Diel pCO(2) fluctuations alter the molecular response of coral reef fishes to ocean acidification conditions

Environmental partial pressure of CO2 (pCO(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 pCO(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 pCO(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 pCO(2) levels (750 and 1,000 mu atm) under both stable and diel fluctuating conditions. We found a common signature to stable elevated pCO(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 pCO(2) in a stable treatment than for fluctuating treatments, but the largest difference in molecular response was between stable and fluctuating pCO(2) treatments. This indicates that a response to a change in environmental pCO(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 pCO(2) treatments, possibly accommodating a more flexible adaptive approach in the response to environmental changes. Our results suggest that environmental pCO(2) fluctuations might enable reef fishes to phase-shift their clocks and anticipate pCO(2) changes, thereby avoiding impairments and more successfully adjust to ocean acidification conditions.

Automated summary

The study found that in a stable elevated pCO(2) environment, both species of coral reef fishes showed a consistent brain molecular response, including the downregulation of immediate early genes. The transcriptional program was more strongly affected by higher average pCO(2) in stable conditions. The largest difference in molecular response was between stable and fluctuating pCO(2) treatments, indicating that the response to environmental pCO(2) conditions differs for organisms living in fluctuating versus stable environments.