Spirals in Time: The Secret Life and Curious Afterlife of Seashells (37 page)

If it wasn’t for the saltwater that covers seven-tenths of the planet, the problems caused by climate change would
already be unspeakably worse than they are today. Every hour, the oceans absorb a million tonnes of carbon. In less than four hours they absorb the equivalent of the annual carbon emissions from a coal-burning power station. We all have a lot to thank the oceans for.

The problem is that carbon dioxide doesn’t just sit unnoticed in the oceans, but it has its own particular effect. When it reacts with seawater, carbon dioxide lowers the pH, making the oceans more acidic. Measurements show that since the dawn of the industrial revolution, ocean pH has fallen by 30 per cent. If we carry on with business as usual and do nothing to cut carbon emissions, experts confidently predict that by the end of the century ocean pH will have dropped by 150 per cent. There’s no question about it: this is purely a case of indisputable chemistry.

The term ‘ocean acidification’ first became popular in 2003, when
Ken Caldeira and Michael Wickett
published a paper in the journal
Nature
. They calculated that if we go ahead and burn all the remaining fossil fuels, the oceans will become more acidic than they’ve ever been in the past 300 million years. Whether things will ever get that bad we’ll see, but the point is that the chemistry of the seas is already changing.

Ocean acidification gets far less attention in the public eye compared to other threats linked to climate change. All we tend to hear about are rising temperatures and rising sea levels. Nevertheless, away from the media spotlight, researchers are beginning to untangle an important, difficult question: how will marine life react to acidifying oceans?

The fact is that the seas aren’t exactly transforming into a caustic acid bath that would strip your skin off when you jump in. Surface waters of the ocean are still mildly alkaline, with an average pH of 8.1, compared to pH 8.2 200 years ago (the pH scale is logarithmic, which is why a drop from pH 8.2 to 8.1 equates to a 30 per cent change). Pure water has a pH of around 7; acids are below that, with milk at pH 6.5, lemon
juice at pH 2 and stomach acid at pH 1. At the other end of the scale are strong alkalis, like household bleach with a pH over 12.

By 2100, average ocean pH could be down to 7.8, which is not exactly stomach acid, but in fact around the same pH as human blood. However, many marine organisms are adapted to living in water that has a fairly constant pH. Even a minor tweak to seawater pH could be enough to throw all sorts of things out of whack.

Laboratory studies on the effects of falling pH on marine life are producing plenty of findings, some of them rather unexpected. In water with carbon dioxide bubbled through it, young clown fish lose their sense of smell and become deaf, making it more likely that in the real world they would blunder into a predator or have trouble sniffing and hearing their way home to a coral reef. If the movie
Finding Nemo
had been set in the future, the little clown fish would probably have stayed permanently lost (or been eaten). Other fish species could become more anxious as the seas’ pH drops. Californian rockfish kept in tanks of more acidic water became uncharacteristically shy, spending much of their time lurking in darkened areas and staying away from the light.

Ocean acidification could also make the seas more toxic in other ways. Lugworms live burrowed into sandy and muddy shores across northern Europe, where they are important food for wading birds and fish. You won’t often see them, but they leave distinctive worm casts across beaches at low tide, like squeezes of sandy toothpaste. Recent studies show that copper, a common contaminant of coastal waters, is much more toxic to lugworms in acidified seawater. When pH drops, copper kills lugworm larvae and damages the DNA in lugworm sperm, making them swim more slowly and reducing their chances of reaching a fertile egg and forming an embryo.

These various subtle effects on behaviour and toxicity are difficult to predict, and researchers have to work backwards,
unpicking the story and figuring out why these changes take place. However, for one particular group of marine species the effects of ocean acidification are much more foreseeable.

Calcifiers are a mixed gathering of marine organisms that all produce calcium carbonate in some form, as exoskeletons or shells. There are calcifiers stationed all the way through marine food webs, from microscopic, sun-fixing plankton, to sea urchins, starfish and corals, crustaceans and worms, and of course all those molluscs with shells. And these carbonate-makers are all in the firing line of ocean acidification.

The calcifiers’ problems begin with the fact that calcium carbonate dissolves in acid. If you place a chicken’s egg (also made of calcium carbonate) in a glass of vinegar you’ll see this happening for yourself, albeit to an extreme degree: the shell dissolves leaving a naked egg, held together by a thin membrane. This sort of approach is the only way, so far, that anyone has investigated how acidifying oceans might affect argonauts. When Jeanne Power studied argonauts in Sicily in the early years of the industrial revolution, she had no reason to think of testing the effect of pH on pieces of their shells. When Kennedy Wolfe at the University of Sydney, Australia tried it in 2013, he found that at pH 7.8, argonaut shell begins to dissolve. This arises from the fact that female argonauts make their shells from an especially fragile form of carbonate, called high-magnesium calcite, which readily dissolves at lower pH. Argonaut shells also lack an outer, organic layer that could help protect other molluscs from acid attack (a spongy, thick protein layer is one reason molluscs can survive the corrosive conditions at hydrothermal vents).

What we don’t know is how argonauts might react to falling pH while they are still alive (the animals are too rare and difficult to keep in captivity, so no one has tried this). Jeanne watched her animals use web-like membranes to fix
damaged shells. Would argonauts do the same thing if their shells began thinning and dissolving in acidifying waters? Perhaps, but there is an added problem. As well as making their shells more likely to dissolve, ocean acidification also makes it harder for molluscs to make and mend their shells.

