[USA] Plankton — tiny organisms drifting in the sea — often are too small to see without a microscope. But with the help of some math and a very powerful imaging device, scientists for the first time have identified a species of plankton from space. Finding out which plankton are proliferating can help researchers learn more about toxic threats in the ocean. For instance, it might help determine if your nearest beach should be closed owing to poisons shed by those microbes.
Plankton in the ocean often bloom — undergo short periods of rapid reproduction. The tiny organisms can increase so quickly that they form a mass big enough to change the color of the water. Affected water can turn red, brown or even green. Regardless of the color, all of these blooms are still called “red tides.”
Some blooms can prove harmful to the environment. They can reduce the amount of sunlight that reaches other organisms. And they can deplete waters of the oxygen that fish and other species need to survive. But some blooms are particularly dangerous for people. That’s because these plankton make a toxin, or poison. Toxins can kill fish, make it hard for some people to breathe and taint the shellfish destined for dinner tables. When health officials are uncertain which species of plankton is behind a bloom, many play it safe and close beaches.
Because of red tides’ potential risks, scientists try to keep track of plankton blooms. Usually, this search takes place at sea. “We go out on a ship and take buckets of water,” explains Heidi Dierssen. She works at the University of Connecticut in Storrs. There, she studies ocean optics — the light coming off the water surface — to gauge what’s living in it.
Dierssen and her colleagues collect water samples from Long Island Sound, the water separating New York and Connecticut. It’s an estuary, where fresh and saltwater mix. Back in the lab, her team uses microscopes to scout for plankton. But because the researchers head out only once a month, they usually miss sudden blooms, even huge ones.
A lucky strike from space
But on September 24, 2012, “one of our colleagues had been out to sea, and she had seen this large bloom,” Dierssen recalls. “[She] collected some water for us and brought it back.”
At the same time, Dierssen was looking through images of the ocean captured from space. A camera aboard the International Space Station had snapped pictures of the same area of Long Island Sound — on the same day.
That camera is called the Hyperspectral Imager for the Coastal Ocean, or HICO. This spectrometeranalyzes wavelengths (colors) of light. HICO was designed specifically to study light from coastal areas.
Most imagers in space “see” only about one square kilometer (0.4 square mile) per pixel. Pixels are the tiniest dots of light on a computer screen. Pictures emerge from viewing thousands of pixels or more. The smaller the area represented by each pixel, the more detailed an image will become.
Imagers where each pixel represents one square kilometer per pixel would portray Long Island Sound as a series of large colored blotches. The sound is only 177 kilometers (110 miles) long. Its maximum width is a mere 34 kilometers (21 miles). But HICO was almost 10 times better than those imagers. It could detect changes over areas as small as 0.00011 square kilometer (1,180 square feet) per pixel. Signs of a red ocean could be seen in this far more detailed image. The imager also could detect a wider range of colors than most similar instruments in space.
So when Dierssen studied the image from the space station, she was able to pick out the same red tide that her colleague had just sampled. “We were lucky,” she adds, because HICO “is no longer operational.” It had only been working a short time as part of a test. Another imager also captured the image, but in far less detail.
Seeing yellow, finding red
Green algae and plants possess chloroplasts (KLOR-oh-plasts). These tiny structures turn sunlight into energy. Being fluorescent, chloroplasts absorb some light, then emit a share of it back into space. Most of them emit light that the imager would read as reddish.
But one type does not. These chloroplasts contain phycoerythrin (FY-ko-eh-RITH-rin). This pigment emits light that the imager sees as yellow. HICO detected that color coming off of Long Island Sound.
Based on that yellow “flag,” Diersson and her colleagues could tell which species of plankton had made it:Mesodinium rubrum (MEZ-oh-DIN-ee-uhm RU-brum). M. rubrum is a zooplankton (ZO-plank-tun), a tiny animal that eats algae. And when it does, this animal keeps their chloroplasts, using them to get extra energy from the sun. It was the algal chloroplasts that these plankton had eaten that had emitted the yellowish glow seen from space.
Diersson’s team confirmed the animal’s ID with its microscopes. M. rubrum does not make toxins. So this red tide posed no danger to swimmers, shellfish or human diners.
The researchers also analyzed the genes — segments of DNA that are unique to each species — of the blooming plankton. These confirmed that the species was M. rubrum. The scientists published their findings November 16 in the Proceedings of the National Academy of Sciences.
“We could see the [red tide] from space,” Dierrsen says. “It’s the first time anyone’s ever done that.” While HICO is no longer working, Dierrsen hopes that future satellite sensors will allow scientists to similarly keep a spying eye out for plankton blooms.
Limitations on the technology
The new study “is a solid paper that addresses a current and topical issue,” say Leslie Brown and Gary Borstad. Both work at ASL Environmental Sciences in Victoria, British Columbia, Canada. They look for changes in the environment that can be detected from very long distances, including from space.
Brown and Borstad think it might be too risky to identify red tides solely from space. There needs to be microscopic identification of what’s causing a bloom, they explain, “especially when human health is involved.” But both agree that satellites and other space-based sensors could offer a valuable early warning of what deserves further study.
“Ships miss these big events,” Deirssen says. With spying eyes in space, “We can find out a lot about what’s growing in the ocean. It can help us find out why [plankton] bloom when they do.”
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