Acidification-mediated bacterial infection determine mortality and growth patterns of early Ostrea edulis life stages

Chair: Sue-Ann Watson

Nuno Caiola (1)*, Patricia Prado (1), Ana Roque (2), Carles Alcaraz (1), Frederic Casals (3), Josu Pérez (2), Carles Ibáñez (1)

1
IRTA Aquatic Ecosystems, Sant Carles de la Ràpita, 43540, Spain
2 IRTA Aquaculture, Sant Carles de la Ràpita, 43540, Spain
3 University of Lleida, Lleida, 25003, Spain

Background
One of the most important shellfish aquaculture in the Mediterranean (including our study site in the Ebro Delta, NE Spain) is the Pacific oyster (Crassostrea gigas) farming. This species is vulnerable to ocean acidification. The once important production of the native European flat oyster (Ostrea edulis) was drastically reduced during the 1970s/1980s due to protozoan disease, but little is known on its resilience capacity to future scenarios of ocean acidification. In addition, studies aimed at investigating how seawater acidification interacts with the bulk bacterial community are also necessary to understand mortality patterns and to secure the continuity of commercial activities and the recruitment of populations.

Methods
The effects of seawater acidification were investigated across larval stages (veliger, umbonate, and pediveliger) and spat of O. edulis in laboratory conditions. Simultaneous experiments were also conducted to investigate the potential effects of reduced pH on bacterial growth that could provide a more comprehensive understanding on the effects of seawater acidification in larval mortality. Larvae and bacteria were exposed to four pH treatments: ambient (7.9) and 3 manipulated conditions (7.6, 7.4, and 7.2). The pH range agrees with the latest global IPCC scenarios (RCPs 2.6 to 8.5). The pH conditions were achieved manipulating the carbonate chemistry of seawater by diffusing pure CO2 in the experimental tanks.

Findings
The survival of larvae increased with pH reductions by ca. 26%. No shell malformations were observed and growth patterns tended to mirror those of survival. This coincided with lower bacterial growth, particularly for Vibrio spp., in the two lowest pH treatments, suggesting that seawater acidification may help to prevent pathogenicity in O. edulis larvae.

Conclusions
Compared to available information on the vulnerability of other commercial bivalves to ocean acidification, our results suggest that O. edulis could be a more resilient species.