Grow in the Dark: Bottom-dwelling bacterium survives on geothermal glow

A microbe discovered in the deepest, darkest reaches of the Pacific Ocean makes its living in an unlikely way—by photosynthesis. The newly described species, announced in the June 28 Proceedings of the National Academy of Sciences, uses faint light emitted by deep-sea hydrothermal vents to power its metabolism.

A host of weird creatures lives at these vents, often called black smokers, where volcanically heated fluids gush from Earth’s crust (SN: 11/24/01, p. 331: Available to subscribers at Tube worms like it hot, but larvae not). But the microbe, the only photosynthetic organism in nature known to use a light source other than sunlight, may be among the most unexpected.

“It expands our vision of possible environments where you could have photosynthesis,” says biochemist Robert Blankenship of Arizona State University in Tempe.

He and his colleagues cultivated what Blankenship describes as a “beautiful, emerald green organism” from water they’d collected from the vents of the East Pacific Rise, which lies 2,500 meters underwater off the Mexican coast. They’d begun searching for deep-sea photosynthesizers a decade ago, after coauthor Cindy van Dover of the College of William and Mary in Williamsburg, Va., discovered that hydrothermal vents emit small amounts of light.

Just as hot electric-stove elements radiate light, black smokers glow dimly as 400°C fluid emerges. Most of the glow lies in the infrared spectrum, which the microbes can’t absorb, but part of the light reaches the edge of the visible spectrum. The microbes appear to eke out a living from that light, which the researchers can see only through night goggles.

According to DNA analysis, the bacterium, designated GSB1, belongs to a group of organisms called green sulfur bacteria. These photosynthetic microbes use sulfur in their metabolism and require low-oxygen conditions. Conveniently, vent fluids tend to be sulfur-rich and oxygen-free.

All green-sulfur bacteria sport sophisticated light-gathering molecules. “They’re the champions of low-light photosynthesis,” Blankenship says. “It makes sense that these would be the group that you’d find down there in that environment.”

Researchers have speculated that organisms first developed photosynthesis near hydrothermal light, not sunlight, says John Allen of Queen Mary University of London. “Photosynthesis might be much older than most people think,” he says.

But DNA analysis suggests that the newly discovered bacterium is a cousin, not a forefather, of modern green-sulfur bacteria that live in sunlit, low-oxygen marine environments. Because all these bacteria are better suited to collect sunlight than infrared light, the just-discovered microbe’s ancestors probably evolved photosynthesis in sunlit regions, says coauthor J. Thomas Beatty of the University of British Columbia in Vancouver. A few pioneer bacteria could then have washed down to deep-sea vents and found a home, he says.

Because the organisms survive at most 2 weeks in open waters, and the next-closest high-sulfur, low-oxygen environment that could support them lies 2,250 kilometers away, Beatty suspects the organisms at the East Pacific Rise vent indeed rely on the vent’s glow to stay alive.

“‘Where there’s light, there’s photosynthesis’ is the take-home lesson,” says Blankenship.