Iron Deficiency and Oceanic CO2 Absorption

          As early as the 1930s, the potential role of iron as a limiting factor in phytoplankton productivity in the ocean was known to geochemists but was not understood. The conventional explanation for low phytoplankton productivity in certain parts of the oceans where high productivity would be expected was that grazing by zooplankton kept the phytoplankton ocean plants in check and prevented them from becoming too populous. Few oceanographers were happy with that explanation, which they thought was insufficient to fully account for the failure of phytoplankton in certain parts of the ocean to bloom into huge colonies.

          In the mid- to late-1980s, John Martin, an oceanographer at Moss Landing Marine Laboratories in California, proposed a controversial theory, that a lack of dissolved iron in the ocean kept populations of marine algae lower than normal and that seeding the ocean surface with iron should make phytoplankton multiply dramatically and absorb so much carbon dioxide, which is dissolved in the seawater, that the Earth’s atmosphere might thereby be cooled. Although Martin’s iron hypothesis excited some ocean scientists it caused great controversy as several prominent oceanographers stated that experimenting with the ocean was folly and involved treating the symptom without addressing the root cause, anthropogenic global warming. Other scientists resorted to less enlightening tactics, ridiculing Martin’s ideas as a Geritol solution. To quiet the criticism, Martin proposed something no previous oceanographer had done before: to conduct a laboratory experiment in the open ocean. But that idea proved even more controversial as it was opposed by many leading scientists who thought such a test would be dangerous to the ocean environment.

          The controversy soon became so heated that the National Research Council and the American Society of Limnology and Oceanography held national meetings to hear both sides. If either group decided that Martin’s idea was unethical or lacked scientific merit, Martin would have been denied funding to test the hypothesis. But to Martin’s relief, the scientists concluded that a small-scale experiment in the ocean wouldn’t threaten the environment and that the iron hypothesis should be tested. Although on June 18, 1993, Martin died from prostate cancer that had metastasized throughout his body, his experiment was carried forward in October of that year by Ken Johnson from Moss Landing Oceanographic Laboratories and Dick Barber of Duke University and scientists from several other universities. To the surprise of many oceanographers, the seeding of a 25-square mile ocean patch off the Galapagos Islands with iron crystals resulted in a three-fold increase of the chlorophyll levels in the water. Although it wasn’t the twelve-fold increase Martin had predicted, it was still a vindication of his ideas.

          In June 1995, Johnson and Kenneth Coale from Moss Landing Oceanographic Laboratories and 35 other researchers embarked on a second expedition to the eastern Pacific to replicate their first experiment. The team reapplied iron crystals twice and the results were dramatic. The scientists observed a 30- to 40-fold increase in chlorophyll levels, well beyond Martin’s prediction of a 12-fold increase. They also determined that particular characteristics of an overlying layer of water in the first experiment off the Galapagos Islands had reduced the effects of the iron. John Martin’s pioneering work had been vindicated.

Several recent research efforts by Kenneth Hutchins and David Bruland, beginning in the late 1990s (Nature, vol. 393, pp. 561-564, 1998) extended Martin’s ideas by attempting to determine exactly how iron deficiency in certain coastal and open-ocean areas inhibited the ability of sea water to store carbon dioxide. They knew that under ordinary conditions, iron enables phytoplankton to draw carbon dioxide from the atmosphere. That process enables ocean water to absorb about 80 times more carbon dioxide than is found in the atmosphere. However, in the absence of iron or with inadequate amounts of iron, that absorption process no longer is effective. As a result, phytoplankton growth is disrupted and the marine food chain is decimated from bottom to top, affecting life as small as bacteria and as large as whales.

          What Hutchins and Bruland found was that although those central California waters adjacent to the Big Sur in the Monterey Bay National Marine Sanctuary (later research confirmed their findings in coastal Peru and the Bering Sea) are rich in such plant “fertilizers” as nitrate, silicate, and phosphate, they contained insufficient iron to enable phytoplankton to use those nutrients through photosynthesis. Since iron-starved phytoplankton populations are unable to photosynthesize efficiently, the entire food chain that uses plankton in various ways as a type of sea-fodder is negatively affected. As a result, less food and energy are available to support predators large and small, across the gamut from cod, dolphin, tuna, marlin, sharks, and whales all the way to humans. That research is important because if near-shore waters fail to effectively absorb carbon dioxide owing to a lack of iron, geoscientists may need to revise existing carbon-cycling models and global climate-change models. Their work specifically demonstrated that iron availability controls the Si:N and Si:C ratios of diatoms, a finding that has been confirmed by many researchers working all over the world. That research has important implications for fields as diverse as algal physiology, carbon and nutrient biogeochemistry, global climatology, and paleoceanography.