Why is so much of the sea so poor in plants? Because it lacks iron, said the late John Martin--and he's just been proved right.
Three years ago 140 biological oceanographers got together in a San Diego hotel for a special symposium convened by their professional organization, the American Society of Limnology and Oceanography. ASLO had sponsored a similar symposium in 1971, when the issue of the day was the slow choking of polluted American lakes by overnourished plant life. Now the issue was no less fundamental and no less politically charged. Why was there so little plant life--so few single-celled phytoplankton--in regions of the ocean where the plankton had all the nutrients and sunlight they needed? Why were there deserts in the ocean where there should have been rain forests?
The question had bedeviled oceanographers for decades. They were gathering in San Diego, though, because one of them claimed to have an answer, and a very simple one. As the symposium participants filed into the conference room, they passed a gray-haired man with a weathered, bearded face, sitting in the back of the room, in the wheelchair to which polio had confined him. Most of the oceanographers didn't look at John Martin, nor he at them. They were skeptical, most of them, and some perhaps were jealous; he was impatient and fidgety. He did not like meetings much.
This past February, three years later to the day, much the same group, only more of them, crowded into another San Diego conference room, standing-room-only out into the hall. Again Martin was the reason for the gathering, but this time he wasn't there. His peers had come together to honor his memory and to hear the results of Martin's last experiment, the one he had been hurriedly planning at the time of his death last June from prostate cancer. It was a first-of-its-kind experiment, done not in the laboratory but in the ocean itself--a controlled manipulation of a small patch of ocean. The results, the leaders of the experiment announced that day, proved unequivocally that John Martin had been right: the reason plants don't grow in vast and apparently fertile swaths of the ocean is that they don't have enough iron.
The idea was not new with Martin--it had been put forth as a speculation and then abandoned 60 years ago--but when Martin resurrected it in the late 1980s it had struck most of his colleagues as counterintuitive, to say the least. Iron is a micronutrient: the amount of it a plant cell needs is tiny compared with the plant's needs for its principal nutrients, nitrogen and phosphorus. But Martin and his colleagues, Michael Gordon and Steve Fitzwater, had shown that all previous measurements of the iron concentration in seawater had been radical overestimates, contaminated by rusty ships and sloppy lab work. When Martin's group tested seawater in a clean room, of the kind normally associated with manufacturing computer chips, they found that the amount of iron in large parts of the ocean--the entire Southern Ocean, the equatorial Pacific, and the Gulf of Alaska--was even less than what plants needed to use up all the nitrogen and phosphorus in the water. And when they took a bottle of that water and fertilized it with a bit of iron, the microscopic plants responded by growing rapidly and turning the water green.
There was enough in that claim for a nice academic debate. It would probably still be sputtering on uneventfully had Martin not raised the stakes. If you could make plankton bloom in a bottle by giving them a tiny amount of iron, he argued, then perhaps you could make them bloom in the ocean, on a much larger scale. And if you fertilized, say, the entire Southern Ocean with a tankerload of iron, then all that luxuriant plant life would draw a whopping amount of carbon dioxide out of the atmosphere-- enough, Martin suggested, to make a dent in the greenhouse effect. Nature had already done the experiment, he claimed: it was cold during the last ice age in part because there was so much less CO2 in the atmosphere, and there was so much less CO2 because stronger winds blowing over larger deserts had fertilized the ocean with iron-rich dust.
Now the debate about what controls phytoplankton growth had become a debate about what controls Earth's climate. That meant it was sure to attract a lot more publicity--and a lot more criticism. At the first San Diego meeting Martin tried to fend off the critics who said his bottle experiments proved nothing: let us test the iron hypothesis in the ocean, he said. The idea did not sit well with many of his critics. Some feared that John Martin's Big Sexy Iron Experiment would suck all the grant money out of a chronically underfunded field; some feared that the experiment might succeed, and nudge the world toward a massive fertilization of the Southern Ocean--a scheme that was abhorrent to most oceanographers, who naturally feel protective of the sea's untouched wildness.
Yet Martin came away from that meeting in 1991 with what he wanted: an endorsement of a small ocean fertilization experiment that would give him a chance to prove himself right--if only he could figure out how to do it. Given that it's so hard to measure iron in seawater, how could he scatter a fine dust of the stuff on a patch of turbulent ocean and then manage to keep track of it long enough to see how the microscopic plants would respond? Martin took his answer from Andrew Watson, a physical oceanographer from the Plymouth Marine Laboratory in England. By tagging the iron patch with sulfur hexafluoride, a harmless chemical that is much easier to detect in water, Watson said, you could keep track of the patch even as the ocean was tearing it apart.
The next question was where to do the experiment. Martin was clear on that: it should be done in the equatorial Pacific, near the Galápagos Islands. The tame ocean currents there would keep the iron patch intact for the longest possible time, and the warm sunshine would enable the plants to respond to the fertilization quickly if they were going to respond at all. Moreover, there was a second good reason to go to the Galápagos. The islands sit in the middle of a band of underachieving ocean that stretches right across the Pacific. Yet immediately to the west of the islands--down current--the plankton bloom profusely, turning the water so green it can be detected by satellite (see illustration). To Martin it seemed clear that iron was getting blown or washed off the Galápagos--that the islands were performing a natural iron-fertilization experiment. By doing his own fertilization near the Galápagos, he could test two ideas on one cruise.
