But in recent years, researchers discovered life also thrives in other, much colder, lightless deep-sea ecosystems. Such habitats are created where faults in ancient sediments allow natural gas (methane) in deeply buried deposits to seep upward to the ocean floor to form methane ices known as gas hydrates.
These "methane seeps" are found all over the world on continental slopes some 500 to 1,000 meters (1,640 to 3,280 feet) beneath the waves. There, where pressures are 50 times greater than on Earth's surface, compressed natural gas becomes trapped in a lattice of water ice crystals to form gas hydrates at temperatures of 7 degrees Celsius (45 degrees Fahrenheit) or even warmer, depending on the crystal structure. The hydrates pile up in layers and mounds several meters (several yards) tall.
To study the ecosystem on and around the gas hydrates, two dozen scientists voyaged into the Gulf of Mexico for almost three weeks last July aboard the research vessel Seward Johnson. The expedition's chief scientists were Patricia Sobecky and Joseph Montoya of the Georgia Institute of Technology; Ian MacDonald, an oceanographer at Texas A&M University; and Mandy Joye, a biogeochemist at the University of Georgia.
The team also visited another type of cold, sea-floor ecosystem, where salt seeps upward to form "brine pools." The pools are puddles of salt water - 60 meters (200 feet) wide and larger - that collect in sea-floor depressions and are five times saltier than surrounding seawater. The Gulf sits over massive salt beds, formed when an ancient sea dried up during the Jurassic Period some 208 million to 146 million years ago. When the sea returned, sediments buried the salt, which often is associated with hydrocarbon-rich sediments. Buoyant salt later rose upward, faulting the sediments so that salt, methane and oil now seep upward to the sea floor. The methane seeps and brine pools harbor their own distinct assemblages of living organisms, although methane also is found at some brine pools.
Without light for photosynthesis, bacteria and archaea engage in "chemosynthesis" near the Gulf of Mexico methane seeps and brine pools, converting methane and hydrogen sulfide into food that supports larger organisms. Around the seeps, those include mussels, clams, shrimp and tubeworms, as well as ice worms that burrow into the gas hydrates. Only microbes can survive within the brine pools, but mussels flourish on the edges.
"On Earth we have an abundance of deep-sea habitats where a primary ecosystem niche is methane ice," Joye says. "Abundant microbial life flourishes in this harsh, extreme environment. It is not too difficult to imagine that similar simple organisms could have evolved in extraterrestrial environments with similar ecosystem niches."
Until the discovery of undersea hydrothermal vents and methane seeps, "the deep ocean floor was seen as a desert," says Joye. "We now know there are oases on the seafloor where the diversity of life is similar to what we see on a salt marsh. Instead of being fueled by photosynthesis, these deep-sea ecosystems are fueled by chemosynthesis - the production of organic matter from inorganic oxidation."
Harvard University biologist Colleen Cavanaugh says the same chemicals that nourish seafloor life around cold methane seeps also make life possible near undersea hydrothermal vents.
"It's the methane and sulfide - not the heat - that provides the energy source for the bacteria that are at the base of the food chain," she says.
In the Gulf, mussels thrive near seafloor brine pools. Bacteria live on the mussels' gills in a symbiotic relationship. The bacteria take in methane and convert it into nutrients that nourish the mussels.
Pink ice worms, 1 to 2 inches long, burrow into the mounds of methane-water ice. This strange, eyeless species of segmented worms was discovered 1997. It is not yet known how - or if - they use the gas hydrates for food, although researchers suspect they may browse on bacteria which, in turn, derive nutrition from the methane ice. The ice worms belong to the same family as eyeless worms found near hydrothermal vents in the Pacific Ocean.
Last summer, MacDonald and the other researchers made 17 dives into this strange environment in the four-person submersible vessel Johnson Sea-Link II, equipped with camera, lights, a robot arm and a device to take core samples of the methane ice.
"You see giant bushes of tubeworms," says Sobecky, a marine microbiologist. "You see bubble streams - globs or oil or clear gas bubbles streaming up from the seafloor. You see mussel beds. You see some shrimp, hiding in exposed lips or shelves of hydrate."
The researchers have years to go before they figure out details of how life works in the dark deeps around methane seeps. They still are analyzing sediments, cores of methane-water ice and other samples.
