The abyss hides a world of light
In the vastness of the ocean, after 100 meters of depth, cold and darkness reign as absolute masters. Distances are infinite. How to communicate in these conditions? The people of the abyss, creatures with strange shapes defying the imagination, manage to do so by means of bioluminescence: to feed themselves, to repel a predator or to reproduce. Off the coast of Toulon, passionate researchers are deploying sophisticated equipment in the depths to observe how the organisms living there relate to each other.
In the ocean world, far from being an exception, bioluminescence is a widespread strategy," explains Séverine Martini, a specialist in this subject at the Mediterranean Institute of Oceanology in Marseille (MIO). Indeed, it has been shown that between 100 and 4000 metres deep, nearly 75% of open-water species over a centimetre  emit their own light". This figure is close to 40% for organisms living on the seabed . "2] "Bioluminescence is likely to be a very effective means of communication. The oceanologist has recently acquired a new working tool: a camera that is very sensitive to bioluminescence. This camera is fitted to "BathyBot", a benthic robot (adapted to great depths) which, early next year, will explore the area around ORCA at a depth of 2,500 metres. Behind this acronym is a large neutrino detector deployed since 2017 off Toulon by the Marseille Particle Physics Centre and belonging to the KM3Net observatory. Marine biology and particle physics are a priori two disjointed disciplinary fields. Not in this case.
Between cosmic particles and marine organisms: a luminous cacophony
Neutrinos are produced almost everywhere in the cosmos, especially in stars like our Sun. They are almost elusive particles because they interact extremely little with matter. When they pass through our planet, some of them leave a cone of bluish light as a trace of their passage through the sea: this is what ORCA's optical sensors are trying to detect. Unfortunately for physicists, the wavelength of this blue, around 490 nm, is the wavelength of light emitted by bioluminescent organisms," says the MIO researcher. A problem that arose during the start-up of Antares", a detector close to ORCA, and which preceded it. Against all expectations, this first detector proved to be a privileged observatory for studying bioluminescence.
Let's take a look back. 2008, Antares is fully operational. Immersed in 2,400 meters of water 10 nautical miles (18.5 km) south of Porquerolles Island, it has 900 blue-sensitive optical sensors spread over 12 lines 400 meters long. The detector is connected by an electro-optical cable to the Michel Pacha Institute in La Seyne-sur-Mer, a marine biology station attached to the University of Lyon since its foundation in 1890; its data transits via the institute to the In2p3* Computing Centre in Villeurbanne, where it is stored. The scientists using the Antares data were faced with excessive background noise over certain periods of time, and the photomultipliers responsible for reconstructing the cones of light were triggered at the same time, making the signals difficult to exploit," explains Rémi Barbier, lecturer at the University Claude Bernard Lyon 1. The hypothesis that deep-sea bioluminescence was responsible for this cacophony of signals was put forward and I was asked to imagine a camera to observe what was happening. »
Enigmatic flashes of light
A specialist in single-photon sensitive imagers, then working at the Lyon Institute of Nuclear Physics (IPNL, now called the Lyon Institute of Physics of the 2 Infinis - Ip2I), Rémi Barbier was inspired by a technology used for night vision and designed with his team the LuSEApher  camera: a prototype equipped with a sensor sensitive to the 480 nm wavelength and capable of producing a snapshot of each particle of light detected. "A signal of 10 bioluminescent photons was enough to trigger the camera automatically and record an image," explains the physicist. A true 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 relative to the current. Some sometimes adopt sudden changes of direction, proof that they are produced by something other than plankton! "In addition to confirming the presence of bioluminescent micro and macroorganisms around the detector, this work has enabled us to determine the light signature of some of the photons.