Ocean floor geysers could hold key to understanding our evolution

Click to follow
The Independent Online

In the foothills of the Troodos mountains in Cyprus, Dr Crispin Little sifts through the debris from an abandoned sulphide mine. Little is searching for the preserved remains of marine creatures that lived in the deep ocean millions of years ago. The sulphide deposits once lay thousands of metres below the surface of the sea and were thrust upwards during huge movements of the Earth's crust.

The deposits were formed by hydrothermal vents, small openings in the ocean floor that spew out hot water. These vents support a bewildering array of animals. They form at the junctions of the tectonic plates that make up the Earth's crust. Sea water seeps into newly formed crust where it gets heated up and strips the underlying volcanic rock of minerals. Eventually this "hydrothermal fluid", which can be as hot as 400C, finds its way back to the surface, where it can gush out like an underwater geyser.

"As the minerals precipitate they form sea-floor structures," says Little, a geologist at the University of Leeds. "Typically these can resemble a lens-shaped mound that could be a hundred metres across."

The first hydrothermal vent was discovered in 1977. Since then, hundreds have been identified. What makes these habitats intriguing for biologists is that they support complex communities of organisms in a way that is seen nowhere else on the planet. Elsewhere, ecosystems depend on sunlight. In hydrothermal vents, however, the primary source of energy comes from dissolved hydrogen sulphide. Microbes can use this as their principal source of energy. Rather than being photosynthetic, they are chemosynthetic.

Some creatures have formed a symbiotic relationship with these microbes. Giant tubeworms, for example, have no digestive system. Instead they harbour huge colonies of chemosynthetic bacteria. The worm provides the bacteria with oxygen from the seawater and hydrogen sulphide from the vent fluid, and in return receives its energy requirements from the microbes. Shrimps and crabs, meanwhile, scavenge on decaying animals.

"These communities should be buffered from the big extinction events that happen on the surface," says Little. "For example, if it was an asteroid impact that wiped out the dinosaurs by causing massive changes in the environment on the Earth's surface, it would not have affected these deep ocean habitats."

Little has found fossilised remains of creatures in Cyprus, California and Ireland, and studied others from Oman, Georgia and Turkey. Species include giant tubeworms, snails and shelled animals called brachiopods, which have not been found in any modern vent communities. "It looks as though there has been a change in the sorts of the animals that you find at vent sites over geological time, although these do not seem to be linked to major extinction events."

But the marine biologist Dr David Dixon is set to cause a stir when he presents evidence of his findings that vents may not be as isolated from the rest of the processes on the planet as is currently thought. "It has been suggested that, because hydrothermal vent animals are independent of solar energy, they would not show any seasonal growth and reproduction," says Dixon, who works at the National Oceanography Centre in Southampton. "But you find very discrete classes of size, as if they were being produced at specific times."

He put vent mussels in cages and placed them at vent sites, hauling them up in winter. He discovered that they were breeding at that time of year. Dixon then examined the annual blooms of phytoplankton in the oceans. "The plankton activity coincided with the mussel breeding cycle," he says. "I think that the mussels release their larvae to coincide with the precipitation of dead phytoplankton to the deep ocean."

This, Dixon says, suggests that vent communities are not as isolated as first thought. "People started talking about quasi-alien life-forms evolving separately and even arguing that these environments could be the crucible in which life on the planet was cooked up. But when you look closely, you see the sorts of creature that inhabit hostile environments."