Acclimation History of Elevated Temperature Reduces the Tolerance of Coralline Algae to Additional Acute Thermal Stress

Increasing atmospheric CO 2 is driving major environmental changes in the ocean, such as an increase average ocean temperature, a decrease pH (ocean acidification or OA), and number severity of extreme climatic events (e.g., anomalous temperature heatwaves). Uncertainty exists capacity for species to withstand these stressors occurring concomitantly. Here, we tested whether acclimation history warming (OW) OA affects physiological responses abundant, reef-building crustose coralline algae (CCA), Porolithon cf. onkodes , chronic acute thermal stress. To address this, exposed varying levels 6 weeks this treatment experiment was followed by exposure event (+4–6°C from temperature). Net photosynthetic rate negatively affected across all treatments increasing during event, however, acclimated control were able maintain rates +4°C above their whereas elevated not. Average relative change O produced resulted 100–175% decrease, with largest found combined reduced pH. We conclude that global (i.e., OW OA) will reduce tolerance P. increases may have implications reef building processes.