In the immensity of the ocean, beyond a depth of 100 metres, the cold and darkness reign supreme. The distances are infinite. How do you communicate in these conditions? The people of the abyss, creatures with strange shapes that defy the imagination, manage to do so by means of bioluminescence: to feed, repel predators or reproduce. Off the coast of Toulon, passionate researchers are deploying sophisticated equipment in the deep sea to observe how the organisms that live there relate to each other.
Far from being an exception in the ocean world, bioluminescence is a widespread strategy," explains Séverine Martini, a specialist in this field at the Mediterranean Institute of Oceanology in Marseille (MIO). Between 100 and 4,000 metres below the surface, nearly 75% of open-water species over one centimetre[1] emit their own light". This figure is close to 40% for organisms living on the seabed[2]. "Bioluminescence is probably a very effective means of communication". The oceanographer recently acquired a new working tool: a camera that is highly sensitive to bioluminescence. This is being fitted to BathyBot, a benthic robot (adapted to the deep sea) which will be exploring the ORCA area at a depth of 2,500 metres early next year. Behind this acronym lies a major neutrino detector deployed since 2017 off the coast of Toulon by the Marseille Centre for Particle Physics and belonging to the KM3Net observatory. In theory, marine biology and particle physics are two separate disciplines. Not in this case.
Between cosmic particles and marine organisms: a cacophony of light
Produced almost everywhere in the cosmos, particularly within stars such as our Sun, neutrinos are almost elusive particles because they interact very little with matter. When they cross our planet, some of them leave a trace of their passage in the sea in the form of a cone of bluish light: this is what ORCA's optical sensors are trying to detect. Unfortunately for physicists, the wavelength of this blue, around 490 nm, corresponds to the light emitted by bioluminescent organisms," comments the MIO researcher. This was a problem that arose when Antares, a neighbouring detector to ORCA and its predecessor, was commissioned. Unexpectedly, Antares proved to be an ideal observatory for studying bioluminescence.
A quick look back. 2008, Antares is fully operational. Submerged in 2400 metres of water 10 nautical miles (18.5 km) south of the island of Porquerolles, it has 900 blue-sensitive optical sensors spread over 12 lines 400 metres long. The detector is linked by an electro-optical cable to the Michel Pacha Institute at La Seyne-sur-Mer, a marine biology station that has been attached to the University of Lyon since it was founded in 1890. Its data is sent via the institute to the In2p3* computing centre in Villeurbanne, where it is stored. Scientists using the Antares data were faced with too much background noise over certain periods, with the photomultipliers responsible for reconstructing the light cones being triggered at the same time, making the signals difficult to use," explains Rémi Barbier, lecturer at Claude Bernard University Lyon 1. The hypothesis that deep-sea bioluminescence was responsible for this cacophony of signals was put forward and I was asked to design a camera to observe what was happening.
Enigmatic flashes of light
Rémi Barbier, a specialist in imagers sensitive to single photons working at the Institut de Physique Nucléaire de Lyon (IPNL, now the Institut de Physique des 2 Infinis de Lyon - Ip2I), was inspired by a technology used for night vision and, with his team, designed the LuSEApher[3] camera: a prototype equipped with a sensor sensitive to a wavelength of 480 nm and capable of producing a snapshot of each light particle detected. "A signal of 10 bioluminescent photons was enough to trigger the camera automatically and record an image", explains the physicist. A real technological achievement. Immersed on Antares between November 2010 and March 2012, LuSEApher recorded more than fifty films: no photos of jellyfish or other organisms posing patiently in front of the lens, but... halos of photons emitted spontaneously and whose source moves very quickly in relation to the current. Some of them even change direction abruptly, proof that they are produced by something other than plankton! As well as confirming the presence of bioluminescent micro- and macro-organisms around the detector, this work has enabled us to determine the light signature of some of them, i.e. the way in which their light diminishes over time," adds the researcher. In this way, a catalogue has gradually been built up".