The combined effects of temperature and CO2 lead to altered gene expression in Acropora aspera

This study explored the interactive effects of near-term CO₂ increases (40–90 ppm above current ambient) during a simulated bleaching event (34 °C for 5 d) of Acropora aspera by linking physiology to expression patterns of genes involved in carbon metabolism. Symbiodinium photosynthetic efficiency (F _v /F _m ) was significantly depressed by the bleaching event, while elevated pressure of CO₂ (pCO₂) slightly mitigated the effects of increased temperature on F _v /F _m during the final 4 d of the recovery period, however, did not affect the loss of symbionts. Elevated pCO₂ alone had no effect on F _v /F _m or symbiont density. Expression of targeted Symbiodinium genes involved in carbon metabolism and heat stress response was not significantly altered by either increased temperature and/or CO₂. Of the selected host genes, two carbonic anhydrase isoforms (coCA2 and coCA3) exhibited the largest changes, most notably in crossed bleaching and elevated pCO₂ treatments. CA2 was significantly down-regulated on day 14 in all treatments, with the greatest decrease in the crossed treatment (relative expression compared to control = 0.16; p < 0.05); CA3 showed a similar trend, with expression levels 0.20-fold of controls on day 14 (p < 0.05) in the elevated temperature/pCO₂ treatment. The synergistic effects of ocean acidification and bleaching were evident during this study and demonstrate that increased pCO₂ in surface waters will impact corals much sooner than many studies utilising end-of-century pCO₂ concentrations would indicate.