The continued increase of atmospheric carbon suggests that by the end of this century the world’s oceans, which absorb 25 percent of our carbon dioxide emissions, could contain twice as much of the greenhouse gas as they do now. Such a steep rise could have significant impacts on some species of marine fish, since the introduction of more carbon dioxide turns seawater acidic and dramatically alters the animals’ sensory response -- changes that a new report published in the journal Nature Climate Change indicates are mediated by a chemical receptor in the brain known as GABA-A.
Since the late 1990s, scientists have known that ocean acidification alters seawater carbonate and aragonite chemistry, which affects the calcification and deposition of shell and skeletal materials in marine invertebrates such as corals and shellfish. In the last several years, however, scientists have also discovered that high seawater carbon dioxide levels, equivalent to those expected at the end of the century, affect fish. Among the behavioral changes observed thus far are disruption of hearing and smell (olfaction) in juvenile orange clownfish (Amphiprion percula) and of lateralization (favored turning direction) in yellowtail demoiselles (Neopomacentrus azysron).
In fish, high carbon levels in water can cause acidosis (excessive acid in body fluids), a potentially life-threatening condition. Fish try to overcome acidosis through acid-base regulation and the accumulation of bicarbonate, which neutralizes acids and thereby prevents body fluids from becoming too acidic. But as the new study, conducted by a team of scientists from Australia, Italy, and Norway, has shown, this process reverses the normal function of the GABA-A receptor.
In the vertebrate brain, the GABA-A (gamma-aminobutyric acid-A) receptor is inhibitory, acting to attenuate the transmission of chemical signals between neurons. This occurs when chloride ions, and to a lesser extent bicarbonate ions, flow through the receptor and into the cell in response to some external signal. When intracellular chloride and bicarbonate concentrations become too high, the reverse happens -- the receptor conducts the ions out of the cell. By doing so, however, the inhibitory effect is lost, and the neurons become excitable. In the study, the scientists hypothesized that this reversal in receptor activity was responsible for the observed changes in sensory behavior in juvenile fish.
To investigate their hypothesis, the scientists reared larval clownfish in either control or high carbon environments and determined the effects of carbon on olfactory responses. Controls (fish raised in a carbon environment similar to that currently found in the ocean) avoided water trails that contained the odor of blue-spotted rockcod (Cephalopholis cyanostigma), a clownfish predator. Fish exposed to high carbon levels, however, were drawn to the odor. This abnormal response was corrected when gabazine, a chemical that blocks the GABA-A receptor, was added to the water.
In another series of experiments, the team investigated lateralization as a measure of brain function in yellowtail demoiselles. They collected wild yellowtails and exposed them to either control or high carbon environments and then recorded observations of turning direction in a T-shaped maze. Under normal conditions, yellowtails show a preference for turning left or right that is greater than expected by chance. Following exposure to large amounts of carbon, however, the researchers found that the fish turned at random. Similar to the abnormal olfactory responses in clownfish, the atypical lateralization effect in yellowtail demoiselles was corrected by gabazine.
Because gabazine binds only to GABA-A receptors, the findings indicate that carbon dioxide interferes with normal GABA-A activity and that this interference produces the behavioral abnormalities observed in coral reef fish. The existence of GABA-A receptors in the brains of vertebrates and invertebrates suggests that increasing carbon dioxide levels in the atmosphere and ocean could have effects across a variety of ecosystems. These effects, however, likely are to be most pronounced in aquatic ecosystems, because carbon dioxide is far more soluble in water than oxygen and because aquatic species tend to have relatively low carbon dioxide levels in their blood.
The researchers suspect, however, that because water-breathing species use different strategies to cope with high acidity, only certain groups of aquatic life may be susceptible to the effects of increasing carbon levels in seawater. The most vulnerable groups would include those species that rely almost exclusively on acid-base regulation, such as teleosts and crustaceans, and species that have unusually high rates of oxygen consumption, such as coral reef fish larvae and pelagic fish.
Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.