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Ocean acidification dissolves algae, deafens fish

As if being a major contributor to global warming wasn’t enough, the increasing amount of carbon dioxide produced through human activity is also acidifying our oceans – and doing so more rapidly than at any other time in more than half a million years. New projections show that at current rates of acidification, clownfish and many species of algae may be unable to survive by 2100.

The carbon cycle is the flux of carbon between land, air, and sea, and the intake of carbon dioxide into the ocean is a normal part of that flux. In the ocean, carbon dioxide is converted into a variety of different molecules, one of which – the hydrogen ion – decreases pH as it becomes more prevalent. Soaring carbon dioxide emissions caused by human-related activities is leading to an increase in hydrogen ions which in turn is lowering the pH and acidifying the world’s oceans.

Since the Industrial Revolution, the concentration of marine hydrogen ions has increased nearly 30 percent, with 10 percent of that increase occurring since the 1990s. Expected to only worsen, the impact of ocean acidification on marine ecosystems is just beginning to be understood, but those investigations which have taken place show a dire future for many species.

Change in pH due to human activity from 1770s to 1990s. From the Global Ocean Data Analysis Project.

A study conducted by researchers at U.K. and Australian universities, and published in Biology Letters, investigated the response of clownfish hatched and raised in differing pH levels. An earlier study had shown that their sense of smell is affected by an acidic environment, but this was the first to test the effects of acidity on the fishes’ ability to hear.

“We kept some of the baby clownfish in today’s conditions, bubbling in air, and then had three other treatments where we added extra CO2 based on the predictions from the Intergovernmental Panel on Climate Change for 2050 and 2100,” said the study’s lead author, Dr. Steve Simpson of Bristol University.

The results were released June 1st and show that the hearing of baby clownfish becomes compromised with the lowering of oceanic pH, disappearing completely when levels reach those of the not-to-distant future.

“We designed a totally new kind of experimental choice chamber that allowed us to play reef noise through an underwater speaker to fish in the lab, and watch how they responded,” said Dr. Simpson, “Fish reared in today’s conditions swam away from the predator noise, but those reared in the CO2 conditions of 2050 and 2100 showed no response.”

Common Clownfish, like these, have the ability to switch sex if the need arises. Will acidification also take that away from them?

Since the ears of clownfish are located deep within the body, these deaf fish indicate that the consequences of growing up an acidic environment aren’t only exacted on the parts of fish exposed to the water, but may profoundly affect the development of the sensory system as a whole. Since clownfish depend on their ability to hear in order to avoid predators and find food, deafness is effectively a fatal condition. The possibility exists that the species may adapt successfully to the rising acidity of their ocean environment, but the likelihood of adaptation is as yet unknown.

“What we have done here is to put today’s fish in tomorrow’s environment, and the effects are potentially devastating.” said Dr. Simpson, “What we don’t know is whether, in the next few generations, fish can adapt and tolerate ocean acidification. This is a one-way experiment on a global scale, and predicting the outcomes and interactions is a major challenge for the scientific community.”

These findings come on the heels of a study which looked into the effects of acidification on widespread species of algae. The paper was published in Proceedings of the National Academy of Sciences and focused specifically on the ability of a crucial component in many algal species, the coccolith, to withstand an acidic environment.

A single-celled coccolithophore and its coccoliths. Photo by NEON Ja and Richard Bartz.

Coccoliths are individual plates of calcium carbonate that surround single-celled species of algae called coccolithophores and are thought to serve a protective, perhaps structural, role. Coccolithophores are widely distributed in oceans worldwide and because of their environmental sensitivity, fossil specimens are often used by paleontologists to reconstruct the prehistoric past. Now, for the first time, researchers are using them to look into the future.

“We know that the world’s oceans are acidifying due to our emissions of CO2 and that is why it is interesting for us to find out how the coccoliths are reacting to it.” explains Tue Hassenkem of the University of Copenhagen’s NanoGeoScience research group. “We have studied algae from both fossils and living coccoliths, and it appears that both are protected from dissolution by a very thin layer of organic material that the algae formed, even though the seawater is extremely unsaturated relative to calcite. The protection of the organic material is lost when the pH is lowered slightly.”

“In fact, it turns out that the shell falls completely apart when we do experiments in water with a pH value that many researchers believe will be the found in the world oceans in the year 2100 due to the CO2 levels.”

Algae is a major player in the global carbon-oxygen cycle, using its photosynthetic ability to produce atmospheric oxygen. It is also a bedrock member of the marine food chain and is important to the survival of a multitude of species.

“These findings underscore that the acidification of the oceans is a serious problem.” explains Dr. Katherine Richardson, professor of biological oceanography at the University of Copenhagen, “The acidification has enormous consequences not only for coccoliths, but also for many other marine organisms as well as the global carbon cycle.”

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