Martin spent two years securing the grant money and organizing the cruise. After his death in June, the leadership was taken over by his colleagues at the Moss Landing Marine Laboratories in California, Kenneth Coale and Kenneth Johnson, and by Richard Barber of Duke University. The Columbus Iselin sailed from Miami on October 11, crammed to the rails with equipment, researchers (23), and computers (51). The ship was so packed you couldn't walk on the fantail, Coale recalls. And it was listing so badly to port that we had to sail without our full complement of drinking water.
The Iselin arrived on-station 300 miles south of the Galápagos on October 23, surveyed the waters for a day and a half, then started dumping iron. For 24 hours the ship steamed back and forth across a five-mile-wide square, taking in seawater, mixing it with iron sulfate, mixing that with sulfur hexafluoride, then flushing the lot into the ship's wake. In 24 hours the researchers dumped just half a ton of iron, spread so fine that its concentration in the water was never more than a few parts per billion. They put a navigation buoy in the center of the iron patch to help the ship stay with the patch as it drifted down current. Then they started measuring the plants' reaction.
It was immediate. Within hours a laser device designed at the Brookhaven National Laboratory on Long Island was showing that the plankton were absorbing more light and photosynthesizing more efficiently than they had been before the iron was scattered. By the next day the fruits of all this efficiency had become apparent: the concentration of chlorophyll--a measure of the amount of plant matter--was twice as high inside the iron patch as outside. You didn't need fancy equipment to see the change, either. I spent a lot of time watching the water, and the color of it was different, says Barber. It was actually visible to the eye. That is, the iron-fertilized water was greener.
By the third day the phytoplankton were photosynthesizing at two to three times their previous rate. A NASA airplane flying over the iron patch and bouncing a laser beam off it found that it could easily detect the lush new growth. IT'S HAPPENING, read the subject heading on the happy E-mail message that transmitted the news via satellite and the Internet to other oceanographers on land and sea. A day later the excited scientists on the Iselin even drafted an E-mail press release. But that idea was vetoed by the keepers of the government purse, who considered it premature.
By last February, however, when ASLO convened again in San Diego, the IronEx Group was ready to go public. One after another they got up to explain their results--but the bottom line was easily stated. The growth rate increases unequivocally confirm the iron hypothesis, said Barber, to a degree that was beyond John's wildest dreams.
Perhaps wildest was an exaggeration. In his wildest moments, after all, Martin believed he had an idea that could help explain the ice ages and combat global warming--although he was always cagey about just how seriously he meant to be taken when he suggested large-scale ocean fertilization. Now that his small-scale experiment has been done, there is less reason than ever to take iron dumping seriously as a greenhouse palliative. The phytoplankton at the Galápagos responded dramatically to their iron supplement, but in the process they drew only a trivial amount of carbon dioxide out of the atmosphere. It was a win-win situation, says MIT oceanographer Sallie Chisholm, who had helped Martin organize the experiment but who shared many of her colleagues' fears about how a successful result might be interpreted by the public. The experiment worked, but not enough to make anyone want to fertilize the Southern Ocean.
Why didn't the iron pull down more CO2? One reason, Barber explains, is that animals eat plants, and in the process they exhale CO2. As the phytoplankton multiplied in the iron patch, so did the microscopic animals--the zooplankton. Says Barber: We added iron, the phytoplankton grew better--and we ended up with a lot of happy little animals.
A second reason the iron didn't have a bigger effect on CO2 is that, being heavy, it apparently sank out of reach of the phytoplankton before they could make full use of it. How would the plants respond, though, to a continuous supply of the stuff? That's the experiment the Galápagos Islands are doing. On the second leg of their cruise, Barber and his colleagues found that the iron concentration in the plankton-rich waters west of the islands was indeed greater than it was to the east, as Martin had predicted. The productivity of the western phytoplankton was twice as great as it was in the researchers' own iron patch. Yet even this continuous input of iron was drawing only a small amount of carbon out of the atmosphere.
So the idea of artificially fertilizing the entire Southern Ocean should probably be considered dead, and things aren't looking too good for Martin's ice-age hypothesis either. But the core of his iron hypothesis-- the statement that the growth of phytoplankton in nutrient-rich regions of the sea is limited by a lack of iron--has been vindicated in the most convincing possible way: by scientific experiment. Every textbook says that phytoplankton in the ocean are regulated by the supply of nitrogen and phosphorus, says Barber. Yet the high-nutrient/low-chlorophyll regions turned out to be a large portion of the ocean. They were an enigma we couldn't explain. Now we've supported an explanation for how a third of the ocean works.
It is not just the content of Martin's hypothesis that will outlive him, however, but the pioneering way he and his colleagues set about proving it. This work opens up completely new horizons for understanding the marine food web, says Chisholm. Before, we always had to put these organisms in bottles. We've never been able to do an experiment in the ocean. That's what's exciting about it. Addressing her assembled colleagues last February, Chisholm made the point more personally. This experiment was John's parting scientific gift to us, she said, and I think it's a gift that will be remembered forever.