Wrought by bacteria: a sea of light

In 1995, the S.S. Lima, a commercial vessel steaming offshore from Somalia, found itself sailing into waters that were not the inky black you’d expect on a moonless night, but glowing a ghostly white, punctuated only by dark patches that turned out to be kelp blocking the light coming up from below.

What they’d found, says Steven Miller of Colorado State University, US, was a rare phenomenon called “milky seas,”. People fortunate enough to encounter milky seas have compared it to sailing through a snowfield extending all the way to the horizon, with light bright enough to read by.

And, it turns out, that same light is bright enough to be seen from space – something that Miller verified a few years ago by reexamining 1995-era satellite images of the night-time Earth.

These images, he says, revealed a diffuse glow covering more than 15,000 square kilometres (nearly a quarter the size of Tasmania), exactly on the course followed by the Lima. “This was the first satellite confirmation of a milky sea,” he said last week in a virtual session of the 2022 Ocean Sciences Meeting.

Since then, he’s found 12 more-recent such events, using even better instruments on the next generation of night-peering satellites. The biggest of these events, imaged from 25 August to 7 September 2019, spanned a whopping 100,000 sq km in the waters south of Java.

The bioluminescence, Miller and his collaborator Steven Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute, in California, say, isn’t like the bioluminescence sometimes seen in breaking waves or the wakes of boats.

That bioluminescence is composed of transient flashes, created when organisms in the water are disturbed by the wave. Milky seas are a steady glow and only appear when the bacteria that produce them are extremely numerous and in close proximity.

Haddock refers to this as “quorum sensing” – meaning that there have to be a lot of bacteria around to induce it. The threshold number, he says, is in the order of 100 million per millilitre of seawater.

That’s an enormous number of bacteria, and consider this: the area of the Java incident wasn’t a quarter the size of Tasmania, but 50% bigger than the island state. If you calculate the number of bacteria needed to produce a milky sea event that large, Haddock says, it comes up at about 1 x 10²³ – as in 100 billion trillions.

“That’s a hard number to intuit,” he admits. But to put it in context, he says: it’s about 10 times more than the total number of bacteria estimated to be, on average, in the upper layers of the ocean, worldwide.

No wonder milky seas events have been so rare. If Haddock’s back-of-the-envelope calculation is even remotely correct, they can only occur when something extraordinary is going on.

Miller’s data indicates that the majority of them occur in the Arabian/Somali Sea (like the one seen by the Lima) or in the waters north of Australia (like the one seen south of Java).

That, he and Haddock say, may be a result of ocean-current circulation, and how and where it produces upwellings of nutrients from deep below the surface.

These nutrients feed algal blooms that, in turn, feed the bioluminescent bacteria. These bacteria then glow in an effort to attract fish that might feed on the decaying algae, because (ecologically speaking) the bacteria aren’t worried about being eaten. In fact, they live better in the fishes’ gut than they do in the open ocean. “That’s actually a preferred habitat for [them],” Miller says.

The details of this are still being determined, but the holy grail is clear. One of these days, satellite instruments are going to spot one of these milky-sea events in a place to which researchers in Australia, India, or Africa will be able to dispatch a research vessel in time for it to be studied not only from space, but also from the ocean, allowing scientists to figure out what exactly is happening, while it’s still happening.

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