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Science 15 April 2011:
Vol. 332 no. 6027 p. 292
NEWS & ANALYSIS
RADIOECOLOGY
[Fukushima Radiation Creates Unique Test of Marine Life's Hardiness]
Sara Reardon
When radiation readings from water monitors around the leaking Fukushima Daiichi plant began rising this month, spiking at 7.5 million times Japan's legal limit for radioisotopes in public water, government agencies reacted sharply. For the first time in history, the Japanese government set a limit on radiation in seafood and began screening fish. India and China recently banned imports of food products from certain areas of Japan, and the U.S. Food and Drug Administration began conducting its own radiation screens on seafood imports.
Monitoring food makes sense, according to radiation geochemist William Burnett of Florida State University in Tallahassee, but he warns against overreacting to the perceived risk of ocean contamination. “Stopping eating sushi, that's crazy,” he says, based on what is known about isotope accumulation in marine animals and reports on radiation in the sea near Fukushima. Although radioactive elements could spread quickly through the food chain, Burnett and others say it's unlikely that would lead to a significant risk for humans. What it could mean for ocean-dwelling organisms is another matter—one that scientists are eager to investigate.
Many marine species are good at absorbing radioactive isotopes from the water, and experts across the world are beginning to prioritize studies they'd like to do in Japan once the situation stabilizes. “It's another opportunity to study impact on the environment, and many countries will use this opportunity,” says marine radioecologist Bruno Fievet of the French Institute for Radiological Protection and Nuclear Safety (IRSN) in Cherbourg. “Probably we will see traces of this accident for many years,” he says.
Little is known about the impact of radioactivity on many marine organisms, in part because they are remarkably resistant. Fievet says that a deadly dose of radioactivity for crustaceans and mollusks is orders of magnitude larger than a lethal dose for humans, which makes it dangerous and inefficient for researchers to conduct lab experiments. Harming organisms in the lab, Fievet says, requires levels of radioactivity that are “very, very high compared to what is observed in the marine environment at Fukushima.”
The creatures' simple physiology and the salty environment also offer them some protection. Just as potassium iodide tablets can saturate the human thyroid and prevent radioactive iodine from binding, the abundant ocean salts reduce the absorption of radioactive ions by sea creatures.
At the same time, Fievet adds, just because there's no immediate hazard is “not a reason not to worry.” Water close to the plant contains particulate radioisotopes, including cesium-137 and iodine-131, which organisms can incorporate into their bodies through their skin or food. Cesium-137 is more worrisome because its 30-year half-life means that it will be around for many decades. The resulting internal dose could induce mutations, stunt growth, and cause reproductive defects. Some organisms are exceptional at taking up isotopes and concentrating them in their bodies. Larger organisms that eat them may receive a high dose of radiation all at once, possibly increasing the risk of harm.
In terms of cleaning up the ocean, however, this bioaccumulation could be a boon. Phytoplankton, the base of most marine food chains, can concentrate plutonium a million-fold as it sticks to the surface of their membranes. Larger organisms that eat phytoplankton can't assimilate plutonium, so they excrete it as concentrated fecal matter that sinks to the bottom of the ocean and away from the vast majority of marine life near the surface. Similarly, brown seaweed, a staple of miso soup and sushi bars, acts as an iodine sponge, concentrating the element 10,000-fold compared with the surrounding water. Some researchers have even proposed planting the ocean full of brown seaweed to soak up radioactive iodine, but Fievet says this idea has been more or less rejected because it would simply transfer the problem from water into seaweed.
Most researchers believe that in the end the ocean will take care of itself through its vast size. Off the coast of Japan, the sea floor drops quickly, providing a large dilution factor. The International Atomic Energy Agency (IAEA) reports that over a span from the coast to 31 kilometers out, radiation drops 1000-fold. After dilution, says marine biologist Nicholas Fisher of Stony Brook University in New York, added radiation quickly becomes indistinguishable from the natural background level. “The bit introduced by man is a drop in the bucket,” he says.
Fievet says that IRSN scientists have begun formulating studies they'd like to undertake with collaborators in Japan. U.S. radioecologists from a number of U.S. universities, the U.S. Department of Energy, and IAEA's Marine Environment Laboratories have also expressed interest in beginning long-term studies. “We are faced with a unique situation” in which an accident is having a direct impact on the sea, says Dominique Boust, director of IRSN's radioecology lab.
In the meantime, Fisher says, “While it's nice to know the isotope concentration in water, it's more critical to know what it is in marine organisms.” The National Research Institute of Fisheries Science in Japan has begun screening a number of species and has found elevated cesium-137 in a few fish. Even so, research from weapons testing and the Chernobyl accident found that large fish that accumulate cesium-137 excrete it over time. Compared with other toxins that humans put into the marine environment, researchers conclude, the danger to Japan's fish from the radiation spill is very low.
