Milky Seas: Vibrio Harveyi Glow Fields Seen From Space, and Really Blue

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Sailors in the northwestern Indian Ocean have long reported nights when the sea turned into a glowing, snow white plain to the horizon. Unlike the flashing dinoflagellates of breaking waves or Scotland's localized glow, a milky sea is a steady, continuous light from the bacterium Vibrio harveyi, which feeds on decaying organic matter such as a collapsed microalgal bloom. Through quorum sensing, the bacteria switch on their luciferase light together once their numbers pass a threshold, and they crowd the surface for oxygen, giving the glow its opaque look. Around 1615 Captain William Keeling of the East India Company sailed through one near Cape Guardafui, and in 1772 a captain proposed that animalcules were responsible. Between 1915 and 1993 ships logged 235 sightings, mostly off Somalia and around Java and the Banda Sea, tied to monsoon upwelling.

In the late 1800s Bernhard Fischer argued that luminous bacteria were the cause, and in 1985 a research vessel in the Arabian Sea sampled an active event and identified Vibrio harveyi. In 2005 Steven Miller matched a 1995 merchant ship's log with Defense Meteorological Satellite Program images and found a glowing field about the size of Connecticut that lasted three nights. Newer day night band sensors later tracked a 2019 event covering about 100,000 square kilometers for more than 40 nights. The light is actually blue: human rod vision and monochrome satellite sensors both render it white. Scientists now link these events to the Indian Ocean Dipole.

• Milky sea fields can reach about 39,000 square miles, roughly the size of Kentucky.
• Dutch sailors called it the winter sea for its resemblance to snowy fields.
• The Somali name translates roughly as glowing sea creatures.
• Jules Verne sent his submarine through a sea of milk.
• Tracking these blooms from orbit could help show how the ocean food web responds to climate shifts.