Drilling Down | The Limits of Life Below the Seafloor
IT USED TO BE ASSUMED THAT DIRECT SUNLIGHT WAS NECESSARY to sustain all major forms of life on Earth.
That was until 7-foot tube worms, giant white clams, eyeless shrimp, and many other creatures were discovered to be thriving in complete darkness nearly a mile below the surface of the sea.
This whole other world was not only flourishing in the dark but in tremendous water pressure—over 3,000 pounds per square inch—in temperatures double the boiling point of water, and where sulfur, iron, copper, and other toxic metals and minerals spew from vents in the seafloor.
These deep-sea hydrothermal vents are a result of volcanic activity that allows seawater to seep through cracks in the seafloor, absorbing minerals and metals from the sediment and becoming super-hot from the underlying magma. The water erupts back out through the vents like underwater geysers.
Scientists had suspected their existence near rift valleys where the seafloor is spreading apart, allowing magma below the Earth’s crust to be pushed up. But they had not expected to find life in these extreme environments.
So, when scientists traveled to the Galápagos Rift in the 1970s in search of hydrothermal vents, they were surprised to find a thriving oasis in the darkness of the deep sea.
Instead of using energy from the sun, like plants on land, bacterial communities near these vents were found to be using energy from the Earth—absorbing sulfur compounds (which are toxic to most land-based life) gushing from the seafloor. This process allows them to grow and reproduce, providing a food source for other creatures.
“We were struck by the thought, and its fundamental implications, that here, solar energy, which is so prevalent in running life on our planet, appears to be largely replaced by terrestrial energy … bacteria taking over the role of green plants,” said the late biologist and oceanographer Holger Jannasch in the Annual Review of Microbiology in 1997.
Jannasch was one of several scientists, along with Bob Ballard, now at the University of Rhode Island’s Graduate School of Oceanography (GSO), to be aboard the 1977 Galápagos expedition that first discovered these hydrothermal ecosystems.
“This was a powerful new concept and, in my mind, one of the major biological discoveries of the 20th century.”
Major, because the discovery of life in such harsh conditions made scientists reconsider environments that were previously viewed as too hostile for life, such as the barren landscape of Mars or the icy moon Europa that orbits Jupiter. But instead of looking up to the stars, many scientists are looking down into the depths of oceans to better fathom the possibilities of where life could exist.
“Understanding the limits of life is fundamental to our exploration for life outside of Earth,” says Art Spivack, an oceanography professor at GSO who was part of an international science expedition in 2016 to examine life beneath the seafloor. “We use extreme environments on Earth to understand the potential for life throughout the rest of the solar system and universe.”
The “deep biosphere,” or the zone of life below the seafloor, also fits the requirements of such an extreme environment. It’s dark, food sources are severely limited, the pressure is very high, and the deeper you dig, the hotter it gets.
“The deep ocean is a good analog of what could go on in other planets,” says Spivack.
Spivack, along with GSO graduate students, was among a team of 31 scientists from eight nations that embarked on a 60-day scientific drilling expedition to the Nankai Trough off the coast of Japan as part of the International Ocean Discovery Program’s (IODP) Expedition 370. The mission was to study the deep biosphere to find out how hot it can get before life can no longer exist and to determine what conditions ultimately limit life on Earth.
The Nankai Trough was chosen for this study because it is a known hot spot at a relatively shallow depth in the seafloor, making it easier for scientists to collect samples from extreme temperatures. This boundary marks a subduction zone right under Japan where one tectonic plate bends down under another less-dense plate.
As the one plate bends down, sinking into the fluid mantle just under the Earth’s crust, the underlying heat is then transferred closer to the surface. Here, scientists were able to take core samples from an environment exceeding 248 ̊F, the highest temperature at which life (in the form of microbes) is known to exist, at a depth just over 1 kilometer— roughly three-quarters of a mile—below the seafloor.
“We found abundant [microbes] all the way to the bottom of the hole,” says Spivack, referring to the 1,200-meter hole the crew dug using special diamond- crystal-coated drill bits to cut through the seafloor for deep core samples. “We don’t know yet how much of those are being actively produced or consumed, or if they’re just relics, and that’s a major part of the continuing effort.”
The presence of these microbes at such depths leaves scientists wondering about how deep in the Earth life can subsist. “Where is the bottom of the deep biosphere? What ultimately limits life here?” asks Verena Heuer, a geochemist at the University of Bremen and co-chief scientist of the international expedition, in the expedition’s video summary.
(Above) The expedition ship, Chikyu, was home to an international team of scientists of different disciplines. Photograph by JAMSTEC/IODP
This kind of expedition wouldn’t have been possible over a decade ago. Advances in drilling technologies, the development of telepresence to connect scientists at sea with colleagues on land, enhanced laboratory capabilities for measuring and storing samples on the ship itself, and especially improvements in analytic methods have allowed scientists to detect microbes in concentrations as low as 10 cells per cubic centimeter.
