On some moonless nights, enormous patches of the Northwest Indian Ocean and seas around Indonesia begin to glow. This event has been witnessed by hundreds of sailors, but only one research vessel has ever, by pure chance, come across this bioluminescent phenomenon, known as milky seas. Thanks to that vessel, samples showed that the source of the light was a bacteria called V. harveyi, which had colonized a microalgae called Phaocystis. But that was back in 1988, and researchers have yet to be in the right place and the right time to catch one of these events again.
Both the bacteria and algae are common to those waters, so it’s not clear what triggers these rare events. To help understand why milky seas form, researchers have gotten much better at spotting these swaths of bioluminescence from the skies. With the help of satellites, Stephen Miller, a professor of atmospheric science, has been collecting both images and eyewitness accounts of milky seas for nearly 20 years. Thanks to improvements in the imaging capabilities over the past decades, Miller published a compilation last year of probable milky seas in the time frame of 2012 to 2021, including one occurrence south of Java, Indonesia, in summer 2019.
But these satellite observations lacked surface confirmation—that is, until the crew of the yacht Ganesha reached out to Miller with their first-hand account of what they had experienced during their trip through the seas around Java that August, which was recently published in PNAS. Their eyewitness corroboration—along with the first photographs of a milky sea—show that these satellites are indeed a powerful tool for spotting these events.
Skies’ eyes
Although milky seas can be massive—greater than 100,000 square kilometers in the case of the 2019 sighting—the intensity of this bioluminescence is still relatively faint. By comparison, the better-known sea sparkle from marine plankton (dinoflagellates) is 10 times stronger—and even that can be hard to spot.
To catch milky seas by satellite, researchers like Miller and his collaborators had to wait for the installation of the Day/Light Band on the latest generation of National Oceanic and Atmospheric Administration (NOAA) environmental satellites. This low-light imager is sensitive enough to capture light 10,000 times weaker than reflected moonlight and about 1 billion times weaker than reflected sunlight. Day/Light Bands have been installed on two satellites: the Suomi National Polar-orbiting Partnership (launched in 2011) and the Joint Polar Satellite System series (launched in 2017).
Thanks to these satellites, Miller could search through 10 years of satellite data, in which he found 12 suspected milky seas between 2012 and 2021. This data showed that the events could last as long as weeks and that they often coincided with regional monsoons and algal blooms resulting from the upwelling of nutrient-rich waters.
“While milky seas are spectacular visual phenomenon with an interesting historical backstory tied to the maritime folklore, I think in modern times we are also very interested in understanding how and why this massive expression of our biosphere, associated with primary production (the very base of the marine food chain), occurs,” writes Miller in an email to Ars Technica. “I would like to translate this to a better awareness of atmosphere/ocean/biosphere coupling in Earth’s climate system, such that we can begin to understand how fundamental components of our planet’s ecosystem may respond in a changing climate.”
But all of Miller’s observations were coming from an altitude of over 800 km in the sky—until he heard from the Ganesha crew.
Lucky sailors
“When waking up at 2200 the sea was white. There is no moon, the sea is apparently full of ? plankton ? but the bow wave is black! It gives the impression of sailing on snow!” the Ganesha crew wrote in their logs.
This glow was continuous as far as they could see, and they also compared the effect to glow-in-the-dark stickers. When they collected some of the water in a bucket they found that the light extinguished when stirred, which is the opposite of dinoflagellate behavior.
“I was surprised by their description of the pinpoints of light that vanished upon stirring and the sense that the glow was coming from depth,” writes Miller. “The disappearing glow may be due to bacterial communities being broken up that made their individual glow less apparent than when concentrated on a particle, or a change in the water that shut off that glow… not sure!”
To date there have been two main hypotheses about the formation of these bacterial communities—one suggests a slick of bacteria on the water’s surface, the other a community reaching further into the water column. The accounts from the Ganesha are some of the first to provide evidence for the latter, even suggesting that the bacteria may extend as much as 10 meters below the surface.
“It makes me wonder what the vertical cross-section of those glowing particles looked like, and what clues that might provide as to their formation mechanisms,” adds Miller.
Particularly in the latter case, scientists think that the bioluminescence is driven by quorum sensing, a form of biological signaling that allows bacteria to determine how large and concentrated their species is in an area. In this case, the bacteria may only glow when they become concentrated enough. In the case of the Java event, researchers estimated the involvement of 100 billion trillion bacterial cells.
Right place, right time
When Miller compared the Ganesha’s course to the satellite data, he found that their ship didn’t even cross through the most brightly lit section of that particular milky sea. Two-hundred kilometers to the north, the intensity was up to five times greater than what the crew had observed.
That said, their human eyes were still more sensitive than the satellites, and they reported seeing a glow in the water again the second night, suggesting that the event might have reached much farther than the roughly 100,000 square kilometers that were detected by the satellites.
Altogether, growing evidence suggests that milky seas can be large and prolonged enough that it might soon be possible to coordinate a research expedition to begin answering some of the many open questions about this phenomenon.
“We’re getting there, slowly but surely!” writes Miller. “I think we still have some work to do on understanding the causality and key antecedent factors that can allow us to better anticipate their formation… after all, algal blooms happen all over the place, all the time, and yet we rarely experience milky seas. So, there must be other factors at play. We need to understand these better before putting in resources to deploy to one—that is a very expensive venture and typically we are working on shoe-string budgets!”
Although it may be some time before funding for a formal expedition comes together, perhaps more awareness will lead to additional reports from camera-ready sailors in the meantime.
“I would also like to point out that while we look in collective awe at the incredible photos from James Webb Space Telescope coming from the edges of our universe, there still somehow remain these fascinating mysteries down below, waiting to be discovered, appreciated, and learned about,” adds Miller.
