Many of the blue holes' microbes aren't known to science. But the colonies that the team was able to identify appear to feed on sulfur compounds, such as hydrogen sulfide, that are toxic to most other forms of life.
The announcement of the hardy new bacteria intrigues not only researchers seeking extreme life on Earth but also those looking for it off-world.
That's because similar conditions might exist in pitch-black oceans millions of miles away—perhaps under the icy crusts of Jupiter's moon Europa and Saturn's moon Enceladus, said Kevin Hand, an astrobiologist and NASA's deputy chief scientist for solar system exploration.
"My ears always perk up when I hear about sulfur-based ecosystems and microbes in extreme environments. Much of the chemistry that may dominate ice-covered ocean worlds is sulfur chemistry," said Hand, also a National Geographic Society emerging explorer who wasn't involved in the new research. (The Society owns National Geographic News.)
"It's through our study of life's extremes on Earth that we can extend our understanding of habitable environments off Earth," he said.
Life Thriving in Isolated Blue Holes
All blue holes were once sinkholes that formed on land but are now filled with ocean water. The deepest known blue hole—Dean's Hole in the Bahamas—plummets 663 feet (202 meters), but most are roughly half as deep.
The original sinkholes formed during past ice ages, when ocean water was locked up as ice, causing sea levels to drop roughly 400 feet (122 meters) lower than they are today.
Freshwater rain at that time chewed through coastal limestone rock, creating caverns. As Earth warmed up again and the oceans rose, many of the openings collapsed and filled with saltwater.
In the dark depths that exist in blue holes today, oxygen is scarce and sunlight is available only near the surface. Less dense fresh water often caps a blue hole's salty marine water, and the two forms of water in the isolated body barely mix.
The conditions mean blue holes exist mostly apart from the potpourri of life—and food sources—found elsewhere around the ocean.
Yet somehow rich ecosystems thrive in the inhospitable depths, including species of shrimp, aquatic mites, copepods, and other crustaceans.
Tom Iliffe, a marine biologist who has dived in hundreds of underwater caves in the past 30 years, has mostly focused on collecting these top-of-the-food-chain creatures—until now.
"The real question is what are they eating? These animals stay close to their food, and there are no plants down there," said Iliffe, who led the recent expedition in the Bahamas.
"The answer was microbial nutrition. There's bacteria and other small forms of life, and animals up the food chain are feeding on them."
Blue Holes Lined With Microbe Mats
Over the past decade, Iliffe and others had found several suspect colonies of bacteria in the Bahamas but had never sampled them, so the new expedition started the search in the island country.
Iliffe and graduate student Brett Gonzalez made dives in three Bahamian blue holes—Sawmill Sink, Cherokee Road Extension, and Sanctuary Blue Hole. The team logged temperature, salinity, and acidity at regular depths, as well as the levels of oxygen and hydrogen sulfide.
Along the way, the researchers also sampled colonies of microbes and ferried them to the surface.
"One of [the blue holes] has mats of microbes completely covering the walls," Iliffe said.
"They were an inch or more thick, and you could just peel the mat off the walls with your fingers and into a sample bag."
Next Stop: Oceans of Europa?
Many of the bacteria Iliffe and Gonzalez pulled from the depths weren't known to science.
But genetic analysis conducted with Jenn Macalady at Pennsylvania State University revealed recognizable species of bacteria able to survive in dimly lit conditions, along with some that feed exclusively on hydrogen sulfide.
In addition, most of the blue-hole bacteria were found to thrive near the halocline—a relatively thin layer where there's some turbulent mixing of freshwater and saltwater.
NASA's Hand said this is precisely the kind of active environment that may exist below the icy moon Europa, whose cracked surface is sprayed by the sulfur-rich volcanic eruptions of a neighboring moon called Io.
"If there's one thing we've learned, it's that life loves to operate at the boundaries, the interfaces. That's where life finds the energy to power itself," Hand said.
(Also see "'Great Lakes' Discovered on Jupiter Moon?")
Several missions to Europa have been sketched out by NASA researchers, including a flyby, an orbiter, and a robotic lander. But it's uncertain if the space agency will choose any to fund.
"With any luck, we'll get one of these off the ground in the 2020 time frame," Hand said. Such a mission would add to our understanding of the moon's under-ice ocean and its potential to hold life.
Study leader Iliffe, meanwhile, plans to sample microbial communities in dozens of other blue holes, compare them, and see how their strategies differ for eking out an existence in such extreme environments.
The blue-hole microbes study was published online in November 2011 by the journal Hydrobiologia.