Botanists using small submarines to prowl the inky depths of the Atlantic Ocean off the Bahamas have found plants growing deeper than any place known on Earth -- on the bottom nearly 900 feet below the surface.

Until now, scientists believed that plants could not grow more than about 690 feet under water because too little sunlight penetrates beyond that depth.

The plants are a newly discovered species of maroon-colored algae that grows in patches, rather like lichens, and uses sunlight to power photosynthesis, the process by which plants combine water and carbon dioxide into carbohydrates.

The plants belong to a group of related species that extract dissolved calcium from sea water and process it into calcium carbonate, or limestone, to line the cell walls. Over the millennia, generations of limestone-forming algae -- like corals, which are animals -- build vast reefs.

The botanists said the plants are growing abundantly in a light level that, to the human eye, seems "pitch black." This is 0.0005 percent the amount of light that strikes the ocean's surface.

Because the plants can grow in near darkness, using light levels only 1 percent of the dim amount that sustains their shallow-water cousins, the researchers said they suspect that the deep plants employ a previously unknown form of photosynthesis 100 times more efficient than known processes.

"It's likely that these plants exist in many parts of the world and that they play a much larger role in the reef-building process than we ever thought," said Mark M. Littler, who made the discovery along with his wife, Diane S. Littler.

Both are algae specialists at the Smithsonian Institution's National Museum of Natural History. He is chairman of the botany department, and she is a research associate.

Littler said the discovery was made last October as he and his wife were exploring off the Bahamiam island of San Salvador in a newly developed submersible craft called the Johnson Lea Link I. The sub, which carries two crew members and two scientists, was developed by Edwin Link, who designed the Link trainer, a flight simulator for aircraft pilots.

The multimillion-dollar sub, equipped wtih a special magnifying videotape camera, sensors to measure light levels and claws for picking up specimens and putting them in sealed compartments, was built by the Harbor Branch Foundation of Fort Pierce, Fla., with funding from Robert Wood Johnson, the Band-Aid magnate.

"Johnson financed the submersible because he was convinced it would lead to new and important discoveries," Littler said. "We were invited to use the sub and, as far as I'm concerned, Johnson was right. This is a very exciting discovery. It's going to force us to rewrite the textbooks on marine ecology."

The algae were on the 45-degree slope of an uncharted seamount, growing to a maximum depth of 884 feet.

Higher up the slope, Littler said, the couple made other significant discoveries. They found another species of lime-producing algae, known as the source of most of the Caribbean's beach sand, growing in deeper water than had been known.

On the seamount's flat top, 265 feet below the surface, they found a community of at least 11 plant species arranged somewhat like those of a tropical forst, with the most light-demanding plants forming an upper canopy and more shade-tolerant species in three lower tiers.

Even on the sunniest part of the seamount, Littler said, the light was a mere 1 percent of the amount striking the surface.