Impacts of Seawater Ph Buffering on the Larval Microbiome and Carry-Over Effects on Later-Life Disease Susceptibility in Pacific Oysters.

Ocean acidification upwelling events and the resulting lowered aragonite saturation state of seawater have been linked to high mortality of marine bivalve larvae in hatcheries. Major oyster seed producers along North America's west coast have mitigated impacts via seawater pH buffering (e.g., addition of soda ash). However, little consideration has been given to whether such practice may impact the larval microbiome, with potential carry-over effects on immune competency and disease susceptibility in later-life stages. To investigate possible impacts, Pacific oysters (Crassostrea gigas) were reared under soda ash pH buffered or ambient pH seawater conditions for the first 24 h of development. Both treatment groups were then reared under ambient pH conditions for the remainder of the developmental period. Larval microbiome, immune status (via gene expression), growth, and survival were assessed throughout the developmental period. Juveniles and adults arising from the larval run were then subjected to laboratory-based disease challenges to investigate carry-over effects. Larvae reared under buffered conditions showed an altered microbiome, which was still evident in juvenile animals. Moreover, reduced survival was observed in both juveniles and adults of the buffered group under a simulated marine heatwave and Vibrio exposure compared with those reared under ambient conditions. Results suggest that soda ash pH buffering during early development may compromise later-life stages under stressor conditions, and illustrate the importance of a long-view approach with regard to hatchery husbandry practices and climate change mitigation. IMPORTANCE Shellfish industries are threatened worldwide by recurrent summer mortality events. Such incidences are often associated with Vibrio disease outbreaks, and thus, it is critical that animals are able to mount sufficient immune responses. The oyster immune system is linked to the microbiome which is laid down during early developmental stages. Consequently, shellfish hatcheries play a key role with regard to shaping the immune competency of later-life stages. This study represents the first in-depth examination of whether the adoption of seawater pH buffering practice by hatcheries for mitigation of ocean acidification may alter the larval microbiome, and thus, have repercussions for adult susceptibility to summer mortality events. Findings demonstrate that even minimal buffering results in a changed microbiome which is paralleled by increased mortality of later-life stages under Vibrio and temperature stressors, highlighting the importance of the hatchery environment with regard to shaping resilience to summer mortality events.