Diel <i>p</i> CO <sub>2</sub> fluctuations alter the molecular response of coral reef fishes to ocean acidification conditions
Celia Schunter, Michael D. Jarrold, Philip L. Munday, Timothy Ravasi
Abstract
Abstract Environmental partial pressure of CO 2 ( p CO 2 ) variation can modify the responses of marine organisms to ocean acidification, yet the underlying mechanisms for this effect remain unclear. On coral reefs, environmental p CO 2 fluctuates on a regular day–night cycle. Effects of future ocean acidification on coral reef fishes might therefore depend on their response to this diel cycle of p CO 2 . To evaluate the effects on the brain molecular response, we exposed two common reef fishes ( Acanthochromis polyacanthus and Amphiprion percula ) to two projected future p CO 2 levels (750 and 1,000 µatm) under both stable and diel fluctuating conditions. We found a common signature to stable elevated p CO 2 for both species, which included the downregulation of immediate early genes, indicating lower brain activity. The transcriptional programme was more strongly affected by higher average p CO 2 in a stable treatment than for fluctuating treatments, but the largest difference in molecular response was between stable and fluctuating p CO 2 treatments. This indicates that a response to a change in environmental p CO 2 conditions is different for organisms living in a fluctuating than in stable environments. This differential regulation was related to steroid hormones and circadian rhythm (CR). Both species exhibited a marked difference in the expression of CR genes among p CO 2 treatments, possibly accommodating a more flexible adaptive approach in the response to environmental changes. Our results suggest that environmental p CO 2 fluctuations might enable reef fishes to phase‐shift their clocks and anticipate p CO 2 changes, thereby avoiding impairments and more successfully adjust to ocean acidification conditions.