Ocean Acidification

A changing climate may tamper with marine animals’ sense of smell and change the shapes of signaling molecules.

A pair of spiny lobsters locks antennae as they battle on the gravel-strewn bottom of an aquarium. The two grapple, grabbing legs and jousting with their long spines. Their aggressive actions extend beyond the show of force: the crustaceans also fire off chemical signals by peeing at each other.

“They’re actively signaling as they’re fighting,” says Charles D. Derby, a sensory biologist at Georgia State University whose lab studies these underwater wrestling matches, along with other crustacean behaviors. Lobster urine, released from the face near the base of the antennae, contains an array of compounds, including chemical cues to an animal’s sex and social status (J. Exp. Biol. 2009, DOI: 10.1242/jeb.026492).

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A changing climate may make chemical communication for sea creatures more challenging. By the year 2100, the pH of the ocean—currently about 8.1—is expected to drop to 7.7 if atmospheric carbon dioxide continues to rise. Over the past decade, research has revealed how sensitive chemoreception systems are and how they may take a hit from human activities that cause pollution, warming waters, and ocean acidification. For instance, higher CO₂ levels in the water cause juvenile sea bass to lose sensitivity to some smells. In the wild, that could make it harder for the fish to find food and can render them more vulnerable to predators, says Peter C. Hubbard, an electrophysiologist at the Center of Marine Sciences in Portugal. And for migratory fish, such as salmon, lowered olfactory sensitivity may mean difficulty in navigating rivers for spawning.

Many studies have reported changes to fish olfaction and behavior with increased CO₂ levels. Recently, a group of marine scientists has raised doubts about the data in some of these papers and the methodology the studies used to test behavioral changes in fish due to increased CO₂, a controversy that has cost the field some of its credibility.

But many researchers agree that acidification would impact ecologically and economically important species through changes to the water’s chemistry. That makes it important to investigate the underlying mechanisms and the magnitude of the effects. Understanding such mechanisms could help make sense of confusing, difficult-to-replicate behavioral findings. So scientists continue to work to untangle the basic workings of marine chemoreception, from identifying signaling molecules to finding the receptors that they bind.

A CHEMICAL LANGUAGE

The molecular words that form the ocean’s chemical language present a scientific puzzle. “The language is very, very complex,” says Julia Kubanek, a marine chemist at Georgia Institute of Technology. Potentially thousands of molecules are important for conveying information to ocean organisms, she says, yet many remain unknown.

Improved analytical techniques are helping pin down those molecules. Kubanek and colleagues have observed that when tiny predatory crustaceans called copepods are around, single-celled organisms called dinoflagellates boost their production of toxic compounds for defense. Using chemical separation techniques paired with NMR spectroscopy and mass spectrometry, the team traced the trigger for the toxicity uptick to a family of lipids in the copepods’ waste (Proc. Natl. Acad. Sci. USA 2015, DOI: 10.1073/pnas.1420154112).

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BODY AND BRAIN

In addition to identifying the chemicals that creatures send out, scientists are looking at how those molecules are sensed by other animals and trying to unveil the specific proteins that act as receptors and their roles in biological processes and behavior. Kubanek says that hundreds to thousands of mammalian receptors are known but that “we’re way behind” for aquatic animals.

Lab studies of fishes’ response to CO₂, for example, have reported anxiety or strange behaviors, such as being attracted to predators’ smells. In this area, researchers aiming to replicate these behavioral studies haven’t always reached the same conclusions (Nature 2020, DOI: 10.1038/s41586-019-1903-y).

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Also, the lining of a fish’s nose links directly to the brain via the olfactory nerve. Zélia Velez of the Center of Marine Sciences, Hubbard, and colleagues monitored the response of the olfactory nerve in juvenile sea bream to elevated CO₂. Unlike behavioral tests, which can be more prone to artifacts of the experimental design or even the mood of the fish, Hubbard says, these tests capture how acidification impacts physiology. Under elevated CO₂ conditions, the scientists observed a decreased neural signal in response to amino acids, which are thought to serve as food cues (Front. Physiol. 2019, DOI: 10.3389/fphys.2019.00731).

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MODIFIED MOLECULES

Climate change could alter not only animals’ ability to sense molecules but also the molecules themselves.

At a lower pH, with more hydrogen ions around, some molecules or functional groups pick up a proton. This addition can change a molecule’s conformation and charge distribution, sometimes enough that a protonated form of a molecule may not bind to a receptor in the same way as the deprotonated version.

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It is unclear how widespread the effects of ocean acidification on signaling molecules may be, Roggatz says. Some chemical cues may even become more potent (J. Chem. Ecol. 2021, DOI: 10.1007/s10886-021-01276-9). “What I have found so far is that when the molecule is sensitive, the changes are potentially devastating.”

Along with the changes to chemoreception and signaling molecules that may accompany acidification, creatures will likely contend with effects due to rising temperatures and hypoxia, a decrease in oxygen levels. “It’s going to be difficult to really predict what the outcome will be,” particularly for different animals, Hubbard says.

Chemical & Engineering News (c&en), 19 September 2021. Full article.