Microorganisms create a line in the ocean.

If you flew over the Pacific Ocean about 100 miles north of the equator, roughly halfway between Hawaii and Tahiti, you might notice something resembling a deep green snake hundreds of kilometers long and less than 10 kilometers wide floating in the sapphire sea. Researchers recently identified this mysterious line as a surprisingly rich collection of single-celled diatoms, a species of the phytoplankton Rhizosolenia.

Sailors and astronauts have reported seeing the green streak, but few oceanographers even knew it existed, asserts James A. Yoder of the University of Rhode Island in Narragansett. He and his colleagues analyzed water samples as well as photographs taken from the space shuttle Atlantis in 1992, satellites, aircraft, and a ship, they report in the Oct. 20 NATURE. They did the work as part of the long-term Joint Global Ocean Flux Study.

Coastal waters commonly harbor unusually dense patches of plant or animal life, Yoder notes. Also, sargassum (gulfweed), jellyfish, and anything else that floats will line up along a current boundary, such as the edge of the Gulf Stream. However, rarely do such living lines form in the open sea.

The diatoms creating the streak in the Pacific congregate and thrive at the convergence of the North Equatorial Countercurrent and the cooler, denser water of the South Equatorial Current sinking below it, the scientists assert. The microorganisms feed in the cooler waters, then float up to the sunlight.

Studies by Tracy A. Villareal of the University of Massachusetts at Boston show that phytoplankton "adjust their buoyancy and migrate vertically between surface waters and nutrient-rich waters deeper in the water column," Yoder and his colleagues note.

No other research has demonstrated on such a large scale how the diatoms' buoyancy influences their concentration, Villareal says.

So-called instability waves, which run east to west, propagate along the boundary of the two currents and probably help the diatoms to accumulate, Yoder suspects. The line appears to form only during summer and fall. At other times, the organisms disperse.

The team speculates that the same mechanisms that help these diatoms gather may have caused a similar accumulation that geologists found recently in deep-sea sediments laid down 4.4 million to 15 million years ago in the same area.