It could be the world’s most elaborate marine biology probe – and it was created by particle physicists.
Spanning 10,000 square metres of the Mediterranean seabed, the Antares telescope is designed to tell us about the cosmos by picking up signs of elusive particles called neutrinos, which fly thousands of light years through space. To physicists’ surprise, however, the underwater particle detector is also providing a unique glimpse of marine life.
It is now being adapted to distinguish between different marine species and monitor the sea’s health. “The deep ocean is actually less well understood than outer space,” says Antares team member John Carr at the University of the Mediterranean (UM) in Marseille, France.
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When complete, Antares will consist of 12 electrical cables sticking up for 350 metres above the seabed, of which eight are now in place. Each one is loaded with light detectors called photomultiplier tubes, which hang from it at regular intervals like pearls on a necklace (see Diagram). The cables connect to a junction box on the seabed, which relays data back to the researchers.
How does that help detect neutrinos? These particles rarely interact with matter and so can travel to us from vast distances without hindrance, bringing with them information about where they originated, such as the outskirts of black holes. Their reluctance to interact also makes them difficult to detect, but occasionally a neutrino does interact with matter as it travels through the Earth, producing a charged particle called a muon. If the muon then shoots up from the Earth’s crust into the sea, it generates a shower of light that Antares can detect using its photomultiplier tubes.
So far Antares has not spotted any neutrinos from distant galaxies. However, it was when checking that the first five lines were working that the team got an inkling that their underwater telescope might double up as a probe for marine life.
The photomultipliers were picking up a large amount of light, and given that neutrinos are notoriously shy about showing up in experiments, the light must have been coming from something else. “We guessed that we were measuring light produced by living organisms,” says Jürgen Brunner of UM. They were picking up both a weak, diffuse light that came in waves lasting from a few hours to a few weeks and also short, strong spikes of light.
The team brought in biologist Christian Tamburini, also at UM, who explained that the longer waves of light are probably caused by free-swimming bioluminescent bacteria. “I was surprised, because we didn’t even know that bacteria could give off luminescence under the high-pressure conditions found at these depths,” he says. He attributed the spikes to passing luminous fish, shrimp or jellyfish, which all house bioluminescent bacteria.
“We didn’t even know bacteria could give off luminescence at these depths”
Before they subtract these signals from their measurements, though, the Antares team need to be sure that Tamburini is right, so they have now connected infrared cameras to the cables. Due to start rolling in November, the cameras will be triggered whenever a photomultiplier detects a flash of light, hopefully catching a glimpse of something living. They might even be able to correlate patterns in the timing of light flashes with the presence of different marine organisms, making it easier to distinguish between light that comes from neutrinos and marine animals.
It is biologists, though, who are really excited about the deep-sea photos. “[We] certainly haven’t yet identified all the beasts that live at these depths, and we don’t have the resources to explore these regions on such a large scale ourselves,” says Tamburini. “We are hoping that we might discover some new, exotic species.”
The detector is also allowing them to monitor the ocean’s health. The amount of light picked up by the telescope has been falling consistently, with 10 times less light produced in spring 2007 than in spring 2006. “Naively, this would seem to indicate that there’s 10 times less life down there now, but of course that’s something we need to check,” says Carr.
Tamburini is also worried. “The drop in bioluminescence could be due to an input of carbon into the deep sea,” he says. Winter in 2005 in the Mediterranean was warmer than expected and that could have affected the way water cascades down to lower levels, having a knock-on effect on the life cycle of the bacteria.
To investigate, Tamburini will be connecting an instrument that measures oxygen levels to the Antares junction box. Like other organisms, luminous bacteria consume oxygen and produce carbon dioxide. If oxygen levels change in tandem with the reduction in luminosity, it could indicate that bacteria are dying.
Alternatively, the reduction might be the result of changes in salt levels and temperature. Tamburini will test if this is likely by monitoring how different bacterial species react to changes in these conditions in high-pressure tanks designed to simulate the deep sea. “It could simply be that the bacteria are too stressed to emit light,” he says.
Tamburini will also take advantage of Antares’s vast network of electrical cables to deploy sensors measuring the speed of ocean currents. In the past, oceanographers have dropped their instruments into the sea from boats and retrieved them months later using submarines, giving only a glimpse of the true situation. Now they will be able to build up a complete and continuous profile.
Preliminary results from these instruments is already proving useful. Brunner has found a strong correlation between the strength of the ocean currents and the number of spikes. “Fish may just be crashing into our equipment, causing it to fire,” he says.
They also picked up unexpected and dramatic changes in spring 2006, with water velocity peaks seven times as high as expected given the weather conditions. “It was like there was an undersea hurricane,” says Brunner. The storm even buried the cables that had been lying uncovered on the seabed for six years during the telescope’s construction. “Above the surface, the weather had been calm for some time,” says Brunner. “We have no idea what caused this.”
Antares will start to tackle the mysteries of undersea storms and the dips in bioluminescence as soon as it becomes fully operational in 2008. The researchers hope to spot some cosmic neutrinos too.
Mysteries of the Deep Sea -The deep sea is one of the harshest habitats on Earth, but is home to many remarkable creatures. Learn more in our comprehensive special report.
Song of the neutrino
What do neutrinos sound like? A lot like whales, according to recent results from the underwater NEMO neutrino telescope off the coast of Sicily, Italy.
NEMO is made up of cables studded with light detectors that can pick up showers of light created by charged particles racing through the water. The telescope is also equipped with acoustic emitters and underwater microphones called hydrophones. The NEMO team uses these to work out the exact position of the swaying cables so that the direction of the light shower can be calculated. The hydrophones can also confirm the presence of charged particles by detecting the shock waves they create.
Now, however, the hydrophones have found an unexpected use. While tuning them, the team realised that they could make adjustments to detect the noises made by marine animals. They are now able to detect whales within 16 kilometres of the detector and dolphins at even longer range. “We identified a group of whales passing close by, which surprised the biologists, who didn’t think they would be moving that near to the coast,” says team member Jürgen Brunner.
