From ancient life to alien life
Living where the sun don't shine
A Caribbean cruise may unlock one of biology’s oldest secrets—both on Earth and elsewhere in the universe
MODERN life is powered by the sun. But photosynthesis, the process that converts sunlight, carbon dioxide and water into plants, is a mere 2.4 billion years old. Life itself goes back at least 3.5 billion years. Before photosynthesis, the energy must have come from something else. Without understanding what that something was, it is impossible to know how life on Earth got going. Moreover, there are those who think that whatever did power Earth-bound life before photosynthesis might also power it on other planets. Which is why, on October 7th, a mission was launched from Cape Canaveral—not from the rocket pads in the north of the cape, but from the docks at its south.
The good ship Cape Hatteras, crewed by Chris German of the Woods Hole Oceanographic Institution (WHOI), in Massachusetts, and his colleagues, will act as a base for the exploration of a region of inner, rather than outer space—specifically, the Mid-Cayman Rise, a submarine mountain range that lies under almost 7km (about 4 miles) of water near Grand Cayman island in the Caribbean. When it gets there, Cape Hatteras will launch Nereus, an unmanned submarine, named after an ancient Greek sea god, that is capable of withstanding the pressure at such depth (it went down 11km on a previous expedition).
The Caribbean may seem an unlikely place to study extraterrestrial life, but this is no junket. The reason for going there is that the Mid-Cayman Rise is part of the 60,000km-long system of mid-ocean ridges that zigzags around the planet like a network of giant zip fasteners which open up regularly to reveal the liquid rock beneath. Moreover, it is a particularly unusual part of that network—one which some believe may also reveal what the earliest life on Earth looked like and thus give hints about how life might develop elsewhere.
When liquid rock wells up at a ridge, it pushes the existing sea floor sideways and thus propels any adjacent continents apart—a process known as continental drift. The process also creates underwater geysers, called hydrothermal vents, which pump out large quantities of energy-rich chemicals such as hydrogen sulphide. These vents are home to a range of unusual creatures that subsist—directly or indirectly—on chemical energy derived from the vents, rather than on sunlight.
The discovery of hydrothermal vents, in the 1970s, caused great excitement. Some biologists thought that the bacteria which extract the chemical energy, and thus form the base of the hydrothermal ecosystem's food chains, might be similar to those that lived before photosynthesis. But most of the necessary chemical reactions (those involving hydrogen sulphide, for example) require oxygen, the waste product of photosynthesis, as one of their components, and so cannot predate it.
The exceptions are those reactions that use hydrogen as the source of chemical energy. Photosynthesis employs light to split water into hydrogen and oxygen. The hydrogen then combines with carbon dioxide to create energy-rich carbohydrate compounds, the building blocks of the rest of biochemistry. So if the elemental hydrogen is already to hand, an energy source such as light or oxidation may not be needed to keep the ecosystem going.
The vents of most mid-ocean ridges rarely produce hydrogen. There is, however, reason to believe that hydrogen will be abundant in the vents of a particular type of ridge called an ultra-slow spreader. All mid-ocean ridges are formed mainly of basalt, the liquid form of which directly underlies the solid rock of the sea floor. Ultra-slow spreaders, however, also contain a lot of peridotite, which comes from deeper down and is much richer in magnesium and iron. These elements react with seawater to release hydrogen. Preliminary work by Dr German and others suggests that the vents at these ridges do, indeed, emit hydrogen. And, on top of that, some geologists argue that the ridges of the early, geologically unevolved Earth would have been peridotite-rich. Modern ultra-slow ridges may therefore be a better model for the past than the common variety.
That is why WHOI, NASA (America's space agency) and several other institutions have got together in a snappily entitled collaboration called Oases for Life and Pre-Biotic Chemistry: Hydrothermal Exploration Using Advanced Underwater Robotics. And the consequence of that is the cruise of Cape Hatteras.
Once it is on station, Cape Hatteras will deploy a package of instruments designed to sniff out a suitable hydrothermal vent by sampling the chemistry and physics of the water. Then the team will release Nereus. The vent-sniffing package should get them within 10km of their prey, but Nereus will still have to do a bit of hunting by itself, using measurements of temperature, water-transparency and chemical composition to close in on the target. Once it has found the vent, it will take a few preliminary photographs and return to the surface.
For the next phase, the probe will have to be put on a leash (actually, an ultralight optical fibre that is only twice the diameter of a human hair), in order for Dr German and his team to be able to give it detailed instructions, and for it to send back its findings as it makes them. For six days, it will collect vent fluids, minerals, hydrothermally altered rocks, large volumes of seawater and any organisms it can catch, bringing up 25kg of samples a day.
In the final phase of the expedition, once these samples are safely aboard, the researchers will put the probe through a testing series of exercises in preparation for its deployment in less benign waters than those of the central Caribbean—for the next target may be the Gakkel Ridge under the Arctic pack ice.
Whatever Nereus finds will be interesting. The ocean floor is the least understood part of the Earth's surface, so any new data are welcome. If the probe does come up with hydrogen-eating bacteria, though, that will bring especial comfort to those who hope to find life on other celestial bodies, particularly ones that are far from the nurturing light of the sun. In that case, maybe, one of Nereus's descendants will explore the hydrothermal vents which many researchers suspect lie under ice even deeper and colder than the Arctic's—that of another name from Greek mythology, Jupiter's moon, Europa.