Spatial and temporal consistency in green algae-induced oyster microbiota dysbiosis and associated increased disease...

Growing interest in microorganisms has extended to the microbiota of hosts and how they interact, as many diseases result from a microbiota imbalance. Recent studies on Pacific oysters Magallana gigas have shown that individuals exposed to particular communities, like green algae holobionts from the genus Ulva, were more susceptible to the infection by the Ostreid herpesvirus type 1 (OsHV-1), coinciding with dysbiosis of the bacterial microbiota and modification of host immune response. Considering that the microbiota of macroalgae and oysters vary with time and geographical location, dysbiosis induced by green algae on oysters may not be systematic and could depend on the origin of the green macroalgal assemblage. We therefore conducted an experiment with macroalgae and oysters from several origins to examine whether the biotic interactions previously observed can be extended more generally and attest to a negative effect of all green algae on several oyster families. Two multiparental lines of oysters with different disease resistance were acclimated with four sources of green algae from different sites in Brittany (France). Then, oysters were challenged with OsHV-1 and we followed host mortality and virus replication dynamics in seawater, while investigating the response of the oyster microbial community. Overall, the results confirm that green algae increase the risk of mortality, irrespective of their geographical origins and oyster line. This coincided with early dysbiosis of the oyster bacterial microbiota. The only exception concerned oysters exposed to green algae from the English Channel which presented mortalities similar to the control, although with a change in microbiota composition and structure, but less pronounced than in other exposures with stronger dysbiosis and levels of mortality.