A common garden comparison of the microbiome and gene expression of intertidally- and subtidally-cultured Pacific oysters (Crassostrea gigas) in relation to extreme warming events

In order to investigate how shellfish culture in the intertidal zone may serve as a means of conferring resilience prior to subtidal deployment for commercial grow-out, a common garden experiment was carried out with juvenile Pacific oysters. Individuals were cultured under intertidal or subtidal conditions for one year, following which intertidal animals were transplanted to the subtidal site and both treatment groups were cultured together under subtidal conditions for a period of five months. During that time, microbiome and gene expression were tracked in relation to a marine heatwave event. Multivariate analyses of samples indicated a significant effect of time on gene expression, but no significant effect of culture treatment prior to and during the heatwave. Significant differences in the expression of oxidative stress and immune response genes in post-heatwave samples in comparison to pre-heatwave and heatwave samples, and between culture treatment groups post-heatwave, were detected. Results also indicated significant differences in microbiome according to culture treatment and timepoint, with increased richness observed in intertidally-cultured oysters and over time. Similarly, while the overall composition of the oyster microbiome did not vary between culture treatments, changes were observed over time. At the end of the field trial, a laboratory-based temperature and Vibrio challenge experiment was conducted to compare genomic responses of both culture treatment groups following 24-h exposure to coinciding acute warming and pathogen stressor conditions. A significant interactive effect of temperature and culture treatment on gene expression was observed, further suggesting that stress response may be influenced by prior culture treatment.