If you’re an ocean creature with a hard shell—like a sea urchin, a hermit crab, or a coral polyp—you prefer ocean water with a pH of about 8.2. This chemistry makes it easy to assemble your armor from carbon-based building blocks dissolved in the ocean. Since the beginning of the industrial age, though, the ocean has been absorbing the greenhouse gas carbon dioxide from the air. The increase in carbon dioxide has made the ocean’s pH more acidic, dropping to 8.05 on average. Biologists like Gretchen Hofmann are realizing that this tiny change is hampering the development of hard-shelled marine creatures, leaving them more vulnerable to environmental stressors. To learn more, Hofmann’s team has recreated an acidic ocean in a lab at the University of California, Santa Barbara, and is testing how the change affects sea urchins.
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In a Future Ocean, It Takes a Thick Skin
The next time you pry a clamshell or crack a lobster claw for dinner, pay a small homage. For many ocean creatures with hard shells, growing that armor is taking more effort than ever. These organisms, called marine calcifiers after the minerals they employ during shell assembly, are struggling in our changing climate. The changes are delivering a one-two punch that’s affecting everyday affairs like growth, development, feeding, and even survival itself. The changes also impact the myriad creatures that depend on marine calcifiers.
Punch one is ocean warming. Like air temperatures, ocean temperatures have increased since humans began burning fossil fuels, which releases excess carbon dioxide (CO2) into the air. Even small differences in temperature can stress many marine species.
Punch two is ocean acidification, which some scientists call “the other CO2 problem.” Climate change is causing seawater to become more acidic, much like squeezing lemon juice into a bowl of soup. About 30 percent of the carbon dioxide released by human activities over the past 200 years has already dissolved into the ocean, and much more will ultimately end up there.
Once sea-bound, the carbon dioxide sets off a chain of reactions that alters the ocean’s pH, a measure of how acidic or basic it is. The pH scale runs from 0 (highly acidic) to 14 (highly alkaline). So far, the excess carbon dioxide has decreased the ocean’s pH from about 8.2 in preindustrial times to about 8.05.
Small pH changes can cause big shifts for ocean creatures. “The organisms that live in a particular medium—seawater, freshwater—adapt to living in the pH zone that they evolved in,” says marine biologist Gretchen Hofmann. “What we’re seeing now is a little bit alarming. Predictions are that the pH of the ocean could change very rapidly over the next 100 years. And we’re not really sure those organisms have enough time to adapt to these rapid changes in ocean chemistry.”
Testing the Waters
If humans continue to burn fossil fuels at the current clip, the ocean’s pH will drop to 7.8 in 100 years. To test what this future might hold, Hofmann has built a model ocean at her lab at the University of California, Santa Barbara (UCSB). Hofmann’s future ocean fills large white plastic buckets fed by seawater piped from the Pacific Ocean. Compression tanks bubble extra carbon dioxide into the mix. By studying how marine calcifiers grow in these waters, the UCSB team can predict the creatures’ fate a century from now.
The future ocean’s main residents are sea urchins collected from the California coast. These spiny balls are the lab mice of the ocean—the best-studied marine invertebrate that exists. “Right now we are looking at embryos and larvae of several different urchin species that we have culturing in our facility here,” says Hofmann. “We’re asking whether or not the embryos can still make their calcium carbonate skeleton under these new conditions.”
Ocean creatures with external skeletons rely on charged molecules, or ions, of carbonate (CO3), which are abundant in seawater. Inside their bodies, they merge the carbonate with calcium to manufacture their hard parts, a process called biomineralization.
When carbon dioxide increases in the atmosphere, a series of chemical changes lowers the amount of carbonate available to marine calcifiers. The carbon dioxide dissolves into the ocean and reacts with water (H2O) in a way that frees up atoms of hydrogen. The more free hydrogen atoms a fluid has, the more acidic it is. The free hydrogen atoms soon bond with carbonate ions, forming other types of chemicals. This process leaves fewer carbonate atoms for marine calcifiers to use during body assembly.
Making the Most of It
As Hofmann’s urchins develop from larvae to adults, her science team takes digital baby photos, hundreds of them. The team measures body size, shape, and proportions in each. The pictures are revealing that the ocean 100 years from now will produce smaller, stumpier young urchins. “Most of the metrics are shrinking by about 10 to 15 percent,” says Hofmann. “They can still make their skeleton. But they are just not quite normal. They don’t swim normally. And they don’t tolerate temperature as well.”
Rob Dunbar, a Stanford University climate scientist and oceanographer, explains that calcifying creatures only have so much energy to go around. “These organisms tend to expend as little energy as possible to get to the end goal of having a shell. They have evolved to be that way,” he says. “When ocean water is less saturated with carbonate, that means it takes that organism more metabolic energy to build its shell.” That leaves less energy for other efforts, like swimming and searching for food.
Such handicaps don’t play well in the competitive ocean. “If you’re a marine plant or animal and your growth rate slows down a little bit, you may be overtaken by somebody else and effectively removed from the community,” says Dunbar. In a highly acidic ocean, he says, “it seems certain that we will see some shifts in the kinds of organisms that we find living in different environments.”
The Ocean Outlook
How might sea urchins fare in a future ocean? “I think at this point the jury is still out,” says Hofmann. “But if the larvae are smaller and they swim differently, they may become smaller adults in the future.” Since many cultures eat sea urchins, changes in the size or abundance of the urchin could affect the life and livelihoods of people who catch, sell, and eat them. The same logic follows for other marine calcifiers we eat, such as shrimps, lobsters, crabs, clams, and oysters.
To marine biologists, a graver concern is the impact of ocean acidification on corals, the architects of the ocean. An estimated nine million marine species rely on complex coral reef structures for food or shelter. Climate scientists have long predicted that decreasing ocean pH would slow reef-building, but only recently are biologists acquiring enough evidence to see such a trend. One example is a recent study of Australia’s Great Barrier Reef by the Australian Institute of Marine Science, which determined that the reef’s massive Poritescorals are growing slower than they have at any time in the last 400 years. A double-jeopardy combination of warming ocean temperatures plus acidifying waters seems to be putting the coral—and the species they support—at risk.
A Future Ocean?
Despite the grim possibilities, Hofmann is hopeful. She points out that not every calcifying organism will lose in the acid ocean of the future. In the upper levels of the ocean, phytoplankton—microscopic plants that form the basis of the marine food web—may actually flourish, because they process carbon dioxide to grow. The same is true for marine plants such as sea grasses.
Furthermore, Hofmann has confidence in humanity’s ability to change our situation. More and more, government agencies are turning their attention to ocean acidification. In October 2008, for example, the National Oceanic and Atmospheric Administration and the National Science Foundation convened the first major nationwide study of how ocean acidification might affect fisheries and marine resources.
“We are incredibly good at modifying our environment,” says Hofmann. “We have changed it now in such a way with our technology that we have just released too much carbon dioxide into the atmosphere and we continue to do it. But we also have the technology and the ability, and if we have the will, to go the other way.”