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These days, it's hard to exhale without thinking about carbon emissions. One major consequence of climate change is ocean acidification, which is an increase in hydrogen ions as water chemistry balances the atmospheric carbon dioxide load. Aquatic animals are affected by ocean acidification because pH (a measure of hydrogen ions) is an important factor in all biological processes. Fish, for example, compensate for a change in environmental pH by adjusting certain ions in the blood and tissues. This shifting of ions is important for the fish to maintain optimal pH inside its body, but there are consequences for cells like neurons that rely on ions for communication. For example, the neurotransmitter gamma-aminobutyric acid (GABA) binds to its cell membrane receptor to allow the passage of chloride ions into the neuron as they flow down their concentration gradient, ultimately stopping that neuron from talking to its neighbors (inhibitory). A change in chloride ion concentration on either side of cell membranes would reverse the direction of ion flow through the GABA receptor and ultimately encourage that neuron to chat (excitatory), and this could result in anxious behavior. Trevor Hamilton and Adam Holcombe at MacEwan University in Alberta, Canada, teamed up with Martin Tresguerres at the Scripps Institution for Oceanography in California, USA, to determine whether ocean acidification influences the behavior of rockfish by shifting the GABA receptor from inhibitory to excitatory.

The international research team collected rockfish off the coast of California, where ocean currents already create intermittent periods of pronounced acidification. Back at the lab, half of the rockfish happily swam in normal seawater while the other half experienced simulated ocean acidification for 1 week. To see how water pH affected the fish's anxiety-like behavior, the team then placed individual fish in aquaria that had either a combination of black and white walls or a novel object in the center, and then they recorded the fish's position in the aquarium for 15 min with a video camera. The researchers then repeated the behavioral testing on the fish from the acidified and normal seawater, but this time they gave the fish one of two drugs – one drug closed the GABA receptor (antagonist) and the other drug opened it (agonist). Hamilton and colleagues predicted that if the function of the GABA receptor is reversed in response to ocean acidification as ions re-shuffle across membranes, then fish from normal seawater will be more anxious with a closed receptor, while fish from acidified seawater will be extra anxious with an open GABA receptor.

Compared with the fish from normal seawater, rockfish from the acidified seawater spent more time near the black walls, meaning they were anxious. Rockfish from normal seawater that were exposed to the GABA receptor antagonist also preferred black walls; so closing the GABA receptor in control seawater prevented its inhibitory function and resulted in anxious behavior. Next, the researchers compared how much time the fish spent near a novel object in the testing aquarium if they were given a GABA receptor agonist prior to the behavioral test. Typical of increased anxiety, rockfish from acidified seawater spent more time near a novel object in the aquarium compared with fish from normal seawater that were also given the agonist.

So, if a rockfish changes its ion distribution across neuron membranes to accommodate ocean acidification, it is more anxious because opening the GABA receptor excites the neuron that should be inhibited, as chloride ions flow out instead of in. Hmmm, this GABA receptor switcheroo sounds a bit like the hokey-pokey… ‘You let your chloride in, you let your chloride out…’

References

Hamilton
T. J.
,
Holcombe
A.
,
Tresguerres
M.
(
2014
).
CO2-induced ocean acidification increases anxiety in rockfish via alteration of GABAA receptor functioning
.
Proc R. Soc. B
281
,
20132509
.