That’s like looking for 10 grains of sand in an Olympic-sized pool.
“The techniques are now 1,000 times more sensitive, allowing scientists to look for microbes at deeper depths where they are less abundant,” says Heuer.
The first inkling of life beneath the seafloor goes back as far as the 1920s, when two petroleum scientists found bacteria in fluids produced from drilling oil wells, says Steve D’Hondt, a GSO oceanography professor, who led the first scientific expedition dedicated to studying life beneath the seafloor in 2002.
It wasn’t mainstream science at the time, he says, but interest grew as scientific ocean drilling began with the Deep Sea Drilling Project in 1968, followed by the Ocean Drilling Program in 1983, and the IODP in 2003. The primary drivers, in the beginning, were focused on geologic questions.
It wasn’t until the 1970s that scientists became aware of the magnitude of life in the deep sea. In the 1980s and 1990s, scientists began to look more seriously at the potential for life beneath the seafloor.
The vast reservoir of microbial life found there is now considered comparable to that found in the oceans themselves and plays an important role in global climate by storing carbon and influencing other chemical processes.
“[Understanding] the range of what’s possible biologically has been expanded tremendously by studying the subseafloor,” says Spivack.
Yet, the full impact of microbial activity on chemical cycling or even how microbes get energy is not fully understood, according to the IODP, but it is what scientists like Spivack are looking to find out from the 13,000 samples still being analyzed from the Nankai Trough. In addition, Spivack admits scientists might find answers for questions they haven’t even asked yet.
“We don’t become limited by the ideas which we came in with,” he says. “We should try to see where the data leads us.”
And to follow the data is an enormous undertaking. To conduct key microbial research, a stable laboratory onshore, equipped with state-of-the-art cell detection and quantification capabilities, was required as well as fresh core samples delivered directly from the ship.
Over 44 helicopter trips were made from the ship to the onshore lab during the 2016 IODP expedition. Nearly 40 scientists of different disciplines from eight nations, in addition to a helicopter crew and drilling technicians, made such a feat possible. Although field operations concluded two years ago, scientists are still analyzing and collecting data from a temperature sensor left inside the drill site to better characterize temperature changes in the subseafloor.
“Studying these extreme environments to better understand how evolution happens in these systems could revolutionize and reframe what we know,” says Justine Sauvage, a Ph.D. student at GSO who focused on how microbes use hydrogen for food.
“How do the organisms survive? We try to interpret the chemistry in terms of the metabolism of the deep biosphere. How much do they eat? What do they eat? How much do they produce?” says Spivack, who headed the inorganic geochemistry analysis of the expedition with Sauvage and fellow graduate student Kira Homola.
But measuring the chemistry is only “half of the story,” says Sauvage.
Takehiro Hirose, a physical properties specialist at the Japan Agency for Marine-Earth Science and Technology, is working with both geochemists and microbiologists to see if there’s a link between earthquakes and microbes. Other scientists are also investigating how microbial activity in the deep biosphere influence the carbon cycle and climate.
“The special thing is, you have scientists from all over the world coming together and just trying to solve one problem,” says Florence Schubotz, an organic geochemist from Marum, Germany. “You have scientists with backgrounds in geochemistry, microbiology, geology, and sedimentology. Every day, you learn something new.”
Building that community was one of the primary goals of the Deep Carbon Observatory (DCO), one of several international organizations that supported the IODP 2016 expedition. The DCO is a 10-year global research initiative to better understand the backbone of all life on Earth—carbon—especially as life in the deep biosphere plays an important role in the fluxes of carbon both below and above the surface.
“It’s about challenging our assumptions about what life is like on Earth,” says Katie Pratt, DCO’s communications director, who is part of the program’s outreach and engagement team based at GSO.
The other primary goal of the DCO is to expand the field of deep carbon research.
“The DCO has done a good job of growing the field and community,” said D’Hondt, who has been involved in its Deep Life program, which has supported postdoc and graduate students to work on relevant topics.
“It provides a great opportunity for students to get involved in international research and build experience to lead future expeditions of this magnitude,” says Spivack. “Justine and Kira were instrumental in producing an unprecedented dataset that wouldn’t have happened without their involvement and drive, and preparation at GSO. It sounds corny, but it’s true.”
Although DCO’s 10-year initiative concludes in 2019, it’s not the end. The program’s website will remain as a “legacy” site, storing all the information collected over the various projects, including the IODP 2016 expedition. The most lasting effect, however, is the network of over 1,000 scientists in 40 countries and counting.
“Even though DCO will cease to exist as an entity, the community of scientists that we’ve built will continue doing this work in this new field of deep carbon science,” says Pratt.
The results—and the questions—that arise from studying life beneath the seafloor will no doubt further science’s continued quest to understand this world and any others that may exist.
– By Meredith Haas