The Light at the Bottom of the Ocean; Oceanographers Struggle to Explain a Strange Glow from Seafloor Vents

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Oceanographers struggle to explain a strange glow from seafloor vents

Thousands of meters below the ocean waves, an exotic source of light shines softly in the stygian depths. Though imperceptible to human eyes, this pale illumination has nonetheless served as a beacon, drawing biologists, physicists, and chemists down to the seafloor in search of an explanation for the enigmatic shimmer.

Discovered 8 years ago, the light issues from ridge-top vents that spew out volcanically heated brines laden with metals and caustic compounds. At first, scientists attributed the light to thermal radiation emitted by the 350#161#C water, much as hot electric elements on a stove give off an angry orange glow.

But recent measurements conducted from the submersible Alvin reveal that thermal radiation alone cannot explain the light. What's more, the new data suggest that there might be enough light to power photosynthesis on the ocean floor-which would make it the first known case of photosynthesis divorced from the sun's rays.

"They've got some very funny light down there. There are some very odd and interesting things going on, and we do not understand them," says Euan G.

Nisbet, a marine geologist at Royal Holloway College in Egham, England.

Cindy Lee Van Dover, a marine biologist at the University of Alaska, Fairbanks, first found evidence of the light in the late 1980s, while studying a seemingly blind species of shrimp, Rimicaris exoculata, discovered just a few years earlier. These shrimps swarm around hydrothermal vents on the volcanically active mid-Atlantic ridge, which forms part of an underwater mountain range encircling the planet like the seam on a baseball.

Oceanographers had named the shrimp exoculata because it appeared to lack eyes, but Van Dover and her colleagues discovered that the animal actually does have vision organs, just not in the usual place. Instead of eyes attached to its head, R. exoculata has evolved oversized light-sensing patches on the back of its shell (SN: 2/11/89, p. 90).

Suspecting that the shrimp uses these eyes to see light coming from the hydrothermal vents, Van Dover, Milton Smith, and John R. Delaney of the University of Washington in Seattle and their colleagues made a rough attempt to take digital pictures of the hot plumes. Although far from ideal, their camera recorded an extremely faint glow coming from the vents. The scientists surmised that the light came from the intense heat of the vent water.

As news of the light spread, physicists and chemists started proposing more extraordinary explanations for the radiation. Their ideas propelled Van Dover to probe deeper into the question by collaborating with geophysicist Alan D.

Chave of the Woods Hole (Mass.) Oceanographic Institution and astrophysicist J. Anthony Tyson of AT&T Bell Laboratories in Murray Hill, N.J. Although they could not secure any funding to study the light, the scientists jury-rigged a photometer that measures radiation at four different frequencies, ranging from visible red light into the invisible near-infrared. Piggybacking their experiment on unrelated Alvin dives, Van Dover managed to collect some data during free moments at the tail end of expeditions to hydrothermal vents in the Atlantic and Pacific Oceans.

The scientists found more light coming off the vents than they could attribute to thermal radiation alone. At one location, they recorded 19 times the expected amount of visible red light and shorter-wavelength infrared.

At another location, the photometer measured more light 10 centimeters above the vent, where the water is cooler, than at the blistering opening of the vent, report Van Dover, Chave, Tyson, and physicist George T. Reynolds of Princeton University in the Aug. 1 Geophysical Research Letters.

These observations suggest sources of light potentially more important than thermal radiation, contend Van Dover and her colleagues. …