Joye says organisms near methane seeps tend to be smaller than those near hydrothermal vents, yet there are similarities. "The tubeworms are in the same phyla" in both environments, she says. "The hydrothermal vent mussel has a similar relative that is present at cold seeps." And at both hot vents and cold seeps, larger organisms like tubeworms and mussels harbor symbiotic bacteria.
Joye envisions possible extraterrestrial ecosystems resembling those at the methane seeps and brine pools. They would be dominated by microbes, would not need sunlight and would require only simple chemical molecules.
Sobecky says: "The fact you see life in methane-seep environments on the seafloor on Earth possibly provides evidence for similar environments on the floor of the presumed ocean on Jupiter's ice-covered moon Europa. Understanding how bacteria adapt to these extreme environments on Earth would provide clues of what to look for in the oceans on Europa."
Joye speculates the bottom of Europa's ocean might harbor hydrothermal vents and cold methane seeps - just as Earth's seafloor does. It is more likely microbial life would originate near the hot vents because heat speeds chemical reactions and spurs abiotic synthesis of simple organic compounds, Joye says. But she believes some of the microbes could drift onto the methane hydrates and adapt to the cold environment.
"Anywhere on Earth people have looked at hydrates for microbial life, they have found it," Joye says.
Earth's methane hydrate deposits are vast. "They are all over the place," MacDonald says, adding that if all the carbon in hydrates, plants, animals, oil and coal on Earth were combined, "organic carbon in gas hydrates is about 60 percent of the carbon."
MacDonald says methane hydrates have been found just about everywhere scientists have looked on Earth's continental slopes. Within the 320-kilometer (200-mile) U.S. territorial offshore limit from North Carolina to the south tip of Florida alone, researchers have estimated gas hydrates could cover more than 130,000 square kilometers (more than 50,000 square miles).
While the environment around methane seeps may be analgous to possible Europan ecosystems, Joye says seafloor brine pools are akin to modern soda lakes and perhaps Mars' ancient lakes or seas. The ancient waters would have become more saline as Mars' climate dried and cooled, and more alkaline as the carbon dioxide dissolved in the water to form bicarbonate.
"If you think about alkaline soda lakes - or oceans of Mars - they are carbonate-rich hypersaline systems just like these [seafloor] brine pools," and all are dominated by microbial life, says Joye.
She notes that Earth's early oceans "could have been very similar to early oceans on Mars," so life on Earth might have evolved both near the ocean surface and near seafloor hydrothermal vents.
But there is a big problem facing those who view seafloor methane seeps and salt pools as analogs for extraterrestrial life. Most bacteria in those environments - particularly those living symbiotically with tubeworms and mussels - require oxygen so they can oxidize methane to produce food and energy, Cavanaugh says. Researchers suspect other worlds will have an inadequate oxygen supply for microbes that consume methane.
But chemical and microbiological evidence in sediments near methane seeps show that bacteria and archaea work together to oxidize methane without molecular oxygen. The existence of such anaerobic microbes "would be more relevant for other planets" than microbes that consume oxygen, Cavanaugh says.
Joye notes plenty of compounds contain oxygen and might be used by microbes to anaerobically oxidize methane on other worlds. Cavanaugh says some other process still would be needed to re-oxidize such compounds, otherwise they would run out, and life would come to a halt.
In the Pacific Ocean off Oregon and in the Gulf of Mexico, scientists have found bacteria and archaea living together in microscopic clumps or "consortia" that convert methane to food without using oxygen.
"It gives us models for possible microbial processes on other planets," MacDonald says.
Joye says researchers do not yet know how this system gets "re-oxidized," although it is "possibly through reactive metal oxides. Many people are trying to figure this out."
The brine pools and methane seeps may only scratch the surface when it comes to life beneath the sea. MacDonald says faults and other conduits that produce hydrocarbon seeps show "the system is open to impressive depths. There is circulation between the seafloor and interior of the Earth extends thousands of meters downwards.… Conditions similar to those we see at the seafloor extend to depth. Whether or not microbial communities can survive under those conditions is an intriguing question."
Joye says some researchers are proposing to drill hundreds of yards deep to study any microbes living on hydrocarbons percolating up through this "deep biosphere."