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Light on a dark romance

Once upon a time, a mobile bacterium was responsible for the subtle camouflage of the sepiola, a small cuttlefish native to Hawaii.

In the cold, dark world of the deep sea, a bacterium named Vibrio fischeri has set its sights on the tiny cuttlefish Euprymna scolopes, initiating a bright symbiotic relationship.

The love story begins at the birth of the cuttlefish. At this stage, the sepiola possess photophores with ciliated arms that move water towards their pores. This movement of water allows the cephalopod to trap hundreds of bacteria, including the species Vibrio fischeri. But the latter is not sociable, and once installed, it modifies the expression of dozens of genes in its host, causing the secretion of a chemical cocktail which is hostile to other bacteria. The pores of the cuttlefish will then close, and once they are lurking in the photophores, the Vibrio will start to emit light.

Indeed, these micro-organisms of the bacterioplankton have the characteristic to be bioluminescent. This emission of light is due to the oxidation of luciferin by the enzyme luciferase, causing the release of photons and therefore of light waves, a precious source of defense for the cuttlefish.

But how does this defense work?

In the middle of the night, the blue light emitted by the bacteria mask the shadow of the cuttlefish and bio-illuminate the sandy bottom, simulating the light of the Moon on the ocean. The cuttlefish, then artificially transparent to its predators, can hunt in peace.

In the early morning, when it is about to go to sleep, the cuttlefish expels the vast majority of bacteria, stopping the emission of light.

The remaining bacteria recolonize the photophores organs during the day. By nightfall, the Vibrio count is high enough that the self-inducers they secrete reach a certain density. Through bacterial-to-bacterial communication, or quorum sensing, the Vibrio activate the genes involved in light emission.

This is how the two beings live a happy cohabitation, the bacteria feeding on the nutritive compounds of the cuttlefish, while the latter takes advantage of the camouflage of counter-illumination provided by the activity of these microorganisms.

This relationship between these two tiny species, rich in lessons, opens the way to understanding the interactions between different organisms, and reflects the importance of ecosystems in which each individual counts.