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 A degrees 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 (pCO(2)) 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 pCO(2) 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 pCO(2) 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/pCO(2) treatment. The synergistic effects of ocean acidification and bleaching were evident during this study and demonstrate that increased pCO(2) in surface waters will impact corals much sooner than many studies utilising end-of-century pCO(2) concentrations would indicate.