Ocean acidification: a natural experiment

GLOBAL warming is not the only environmental change that is being wrought by rising emissions of carbon dioxide. This gas, acidic when dissolved in water, is also lowering the pH of the world’s sea water—a phenomenon known as ocean acidification.

How much to worry about this acidification (or, strictly, reduction in alkalinity, for there is no risk of the sea actually becoming acidic) is a matter of debate. The threat most talked of is to creatures that make shells out of calcium carbonate. As school chemistry experiments with chalk and vinegar demonstrate, calcium carbonate dissolves in acid, so an ocean less alkaline than it used to be might make life harder for shell-forming animals. Numerous laboratory experiments agree. There is also evidence that the shells of several widespread marine species are thinner and weaker now than they were a few decades ago. What there has not been, though, is a controlled study in the wild—at least, not until now.

The gap has been plugged by Miles Lamare of the University of Otago, in New Zealand, and his colleagues, who have just published their study in the Proceedings of the Royal Society. Dr Lamare observed that there are several places in the sea where acidification is happening naturally, because low-level volcanic activity is releasing carbon dioxide from submarine vents. Two such vents are located off the coast of Papua New Guinea. These, he thought, would be a good place for an experiment.

Marine biologists suspect that the threat of acidification is most serious to an animal when it is a small, planktonic larva. Dr Lamare and his colleagues therefore carried out their experiment on the larvae of Echinometra, a type of sea urchin. They hung cages containing these larvae, newly hatched from freshly collected adult urchins, in the water above the vents, and also in nearby water of normal pH, to act as a control. They then left the cages for a day or two, to let the larvae grow, before examining their charges under the microscope.

At the first vent site, the differences were startling. In this case all of the larvae came from adults collected in the control area, ie, living in water of normal pH. Those raised in the cages over the vent grew much more slowly than those in the control area. They were also more prone to develop asymmetrically.

At the second site, the picture was more complicated. In this case Dr Lamare carried out a more sophisticated experiment on larvae collected from adults that dwelled in the vents as well as from the control area. It tested both sorts of larvae in both locations, to see if the young of adults that had been living in the vent were inured to less alkaline water. Surprisingly, in light of the earlier result, pH made no difference to the growth rates of either sort of larva, though it still affected rates of asymmetry.

And that was not the only surprise. Dr Lamare also found that larvae whose parents had come from the vent grew larger than those whose parents had not, regardless of the site where they were raised. That does hint at genetic differences between vent-dwelling and non-vent-dwelling Echinometra—just not the one, namely acid resistance, that might have been expected.

As these somewhat confusing results show, a single experiment like this can yield only limited information. But what really counts is that scientists have now discovered an important natural laboratory in which to investigate the effects of ocean acidification further and, hopefully, find more definitive answers about what many see as a worrying problem.

The Economist, 3 December 2016. Article.

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Ocean acidification in the IPCC AR5 WG II

OUP book