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Wednesday, June 19, 2024

The Extraordinary Shelf Life of the Deep Sea Sandwiches

In the late 1960s, a submersible named Alvin suffered a mishap off the coast of Martha’s Vineyard. The bulbous white vessel, holding a crew of three, was being lowered for a dive when a cable snapped. Suddenly, Alvin was sinking. The scientists clambered out, shocked and a little bruised, as the vessel plunged, hatch ajar, eventually settling in the seabed some 4,500 feet below. Alvin was in a slightly embarrassing situation. Though the sub was only a few years old, it had an eclectic résumé that included, in 1966, helping to recover a 70-kiloton hydrogen bomb that was dropped when two military planes collided over the Spanish coast. Now it was the one that needed saving. 

Ten months later, Alvin was pulled from the depths—a blip in the life of a vessel that makes dives to this day (though a steady replacement of parts means none of the original sub remains). But the accident left behind its own legacy in the form of a mysteriously preserved lunch. In their frantic escape, the crew had left behind six sandwiches, two thermoses filled with bouillon, and a handful of apples. After retrieving Alvin, researchers from the Woods Hole Oceanographic Institution marveled at the state of this waterlogged feast. The apples looked slightly pickled by the briny water, but otherwise intact. The sandwiches smelled fresh, and the bologna (this being 1968) was still pink. They even still tasted good, the researchers confirmed upon taking a few bites. Similarly, although the thermoses had been crushed by the water pressure, the soup, once warmed up, was deemed “perfectly palatable.”

Those observations were published in the journal Science in 1971, after the surprised scientists raced to study the meal before it spoiled—which it did, within a few weeks under refrigeration. In addition to nibbling the bologna, the researchers measured the chemical properties of the food and the activity of the microbes gathered on it. Eventually, they concluded that the spoilage had been happening at 1 percent of the rate it would have at the surface, controlling for temperature. The question—one that has vexed researchers for decades—was why. In the 1960s, researchers had little experience in the cold, highly pressurized deep ocean, but they expected it to be filled with microbes ready to break down organic matter, even in extreme conditions. Perhaps there were fewer of those microbes than they thought, or not the right kinds. Or maybe not enough oxygen. Or it was just too cold or too pressurized. The answer was difficult to pin down.

Over time, the question at the heart of the preserved-lunch mystery has become more urgent as scientists have come to understand the role that the oceans play in sequestering carbon. Around a third of the carbon people have put into the air has been sucked back out of it by the oceans—and much of it is thought to be stored in the deepest pools of water. So an accurate picture of how much carbon goes in and how much escapes back into the air is important. It’s especially important if you want to manipulate that process, as some do, by doing things like growing seaweed—which removes carbon from the air through photosynthesis to build its tendrils—and then sinking it into deep ocean trenches to store that carbon away.  

In large part, the difficulty for researchers studying deep water carbon is that conditions at the seafloor are hard to replicate at sea level. Typically, researchers pull water up to the deck of a research vessel where they have equipment that can measure microbial activity. But this has resulted in a mismatch, says Gerhard Herndl, a bio-oceanographer at the University of Vienna. Aboard a ship, microbes are generally happy to chomp down on the nutrients available to them. Their appetite is so great, in fact, that it doesn’t make much sense, because it is far greater than the nutrients found in the deep ocean can provide. “When you do these measurements at the surface, there is always a gap,” he says. 

So instead, following the long legacy of the Alvin sandwiches, Herndl’s team tried a new experiment. By sending autonomous instruments to incubate microbes where they actually live, they quickly found that microbes in the depths were far less happy and hungry. The differentiating factor, they wrote in a study recently published in Nature Geoscience, was pressure. Some organisms like being under extreme pressure—they’re what’s known as piezophilic—and happily metabolize material in the deep. But they represent a small slice of the microbial communities Herndl studied—about 10 percent. The rest were ill-adapted; chances are they were suited to some other, shallower environment and had floated their way down. 

In a rare opportunity, Herndl’s team repeated these experiments around the world, taking samples from a global conveyor belt of nutrient-rich water (which includes the Gulf Stream) that connects the world’s ocean basins and takes more than a thousand years to wrap its way around. They had the benefit of time, Herndl says, on voyages that were just for deep-sea scientists—with no pesky shallow water, algae-studying scientists getting impatient while they performed experiments 4,000 meters down and spent hours pulling water from the depths. The result of those newer methods is data that shows a major gap in prior studies, says Hilary Close, an oceanographer at the University of Miami who wasn’t involved in the study. “It turns out that those past measurements were flawed,” she says. In the deep, pressure is holding the microbes back.

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There are many other factors that control how that material breaks down. One is that much of the carbon down in the deep is old—tens of thousands of years old, according to research at Woods Hole. Over time, those carbon-based molecules have been oxidized and are no longer appetizing to the microbes down there. There’s some newer and tastier material, but one theory is that the diversity of microbes slows down its degradation, because only some of these organisms are suited to breaking down certain molecules. Add to that the pressure constraints, and it doesn’t get transformed back into carbon dioxide particularly fast. (It turns out many of the original hypotheses about the Alvin lunch were at least partially correct.)

Altogether, that might look good for efforts to deliberately sink carbon in the deep. Essentially, if the lunch (or seaweed, or any other biomass) gets broken down by microbes that respire, like people do, then the carbon is more likely to escape back into the atmosphere as a gas. But if they snub the sandwiches, that’s good, right? The biomass stays where it is. Herndl once believed that his research was making that case. But now he’s skeptical of deliberate sinking. There are too many complexities to introducing a bunch of biomass into the sea, he says. If someone suddenly dumps in a pile of seaweed or the carcass of a dead whale, chances are they’ll still stimulate an unusual flurry of microbe activity.

There are a few reasons why. One is that the biomass—say, the whale carcass—may already have a bunch of microbes it picked up in shallower water riding along with it. They’ll be slowed down by the extreme conditions, but they’ll still be there, and hungry. Or maybe the carcass will get broken down before it sinks deep enough for the carbon to really get trapped by the pressure of the water above. Or maybe it depends on where the whale falls to the seafloor, and the exact makeup of the community of critters looking for a meal there. It’s nuanced, and highly specific, says Close, and not particularly well understood. “We need to know what controls the metabolic rates of microbes in the deep ocean,” she says. “What exact kind of organic matter are they receiving, and are they adapted to degrade that type of organic matter?”

Plus, while the processing of nutrients may be slow in deep open water, portions of the seafloor are comparatively teeming with life. Herndl points to observations of whale carcasses on the seafloor. “They’re degraded surprisingly quickly,” he says. “This will also happen if you dump seaweed. So I’m really skeptical of these geoengineering ideas.” 

That skepticism was also present in the 1970s, when the Woods Hole researchers examined the lunches abandoned aboard Alvin. Back then, talk of a different form of geoengineering was making the rounds: fertilizing the ocean by sinking large amounts of organic waste, which would perhaps support the food chain and rebuild fish populations. At the time, the researchers saw the preserved lunch as a cautionary tale—a reminder that the deep ocean remains mysterious and surprising, a place of chemical and biological processes that we do not fully understand. Even as some mysteries are solved, that remains very much true.

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