Impact of seawater temperature on the Pacific oyster (Crassostrea gigas) microbiome and susceptibility to disease associated with Ostreid herpesvirus-1 (OsHV-1)

Context: Intertidal estuarine environments expose oysters to wide temperature variations. This can shift their microbiome composition towards pathogen-dominated communities. Understanding the impact of temperature on the microbiome will facilitate oyster health management. Aims: The present study was conducted to (1) assess the Pacific oyster microbiome at different temperatures (21°C, 22°C, 26°C and diurnal fluctuation between 22°C and 26°C) and (2) investigate microbiome changes in response to exposure to the pathogen Ostreid herpesvirus-1 (OsHV-1) at different temperatures. Methods: Pacific oysters (15 months of age; n = 480) were acclimated to different temperatures in laboratory aquaria. Samples were obtained before and after acclimation and after OsHV-1 exposure for quantification of OsHV-1, total bacteria and Vibrio, by quantitative PCR. Bacterial 16S rRNA gene (V1–V3) sequences were used to characterise the gill bacterial community. Key results: The alpha diversity (number of observed amplicon sequence variants) and total number of bacteria associated with the gills of oysters did not change with acclimation to different water temperature profiles, but there was variation in beta diversity. The highest mortality after OsHV-1 exposure occurred at 26°C and these, together with oysters at 22/26°C, had a higher concentration of OsHV-1 DNA compared with to the ones at the lower constant temperatures (P < 0.05). The total bacterial quantity increased after the OsHV-1 challenge in oysters at 22/26°C. The alpha diversity of microbiota increased after the OsHV-1 challenge in oysters at 21°C and 22/26°C. The beta diversity changed both after acclimation and OsHV-1 challenge. The highest abundance of Vibrio and higher OsHV-1 loads were seen in OsHV-1-challenged oysters at 26°C (P < 0.05). Conclusions: The gill microbiome altered with seawater temperature and OsHV-1 challenge. Higher mortality following OsHV-1 exposure was associated with a higher water temperature and greater abundance of Vibrio spp. arising from the microbiome. Implications: Higher seawater temperature can be considered a key risk factor influencing oyster health by altering the microbiome, increasing susceptibility to OsHV-1 and increasing the Vibrio fraction in the oyster microbiome