamics of viral communities in the Charante estuary in order to define the appropriate seasons to conduct in vitro viral enrichment experiments. These experiments, that constitute the second part of this study, were performed to assess the impact of freshwater viruses carried by the Charante River on the dynamics and composition of coastal bacterial communities. With abundance ranging from 1.4 x 107 to 208 x I07 VLPml-1. viruses are the most abundant component among plankton microorganisms in surface waters of the Charente estuary. Thre temporal dynamics of this component reflects the indirect influence of temperature through the control of bacterial abundance, activity and diversity. The strong similarities found between freshwater and coastal viral communities suggest a seaward freshwater virus transfer. Like autochthonous viruses, freshwater viruses were able to infect, to lyse (71 % of bacterial production within the first six hours) and to modify the composition of coastal bacterial communities. The fact that a significant subset (50%) of dominant phylotypes were found both in the river and in the basin explain the capacity of freshwater viruses to infect allochthonous bacterial population In these experiments, viruses were also able to maintain the species richness of bacterial communities that is in agreement with the "kill the winner" theory. Although many criteria of the viral infection are still unclear (resistance mechanisms, role of lysis products, importance of lysogeny...), our results suggested that Charente vital input might have a non negligible impact on bacterial productivity in the Marennes Oléron Basin.