Scientists are deciphering the secret of the “milky seas” made up of billions of trillions of luminous bacteria

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This milky sea off the coast of Java was the size of Kentucky and lasted more than a month. Photo credit: Steven D. Miller / NOAA.

For centuries, seafarers have been reporting a strange phenomenon: the entire ocean, which stretches as far as the eye can see, shines in glorious white light, which has earned it the name “milk lake effect”. These very rare sightings could be dismissed as crazy ramblings of drunk sailors if not confirmed by entire crews.

These milky seas, sometimes called mareel, can span tens of thousands of square kilometers (the size of some countries) and are bright enough to be seen by modern satellites. They have been described as sailing through a snowfield or cloud peak. Scientists have now scoured countless satellite images in search of milky seas and, for the first time, have been given the opportunity to study these fleeting phenomena directly.

Scientists have confirmed long-standing assumptions about milky seas caused by bioluminescent bacteria. Billions of trillions of them, actually. And that is just the beginning.

Make the lights on the dark sea

“The sea shone in spots and in the wake of the ship with a uniform, slightly milky color. When the water was filled into a bottle, there were sparks … “

The above quote is actually the very first entry in Charles Darwin’s zoological notebook, written during his iconic voyage aboard the Beagle on January 6, 1832 off the coast of Tenerife. What the British naturalist saw was a prime example of bioluminescence, which refers to light created by a chemical reaction in a living organism.

Bioluminescence is so common and useful to living things that scientists believe it evolved independently at least 40 times, both on land and in the ocean.

It is estimated that more than 75% of deep-sea inhabitants produce their own light. The anglerfish, for example, uses bioluminescent baits, which are similar to fishing rods, to attract prey. Closer to the ocean surface is called bioluminescence by plankton. generated Noctiluca scintillans, commonly known as “Sea Sparkle”.

Milky seas, however, are a particular treat as they are incredibly rare. On average, they are only sighted twice a year, mostly in waters around the northwest Indian Ocean and just off the coast of Indonesia. They also cover much greater distances than plankton lightbulbs.

Satellites emit light

The images on the left show milky seas captured by ancient satellites. The much higher resolution images on the right used the day / night band sensor. Photo credit: Steven D. Miller / NOAA.

Because they rarely appear, milky seas have always proven to be a mystery, but modern satellite measurements can change that.

Using NOAA weather satellites, researchers have trained computers to identify milky seas in real time, which opens up the possibility of studying these elusive phenomena before they dissolve.

“Now we have a way to proactively identify these milky seas candidate areas,” said Steve Miller, lead researcher at Colorado State University and lead author of the new study published in. has been published Scientific reports. “If we have assets in the region, the assets could be deployed forward in a SWAT team-like response.”

Miller explains that milky seas are produced by the glowing bacteria Vibrio harveyi. These bacteria remain dormant until their numbers exceed a critical threshold – 100 million cells per milliliter of water. Once this threshold is crossed, it’s like flicking a biological switch and the bacteria turn the ocean into a milky sea.

Scientists believe the bacteria do this to get the attention of fish that eat them. That sounds very strange and counter-intuitive at first, but it’s actually a pretty clever trick. The bacteria swell to tremendous amounts by feasting on massive algal blooms, but that food soon runs out. Once in the fish’s stomach, the bacteria thrive in the gut, just like many other bacteria live in our bodies.

Miller first began using satellite imagery in 2004 to study the milk lake effect. However, the technology at the time was subpar, so the observations have proven obscure and impossible to work with. That changed with the day / night band instrument that went into operation in 2011 on board NOAA’s Suomi National Polar-orbiting Partnership (NPP) and the satellites of the Joint Polar Satellite System (JPSS). This day / night band (DNB) instrument can break down light into gradients so that satellites can see through the lights of cities and forest fires.

Bioluminescent bacteria like those that cause milky seas. Credit: Steve. HD haddock / MBARI, CC BY-ND.

The researchers identified the places where milky seas were most commonly sighted over the past 200 years and then focused on those regions. That hunt proved fruitful, and Miller and colleagues identified over a dozen milky marine events between 2012 and 2021, the largest of which occurred south of Java in 2009.

NOAA satellite imagery confirmed that this particular milky sea reached monstrous proportions, stretching over 100,000 square kilometers, roughly the size of Kentucky. The number of bacteria that take part in this event is simply unimaginable. Perhaps 100 billion trillion cells were involved, about as many as there are stars in the observable universe.

These observations helped unravel many previously mysterious aspects of the milky sea formation. For example, water temperature and chlorophyll are very important.

However, other questions remain unanswered. These include the thickness of the milky broth and uncertainties as to whether algal blooms are actually the main food source for the bacteria. Scientists would have to find themselves in the middle of such a milky sea to better answer these questions.

“Perhaps the most practical revelation is how long a milky sea can last. While some lasted just a few days, those near Java lasted over a month. That means there is an opportunity to deploy research vessels to these distant events while they are taking place. That would allow scientists to measure them in such a way that their full composition, formation, rarity, and ecological significance in nature are revealed, ”Miller wrote in an article.


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