Synergistic pathogenicity between Herpesvirus and Vibrio aestuarianus in killing juvenile oysters during spring in natural conditions

Young oysters of Pacific oyster Magallana (Crassostrea) gigas are regularly impacted by summer mortalities since 2008, caused by the infection by the Ostreid herpesvirus 1 (OsHV-1) when seawater temperature increases in summer. Adult oysters rather suffer from a vibriosis caused by the bacterial primary pathogen Vibrio aestuarianus francensis. Although these diseases are now well described, little is known about their contribution and possible synergy in natural mortality events of juvenile oysters. Using an in-situ approach conducted in the Thau lagoon, we investigated the potential implication of Vibrio aestuarianus in the juvenile oyster's mortality, as well as the virus OsHV-1, throughout a large sampling period from winter to late summer, after the mortality event. We further determined their respective effects on the microbial abundance and diversity on alive and moribund oysters. We demonstrated that microbiomes are dynamic throughout seasons and tend to become more similar in spring with a dominance of Mycoplasmataceae in alive oysters. Surprisingly, our data highlighted the prevalence and high loads of Vibrio aestuarianus in all moribund oysters. In contrast with the literature, we found that the microbial communities of moribund juvenile oysters were significantly influenced by both Vibrio aestuarianus and the virus OsHV-1. The moribund bacterial indicators were detected in abundance during mortalities in the surrounding environment of oyster farms and further (700 m far). These results highlight the connectivity between the different compartments of the lagoon and raise questions about the potential spread of the primary pathogen Vibrio aestuarianus from adult to juvenile oysters in farming areas located in lagoons.