Whenever scientists look for life in Earth’s most inhospitable nooks—acidic hot springs, suffocating subterranean chambers, and hundred-million-year-old rocks beneath the seafloor—they frequently find microorganisms adapted to the extreme environments. And now, research from the DEEPSEA CHALLENGE expedition to the bottom of the Mariana Trench suggests the deepest realms of the ocean can also power diverse assemblies of organisms.
While exploring the Sirena Deep portion of the trench with a robotic lander, scientists found what they believe is evidence of a thriving microbial community clinging to the rocks in the abyss. Microbes and larger organisms, such as shrimp-like amphipods, have been found in the mud of the Mariana Trench before. But unlike other living things in this deep watery realm, which subsist off the “marine snow” of dead organisms and debris drifting to the ocean floor, these microbes appear to feed off chemicals produced when seafloor rocks react with water, the team reports in the journal Deep-Sea Research I.
Because they don’t rely on any life swimming above, these green, fuzzy mats affixed to rocky outcrops may offer clues about the kind of alien life-forms that could survive in the ocean depths of moons in the outer solar system, such as Jupiter's moon Europa or Saturn's moon Enceladus.
“It’s a glimpse of how life might exist billions of miles away from us, right now,” says study co-author, filmmaker, and National Geographic Explorer James Cameron in an email. “And it may also be a glimpse back through time, four billion years, to the dawn of life itself.”
The microbes likely represent the deepest chemosynthetic community ever discovered, feeding off the molecules released by geologic processes more than 6.5 miles beneath sea level. “The discovery of this putative microbial ecosystem thriving on chemosynthesis in the deepest, darkest, highest pressure region of our ocean serves as a guide for the potential habitability within the ocean of Europa,” says Kevin Hand, a NASA astrobiologist, National Geographic Explorer, and lead author of the new study.
However, Hand and other experts caution that the discovery, based on images from the seafloor and samples of water and sediment, needs to be confirmed by collecting a sample from the stringy material.
“They didn’t actually get a sample of the mat itself,” says geomicrobiologist Jenn Macalady of Pennsylvania State University, who has spotted similar-looking microbial communities in underwater caves but did not participate in the new study. Still, she says, the fuzzy patches are very likely alive.
“I’ve seen things that look like that in so many dark, underground places,” she says. It “looks every bit like slime I’ve seen in flooded caves in the Caribbean.”
Journey to the bottom of the ocean
Near Guam in the western Pacific Ocean, the Mariana Trench is a curving chasm at the tectonic seam where the mammoth Pacific plate dives underneath the smaller Mariana plate. The trench is so deep that the summit of a submerged Mt. Everest would sit more than a mile beneath the waves. Whatever survives there must withstand perpetual darkness, temperatures just above freezing, and pressures more than 1,000 times greater than at Earth’s surface.
Hand and Cameron journeyed to the waters above the Mariana Trench in 2012 as part of a National Geographic-funded expedition. During that trip, Cameron made the first solo dive to the deepest part of the trench, Challenger Deep. At the bottom, 35,787 feet below sea level, he encountered an utterly bland seascape dominated by drifting beige sediments and sparse signs of life.
“We didn’t find much visible-scale life on the bottom in Challenger Deep,” Cameron says. But later, during the same expedition, the team dropped a remotely operated, phone-booth size lander into the Sirena Deep, a neighboring part of the trench that extends to depths of 35,029 feet. They targeted this location—the third deepest in the ocean—because it is seismically active, potentially volcanically active, and likely more nutrient-rich due to ocean currents than other portions of the trench.
Together, these qualities hint that the Sirena Deep might harbor “a more vigorous biological presence than elsewhere along the Mariana Trench,” says the University of Hawaii’s Patricia Fryer, a co-author of the new study who has closely studied this portion of the trench.
Chemical reactions in the deep
Coming to rest on a sloping incline, the lander’s onboard cameras revealed the first instance of something other than murky sediments near the trench’s bottom. A heap of rocks—either an outcrop protruding through the sediments or a pile of talus that had fallen from a cliff above—greeted the deep-sea lander. Either way, the rocks would have been forged in the furnace of Earth’s mantle and thrust upward by the shifting crust.
“Up until this dive, we had no indication of actual native rocks—outcrops or talus boulders—at that depth,” Hand says. “The reason that’s important is, we want to understand the geologic and geochemical context for what might be helping any microbial ecosystems down there survive.”
Without access to sunlight for photosynthesis, microbes have a couple ways to make a living. “There’s being a recycler, and chewing on food that someone has already made”—for example, food that has fallen from the sunlit waters near the sea surface—"or there’s living off rocks and stuff dissolved in the water,” Macalady says.
The first kind of microbe was discovered decades ago in the Mariana Trench. But the new study provides evidence for the other kind of life, living off chemical reactions with the rocks.
In caves and other dark corners of the planet where iron- and magnesium-rich rocks meet seawater, a chemical reaction called serpentinization has been shown to fuel microbial metabolisms. The reaction produces a tiny bit of heat and generates compounds like methane and hydrogen that can feed microbial metabolisms.
“Bacterial communities living off of serpentinization are a whole different story and can exist even where solar-powered life doesn’t,” Cameron says.
The team discovered clear signs of serpentinization in the rocks of the Sirena Deep, both in the chemistry of the seawater and in visible alterations to the rocks themselves. The site, therefore, appears to have the fuel that chemosynthesizing microbes require. And though the pressures in this part of the ocean are high enough to crush a military submarine, microbes are so small that they “don’t give a damn” about such extreme forces, says Penelope Boston, an astrobiologist at NASA’s Ames Research Center who did not participate in the study.
Pictures of life
When the team zoomed in on images taken by the lander, something peculiar popped out: greenish, droopy, finger-like filaments clinging to the rocks. The team suspects this assemblage is a microbial mat, a complex structure of many layers of bacteria.
“We initially referred to them as the bearded rocks of the Sirena Deep, because it looks like a whiskery beard on the bottom of the ocean,” Hand says.
The team suspects that, if these truly are microbial mats, they’re feeding solely off the chemical byproducts of serpentinization and surviving independently of matter drifting down from above. Relying only on energy and chemicals produced by geologic processes on the seafloor, these microbes could be similar to the first life on Earth—or to microbes that might exist in the ice-encrusted ocean of Jupiter’s moon Europa. There, Hand says, the water extends as deep as 100 miles, but the moon’s lower gravity means the pressure is about the same as on the seafloor of Earth.
“The Sirena discovery tells us much about what we might find under alien oceans, but it’s just the first glimpse,” Cameron says. “From studying our own ocean’s greatest depths, we will learn to build the vehicles and instruments needed to explore those alien deeps.”
The supposed microbes in the Mariana Trench could even be the base of a food chain in the deepest, darkest part of the ocean, Hand says, feeding other critters such as shrimp-like amphipods. But without a sample of the fuzzy material itself, scientists cannot say with absolute certainty that the structures are living microbes.
"I have spent a lot of time staring at the seafloor, thinking an object was one thing or another, and I am often wrong through the lens of a camera or porthole or whatnot,” says Julie Huber of the Woods Hole Oceanographic Institution who did not participate in the study. “Physically collecting and querying a sample is extremely important.”
Tantalizing evidence from seawater collected by the lander suggests that numerous microbial species are present near the fuzzy rocks—species that are common in other parts of the deep ocean, including members of the Rhodobacteraceae and Shewanellaceae families.
“The Mariana Trench is very deep, but it’s ocean bottom,” Boston says. “So it’s not like it’s different ocean bottom—it’s just deeper ocean bottom. From the microbes’ point of view, it’s probably a great place to live.”