Humanity's extra CO2 could brew a new kind of sea
Susan Milius, Science News 15 Mar 08;
Terrie Klinger is starting to wonder about the future of kelp sex. It's a delicate business in the best of times, and the 21st century is putting marine life to the acid test.
Klinger, of the University of Washington in Seattle, studies the winged and bull kelps that stretch rubbery garlands up from the seafloor off the nearby Pacific coast. These kelp fronds do no luring, touching, fusing of cells or other sexy stuff. Fronds just break out in chocolate-colored patches.
The patches release spores that swim off to settle on a surface and start the next generation. The new little kelps don't look as if they belong to the same species, or even the same family, as their parents. The little ones just grow into strings of cells, but these are about sex.
"Those of us who have spent far too long looking at this can tell the males from the females," says Klinger. The subtly female-shaped filaments form eggs and release kelp pheromones to call in the male filaments' sperm.
Sex filaments have kept kelp species going for millennia, but Klinger says she wants to know what's happening now that carbon emissions are changing seawater chemistry. The intricate reproductive cycle of kelp is an example of a delicate system that can experience big effects from seemingly small changes in ocean chemistry.
This chemistry is already shifting, powered by the increased concentration of carbon dioxide in the atmosphere from human activity. Not all the carbon dioxide from burning fossil fuels stays in the air. The oceans have absorbed about half of the CO2 released from burning fossil fuels since the beginning of the industrial age, says Richard Feely of the National Oceanic and Atmospheric Administration in Seattle. The ocean takes in about 22 million tons of CO2 a day, he says.
The influx causes what scientists call ocean acidification. It's a term of convenience. The ocean isn't acid now, nor do Feely and other ocean chemists expect that seawater will become acid in the foreseeable future. However, the extra CO2 is driving the oceans closer to the acidic side of the pH scale. By the end of this century, Feely says, the upper 100 meters or so of ocean water will be more acidic than at any time during the past 20 million years.
Klinger is just one of the biologists trying to figure out what a shift in seawater chemistry will do to seaweed, corals, fish, and other marine life. The filaments of both bull and winged kelps grow noticeably slower in acidic seawater, she reported last week at the 2008 Ocean Sciences Meeting in Orlando, Fla.
Biologists are discussing what the chemistry change will do to marine creatures: It looks like bad news for calcium users and a new dawn for slimy rocks. It could begin an age of simplification for ocean ecosystems. Either way, there's a rising consensus that, by changing the oceans' chemistry and biology, burning fossil fuels is essentially making new oceans.
Sea change
Researchers say the oceans of today already register a chemical change, though it may sound deceptively small at first.
Feely now rates the upper layer of seawater on average at 8.10 on the pH scale. That scale goes from 14 to 0 and describes the increasing concentration of hydrogen ions. Plain water, defined as neutral, ranks as 7, and lower numbers indicate increasingly strong acids and larger numbers of hydrogen ions. Since the beginning of the industrial age, Feely says, the seawater pH has slipped about 0.11 of a pH unit.
That's a considerable change, says a 2005 report on ocean acidification from the United Kingdom's Royal Society. The pH scale works logarithmically, so 7 means 10 times more ions than 8. The industrial age has increased the concentration of hydrogen ions by roughly a third.
The pH change from this century could be even bigger. The business-as-usual scenario for carbon emissions will drive the pH of the ocean surface waters down another 0.3 to 0.4 units by the end of the century, says Feely.
That's still not acidic, though. To push the ocean pH below 7, models predict that people would have to burn all of the fossil-fuel carbon on the planet plus a good deal of methane hydrates, he says.
Still, describing the process as ocean acidification isn't wrong. Seawater is acidifying in the sense of creeping toward the acid zone on the scale. Even if the ocean isn't turning into lemon juice, biologists predict that smaller dips in pH could do big things to marine life. It's a peril humans easily fail to appreciate. We can bathe in milk (pH 6.7) or chug orange juice (pH 3 or 4) and call ourselves refreshed. Thanks to fancy protective coatings, such as skin, and robust physiological mechanisms, a milk-soaked juice drinker's blood still hovers around pH 7.35 to 7.45. But our bodies don't have to build coral reefs.
Marine species from corals to snails to floating dots of life called coccolithophores create structures of calcium carbonate. A CO2 boost makes this job harder.
A key ingredient in making calcium carbonate is the carbonate ion, CO3–2. When it reacts with water, CO2 forms carbonic acid, H2CO3. "It's the same as adding CO2 to pop to make it fizzy," says Feely. The carbonic acid dissociates, releasing hydrogen ions that react with the carbonate ions in the water—thus making them unavailable to calcifiers such as corals building reefs. Feely says the carbonate concentration in the warmer waters where corals live today has already decreased 16 percent since the preindustrial era.
Not-ok coral
The future of corals depends on just how much CO2 ends up in the atmosphere, says Ove Hoegh-Guldberg of the University of Queensland in St. Lucia, Australia. During a conversation in Boston last month at the annual meeting of the American Association for the Advancement of Science, he refers to his most recent paper. In the Dec. 14 Science, he and 16 other scientists summarize their predictions of three possible futures for corals.
Hoegh-Guldberg flips to a triptych of photographs of coral reefs. In the first, multicolored fish swim over a mosaic of nubby tan and brown corals crowding against each other, the classic postcard of a diverse reef. The scene represents a world where humanity freezes carbon emissions now. The CO2 in the air stabilizes at its current concentration of 380 parts per million (ppm). Some changes for ocean ecosystems are already inevitable, but for most of the world's current reefs, corals will remain the dominant species.
The second image represents the world with atmospheric CO2 concentrations bumped up to between 450 and 500 ppm. Swaths of ocean once hospitable to reefs become so starved of carbonate that more and more corals in the upper 100 meters or so of water can no longer add to their skeletons. The colorful fish have dwindled as the crumbling reef no longer offers them habitats. Big, shaggy species of macroalgae muscle in over the diminished corals, making it ever more difficult for coral larvae to find a home.
The last image, for the 500-plus ppm world, shows a murky slope of eroding rubble. It doesn't actually have an old tire in it, but that's the mood. As Hoegh-Guldberg puts it, "You've got slimy rocks."
This ocean could be real by the end of the century. Even one of the more optimistic scenarios from the Intergovernmental Panel on Climate Change puts the atmospheric concentration of CO2 at 550 ppm in the year 2100.
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.
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