When carbon dioxide reacts with water it not only releases hydrogen ions, causing a drop in pH, but also reduces the concentration of carbonate ions (this happens because they react with hydrogen ions, forming bicarbonate). The problem for calcifiers is that carbonate ions are the basic building blocks they use to produce their shells. Many species need seawater to be supersaturated with carbonate ions to be able to form enough calcium carbonate for their skeletons and shells. As the concentration of carbonate ions drops, and seawater becomes undersaturated, calcifiers must devote more energy to pumping ions around their bodies and maintaining the process of shell-making. Molluscs have to concentrate carbonate ions in the gap between their mantles and their shells where new shell material is made. This can drain energy away from other vital functions, like reproduction and growth.

To make matters worse for shell-making molluscs, carbon dioxide also diffuses from water directly into their bodies, mostly through their gills. Left unchecked, a drop in the pH of body fluids can impact all sorts of important processes, in particular the functioning of enzymes. These proteins govern reactions around the body and they work best within a narrow pH range and will slow down or even stop if their surroundings become too acidic or too alkaline. As a consequence, organisms have evolved complex balancing mechanisms to maintain the right pH. Imagine a living body is a room, and acid-causing hydrogen ions are tennis balls that pour in through an open window; to prevent the tennis balls filling the room, and lowering the pH, you have to push them back out through the letterbox. Living bodies have various ways of keeping pH in balance, but they require yet more energy.

Lots of studies have tested how all sorts of calcifiers respond to falling pH and falling carbonate ion concentration. Coral reefs are a major focus for these studies because various components of these important tropical ecosystems form carbonate skeletons. This includes hard corals, the ‘bricks’ that form a reef’s foundations, together with encrusting coralline algae that cement the reef together. It’s possible that corals may adapt to gradual acidification and survive, but it’s easy to be pessimistic about the future of reefs. The combined impacts of overfishing, coastal pollution, acidification and warming seas (which cause corals to lose the colourful, microscopic algae in their tissues, bleaching them white and in many cases killing them) lead many experts to think that coral reefs as we know them could be extinct by the end of the century.

Molluscs have also been the subject of extensive acidification research, in part because the valuable seafood industry could be left in ruins if edible species start disappearing. Clams, mussels, conchs, scallops, oysters and many more have all been plucked from their salty homes, moved into laboratory aquariums and exposed to seawater at various pH and carbon dioxide levels while scientists watch to see what happens. Initially, researchers mostly used mineral acids to simulate ocean acidification. They now tend to bubble carbon dioxide through water to more accurately mimic the real world. In most studies, as pH drops, the molluscs get in all kinds of trouble.

Flimsy and misshapen shells and lower calcification rates (the laying down of new shell material) are commonly seen in molluscs kept in seawater of lower pH and higher carbon dioxide than they’re used to. Mussel byssus threads lose their stickiness, and many molluscs suffer from a suppressed immune system. Some researchers have observed molluscs swimming and crawling more slowly in acidified waters. Embryos and juveniles seem to be especially vulnerable. They take longer to mature and many don’t survive.

It follows that with all the demands on their energy supplies, molluscs commonly respond to acidifying waters by boosting energy production or metabolic rate. They need energy to grow, to patch up shell damage, to try desperately to maintain their pH balance and ultimately to stay alive. For many species, all of these demands can become too taxing and they suffer, but this isn’t always the case. Lab studies of ocean acidification regularly throw up unexpected and contradictory results.

Some molluscs seem quite unfazed by lowering pH and rising carbon dioxide, and some positively thrive. It seems to depend partly on where in the world the molluscs come from. The Blue Mussel is one species that confounds scientists by behaving differently in acidification studies around the world; in some places they are robust, elsewhere they do badly. It suggests that there is some degree of local adaptation to varying baseline conditions – pH is not the same everywhere in the oceans – and that some populations could be more likely to survive than others.

Slipper Limpets are another odd species. They have continued to grow happily when carbon dioxide levels around them were ramped up to 900 parts per million (or ppm; currently, the atmosphere is around 400ppm). When Common Cuttlefish are exposed to carbon dioxide at a massive 6,000ppm, some individuals remain unaffected and some actually do better than others kept in normal, mild conditions. After six weeks at extreme carbon dioxide levels, their internal cuttlebones, made of calcium carbonate, are bigger and heavier. Cephalopods, including these cuttlefish, are generally thought to have more sophisticated internal balancing mechanisms than other molluscs. They are also good at boosting their metabolism when they need to, which goes some way to explaining why cuttlefish get on so well in such extreme conditions. But plenty of puzzles still remain.

As it stands, the prognosis for shelled molluscs in acidifying oceans is mixed. Some species may be able to tough it out,
while others will come to grief. But for sea butterflies in particular the prospects aren’t looking too good.

Sea butterflies have been labelled the ‘canaries in the coal mine’ of acidifying oceans. These sensitive creatures could be the sentinels, warning of dangers ahead. In the first half of the twentieth century, miners would take down caged birds with them to detect toxic gases, mainly carbon monoxide; when the birds passed out and died, the miners knew it was time to put on breathing apparatus and make a quick escape. For sea butterflies in acidifying seas, the coal-mine analogy is rather ironic – or perhaps poetically dismal – seeing as it was coal that kick-started the problem of ocean acidification in the first place.

With their dainty, thin shells it comes as no great surprise that sea butterflies are among the more sensitive molluscs; they don’t have much to lose shell-wise in the first place, so when exposed to acidifying waters they are especially vulnerable. Dire predictions suggest that swathes of the ocean could be out of bounds for sea butterflies in the years ahead.

Problems are likely to be most severe in polar seas, where acidification is expected to hit soonest and hardest, because cold water naturally holds more carbon dioxide. In parts of the Arctic and Antarctic, it’s predicted surface seawater could become undersaturated with carbonate ions – and corrosive to unprotected shells and skeletons – within the next few decades. These frigid seas are also important parts of the sea butterflies’ domain.