Altered brain ion gradients following compensation for elevated CO₂ are linked to behavioural alterations in a coral reef fish

Neurosensory and behavioural disruptions are some of the most consistently reported responses upon exposure to ocean acidification-relevant CO₂ levels, especially in coral reef fishes. The underlying cause of these disruptions is thought to be altered current across the GABA A receptor in neuronal cells due to changes in ion gradients (HCO₃-and/or Cl-) that occur in the body following compensation for elevated ambient CO₂. Despite these widely-documented behavioural disruptions, the present study is the first to pair a behavioural assay with measurements of relevant intracellular and extracellular acid-base parameters in a coral reef fish exposed to elevated CO₂. Spiny damselfish (Acanthochromis polyacanthus) exposed to 1900 μatm CO₂ for 4 days exhibited significantly increased intracellular and extracellular HCO₃-concentrations and elevated brain pH i compared to control fish, providing evidence of CO₂ compensation. As expected, high CO₂ exposed damselfish spent significantly more time in a chemical alarm cue (CAC) than control fish, supporting a potential link between behavioural disruption and CO₂ compensation. Using HCO₃-measurements from the damselfish, the reversal potential for GABA A (E GABA) was calculated, illustrating that biophysical properties of the brain during CO₂ compensation could change GABA A receptor function and account for the behavioural disturbances noted during exposure to elevated CO₂.