The great glowing ocean – how the ‘milky sea’ was discovered

Bioluminescent bacteria, Krabi, Thailand.

Long ago, sailors would return from sea with stories of glowing waters far out in the open ocean. They recounted suddenly coming across miles upon miles of pale, glowing waters, often as far as the eye can see. These accounts were dismissed as fabrications, or simply figments of delirious sailors’ imaginations. Known as the ‘milky sea’, it has long been a part of folklore within the seafaring world, even appearing in Jules Verne’s Twenty Thousand Leagues Under the Sea.

The scientific community generally ignored these reports, as the concentration of bacteria needed to create such a massive glowing area was thought to be impossible. Until, in 2005, a scientist named Steve Miller decided to check.

So how did he do it? He found the log of a British merchant vessel, the S.S. Lima, which reported a sighting on 25th January 1995, “on a clear moonless night while 150 [nautical] miles east of the Somalian coast, a whitish glow was observed on the horizon and, after 15 minutes of steaming, the ship was completely surrounded by a sea of milky-white colour … It appeared as though the ship was sailing over a field of snow or gliding over the clouds”.

While it was presumed that no area could possibly sustain enough glow, or be large enough, to be captured by satellite imagery. Miller used the Defence Meteorological Satellite Program (DMSP) and its polar-orbiting satellites to detect this ethereal event, he matched the coordinates recorded by the ship to the date.

And there it was, 15,400 km2 of glowing Indian ocean. It glowed for 3 consecutive nights in January, the first event to be recorded by science as well as sailors.

The ‘milky sea’ was no longer just a tall tale, it is believed to be caused by the bacteria Vibrio Harveyi. These bacteria produce a sustained glow, using two substances in a chemical reaction to create light: A light-producing substance (luciferin), and an enzyme that catalyses the reaction, producing light as a by-product.

But, producing light takes energy, so why do they shine? It is possible that they use this glow to attract fish, seeing as they want to live inside a fish’s gut. But, these bacteria are tiny, so can only produce a very faint light on their own. It seems they must gather together to become visible, and the collective light of 40 billion trillion bacteria can make quite an impact in the open ocean.

It remains a mystery how such a large conjugation of bioluminescent bacteria can exist. But now scientists know that it can exist, it is hoped that there will soon be more conclusive answers to explain this otherworldly sight.

5 years ago

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