THE LOST OCEAN

THE LAST OCEAN

LOSERS AND WINNERS. More acidic waters could be tough on the tiny coccolithophore Emiliania huxleyi (left), which builds a shell of calcium-carbonate platelets; but comfy for nitrogen-fixing cyanobacteria such as Trichodesmium (right).Björn Rost; David Caron/Univ. of Southern California Adding heat Increased CO2 also means the corals will have to contend with temperature increases. Depending on the coral species and the place, 3 to 4 weeks of temperatures a degree or two Celsius above current summer peaks can turn a reef into a spooky white sculpture of itself. This bleaching comes from the breakdown of the partnership between warm-water, soft-bodied corals and their colorful live-in algae, or zooxanthellae. They photosynthesize, and the host corals take a share of the lunch. Sometimes the partners get together again after a bleaching break-up, but prolonged absence of zooxanthellae kills a shallow-water coral. Studies of zooxanthellae during the past decade have revealed unsuspected variety in the alga's capacity to endure heat. Corals primarily colonized with a variant called the D strain withstand heat better than others, according to Ray Berkelmans of the Australian Institute of Marine Science in Townsville. Researchers including Andrew Baker of the University of Miami in Florida are working to develop reef-saving therapies that swap out fragile zooxanthellae strains for heat-savvy ones. The strategy doesn't brighten Hoegh-Guldberg's view of coral futures if carbon emissions keep soaring. Heat waves have bleached corals widely in recent years, but Hoegh-Guldberg hasn't seen the zooxanthellae adapting naturally. "Everyone's had enough time to show magical adaptation of corals," he says. Another hope for adaptation swirls through conversations about coral reefs, but it doesn't cheer Hoegh-Guldberg either. Atmospheric carbon dioxide has spiked and ocean pH has plunged before in Earth's history. So the question arises of whether corals could just do whatever it was they did to survive last time. "That's crap," says Hoegh-Guldberg. Ancient corals would have had more time than today's to get up to speed on hot, lower-pH life, he says. Again he flips open the Science paper and jabs a finger at some data. He and his colleagues used published measurements from air bubbles trapped in ancient ice to calculate rates of change for CO2 concentrations in the atmosphere. The concentrations have risen more than 1,000 times faster per century during the industrial revolution than during the previous 420,000 years, the team concludes. Also, Hoegh-Guldberg says he's not convinced that calcifying organisms did manage to laugh off earlier planetary burps of greenhouse gases. During the early Triassic, for example, CO2 concentrations reached levels five times as high as today's. He notes a gap in the fossil record during this time of evidence for both the reef-building corals and the algae that sculpt carbonate. Some lineages of today's corals are ancient enough to have survived hot spells with funky ocean chemistry. Yet those lineages that survived may have done so without calcified skeletons. "They essentially became anemones," he says. That's survival for lineages that can do it, but it's still not a happy ending to Hoegh-Guldberg. Even if all today's corals successfully turned into naked, soft-bodied bits—more magic adaptation perhaps—other reef species would still end up homeless. The intricate crags and crevices of reefs shelter much of the biodiversity of oceans, perhaps a million species. Without complex reef habitats built by corals, it will be a simpler ocean, he says. Floating hubcaps Beings smaller than corals, some of the mere specks of life that drift in the seas as plankton also need calcium carbonate to build. Microscopic coccolithophores, up until now not exactly famous, have become iconic in the study of ocean pH change, thanks to Ulf Riebesell of the Leibniz Institute of Marine Sciences in Kiel, Germany. The celebrity plankton look like a craft project of hubcaps welded around a giant beach ball. The ornate hubcaps, platelets made of calcium carbonate, enclose a photosynthetic cell. Springtime blooms of coccolithophores such as Emiliania huxleyi can spread over an area the size of Ireland. Light glinting off all the platelets makes milky blue streaks in the sea visible from space. E. huxleyi doesn't follow the corals' recipe for calcifying structures. Yet the coccolithophores also fail to grow normally in low-pH seawater, says Riebesell. In experiments simulating such water, he's seen runt cells with flimsy or even deformed platelets. Growth anomalies are showing up in other marine builder species, such as oysters. And in one of the few studies focusing on larvae, Gretchen Hofmann of the University of California, Santa Barbara, reports difficulties for very young sea urchins. Normal larvae look like alphabet soup "A's." In seawater dosed with extra CO2, though, the larvae grow "shorter and stubbier," she says. Outside the shell Much of the first wave of research on the next ocean has focused on the future of calcification. Not that that's silly. Creatures accounting for 46 percent of the annual U.S. seafood catch form some kind of calcified structure, such as clam shells, says Scott Doney of the Woods Hole Oceanographic Institution in Massachusetts. Adding in species that eat the calcifiers, such as pink salmon fattening up at sea on swimming snails called pteropods, would boost the percentage. Still, water chemistry could affect uncalcified aspects of life for marine species, and research is now branching out into these matters. For example, moving around seems to get more difficult for squid in lower-pH water, according to ongoing research by Brad Seibel of the University of Rhode Island in Kingston, and others. The dip in seawater pH disturbs the oxygen transport in squid blood, and squids get sluggish. That odd future ocean means good news for some species, particularly among the noncalcifiers, says David Hutchins of the University of Southern California in Los Angeles. Nitrogen-fixing cyanobacteria grow better in experiments that mimic ocean acidification. "They really love the CO2," he says. The cyanobacteria's cells, such as those in a Trichodesmium species, don't transport CO2 efficiently from the outside world to their internal energy trapping machinery. A richer mix of the gas outside makes the cells more productive. Who flourishes and who fades among the plankton in the new ocean matters to bigger creatures. The marine grazers that feed on plankton prefer some kinds and shun others. If the plankton equivalent of broccoli gives way to a brussels sprouts equivalent, grazer populations change too. Preferences work their way up to top predators, including those on dry land about to pick up a fork. Considering lab and field experiments simulating future oceans, Hutchins speculates that plankton shifts will mean more microbial predators and less fish in the future oceans. "It's not necessarily going to be a world we particularly like," he says. Whether kelps will like it remains to be seen. Kelp biologist Klinger emphasizes that she's just getting started in answering this question. She puts in a plug for the importance of understanding what will happen to kelp. Much like reefs, clusters of fronds offer complex habitats, with hidey-holes for fish and highways for snails. Also one could argue that a future ocean would be a little less interesting without kelp sex.