Sea change

The increasing acidity of our seas is a threat to marine life that for many species may be impossible to overcome.

Female shore crabs are diligent mothers. They produce around 100,000 eggs at a time, which they hold as an orange–brown mass on the underside of their abdomen, using leg-like appendages known as pleopods to pump water over the eggs to ensure a steady flow of oxygen as the young inside develop.

At least, that’s what happens in normal sea water, which has a pH of about 8.1. If the pH falls to 7.7 or so — conditions that could arise in the oceans by the end of the century, owing to the anthropogenic release of carbon dioxide (CO2) into the atmosphere — the shore crabs (Carcinus maenas) don’t tend their eggs. “They just doze,” says Jörg Hardege, a chemical ecologist at the University of Hull, UK.

The reason, Hardege and his colleagues reported1 in 2016, is that the more acidic water alters the shape and electric charge of a molecule, released by the eggs, that stimulates the female crab to pump water. She can no longer recognize the molecular signal, and therefore neglects her eggs.

The shore crab’s lazy mothering is just one of a rapidly expanding set of biological effects that researchers are documenting in the oceans — involving organisms that range from the single-celled phytoplankton at the base of the marine food web to the fish at its peak — that could stem from the ongoing decrease in the pH of sea water.

Not all species are equally vulnerable to this acidification of the oceans, a fact that has the potential to restructure marine ecosystems. More broadly, as well as scrambling signals used in communication, changes to ocean chemistry in the coming decades could dissolve the skeletons of marine organisms, deplete the energy reserves of such organisms and even interfere with the function of their nervous systems.

Acid test

As the burning of fossil fuels increases the concentration of atmospheric CO2, the amount of CO2 dissolved in sea water increases in tandem. The oceans are Earth’s largest carbon sink — by some estimates, about 40% of anthropogenic CO2 has been absorbed by the seas.

Some of that extra dissolved CO2 reacts with molecules of water in a series of reactions that leads to the formation of carbonic acid, and bicarbonate and hydrogen ions (see ‘Ocean impact’). The greater the concentration of hydrogen ions, the lower the pH of the water will be and the more acidic the conditions.

At the start of the Industrial Revolution, the global average ocean pH was about 8.2 — around 0.1 units higher than it is today. That may not seem like a big difference, but a similar decrease in the blood pH of humans can lead to dire consequences, including seizures, heart arrhythmia and coma. Because pH is measured on a logarithmic scale, every change of 0.1 units results in a 26% increase in concentration of hydrogen ions.

“If you think about the enormous scale of the oceans, it’s a tremendous amount of CO2 that the oceans have taken up” to yield even such a small shift in global average pH, says George Waldbusser, an ecologist at Oregon State University in Corvallis.

Further change is likely to come. If carbon emissions continue at the present rate, the global average ocean pH will probably fall to around 7.7 by 2100. And even if nations drastically curtail emissions to meet their commitments under the 2015 Paris climate agreement, the pH of the oceans will still be about 7.9 at the century’s end.

Moreover, after CO2 has dissolved in sea water, it can stay there for a long time, even if its concentration in the atmosphere decreases, owing to patterns of circulation that cause surface water to sink deep into the oceans. “What we do in the next few decades will impact the oceans for tens of thousands of years,” says Carol Turley, a biogeochemist at Plymouth Marine Laboratory, UK.

Shell shock

An understanding of ocean acidification and its biological effects was slow to emerge, especially compared with that of the increase in global temperature caused by rising levels of atmospheric CO2. The Intergovernmental Panel on Climate Change didn’t mention ocean acidification in its reports until 2007. Most research on the topic has unfolded in the past 20 years.

Initial concern, and still probably best known to the public, focused on calcifiers — a diverse group of marine organisms that make shells or skeletons from calcium carbonate. A 1998 study2 suggested that increased levels of CO2 in sea water could hamper calcification by reef-building corals in tropical waters. In 2000, coccolithophores — single-celled algae that build striking shells from overlapping plates of calcium carbonate — were reported3 to form their shells more slowly when exposed to water high in dissolved CO2.

Ocean acidification interferes with shell formation because hydrogen ions in sea water react with carbonate ions, forming bicarbonate ions. More-acidic sea water means that there will be fewer carbonate ions to incorporate into calcium carbonate, and therefore less raw material for building shells. Waldbusser and colleagues have found that in waters undersaturated with carbonate, the larvae of bivalves such as clams and oysters have trouble starting their shells, which form more slowly or turn out misshapen. Such effects are thought to explain why shellfish hatcheries on the west coast of the United States are struggling, Waldbusser says.

Stronger evidence for the perils of acidic oceans comes from a 2016 study4 in which the chemistry of the water around small portions of the Great Barrier Reef in Australia was returned to that of before the Industrial Revolution. The corals in those areas grew about 7% faster than elsewhere on the reef. “That was an extraordinary experiment because it actually showed that [ocean acidification is] already having an impact,” Turley says.

As well as affecting the formation of skeletons and shells, acidic sea water can cause shells to dissolve. For most species, difficulties in shell formation will happen at a much lower threshold of acidity than shell dissolution. But pteropods — tiny, free-swimming sea snails that are an important component at the base of the food chain — seem to be especially vulnerable to the corrosive effects of acidic water. A 2014 study5 found that more than half of the pteropods collected from waters off the US west coast had severely dissolved shells.

Energy demands
When carbonate is scarcer, organisms have to expend more energy to gather material for their shells. As research into ocean acidification has accelerated in the past decade, it has emerged that many of its other biological effects also boil down to energy imbalances.

“The basic problem is the same for all marine organisms,” says Felix Mark, a marine ecophysiologist at the Alfred Wegener Institute in Bremerhaven, Germany. Many organisms have mechanisms to regulate internal pH, such as pumping hydrogen ions in or out. As the pH of sea water falls, organisms have to work harder to keep their own pH constant. “It’s going to cost you energy. As simple as that,” says Mark.

Mark and colleagues have found that the energy strain imposed by ocean acidification can particularly affect young fish, which otherwise tend to put all their resources into growth. In laboratory experiments, Mark’s group has shown that at concentrations of dissolved CO2 expected by the end of the century, the larvae of some species of fish, including the Atlantic herring (Clupea harengus), show slower growth and higher mortality.

Other forms of ocean life, however, may find that their energy budget actually benefits from such shifts in ocean chemistry. For example, as the concentration of dissolved CO2 increases, phytoplankton won’t have to work as hard to get the CO2 they use to produce energy through photosynthesis.

DeWeerdt S., 2017. Sea change. Nature 550: S54–S58. doi:10.1038/550S54a. Full article.

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Ocean acidification in the IPCC AR5 WG II

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