For centuries, scientists on expeditionary voyages have made fascinating discoveries about the natural world. The deep sea remains one of the last unexplored frontiers – what secrets does it keep?
The small plastic bag, containing two litres of seawater from the bottom of the Atlantic, was carefully placed on the swaying aft deck of the research vessel Atlantis. In that moment, Dr William Brazelton was struck by how small the sample truly was and what a monumental effort it had been to recover it. Though seemingly insignificant in appearance, this one plastic container was incredibly difficult to replace and precious for what it might reveal about the mysteries of a hidden world lying several hundred metres below Brazelton’s feet.
Atlantis had arrived at the sampling location in the mid-Atlantic a few days prior, after a lengthy journey sailing east from Cape Cod, Massachusetts. Normally it would have taken six days of the month-long expedition to reach the Mid-Atlantic Ridge. This time, it took eleven. After leaving port on September 8th 2018, the research vessel had maneuvered to stay out of the path of multiple hurricanes – Florence, Helene, and Joyce – which were all churning simultaneously through the Atlantic basin. Brazelton’s students had spent the journey training in the correct methods of collection and sampling procedure while trying to overcome the unpleasant effects of seasickness. But in truth, preparations for this expedition had begun years earlier.
Life and heat on the seafloor
The ocean is truly one of the most remote and unexplored places on the planet. Oceanographers first started to probe the deep ocean in the late nineteenth century, but the limitations of early scientific instrumentation provided only snapshots of the contours of the seafloor and the global distribution of deep-sea marine life.
Though the number of depth measurements (known as “soundings”) increased over time, it was only in the mid-twentieth century that improvements in sonar instrumentation provided a more accurate picture of the bottom of the Atlantic, and only in the last few decades that some of the detailed features of this submarine topography have come into focus.
We now know that the bottom of the Atlantic is home to a massive geological feature extending out for thousands of kilometres into every ocean basin on the planet. It is sometimes referred to as a mountain range, but this description is too simplistic – instead, it might better be visualised as an extensive rift valley, crisscrossed by transverse canyons and interspersed by submarine mountains. This rift valley is a place where two tectonic plates are pulling apart and new crust is rising to the surface as searingly hot molten rock.
An oasis of undersea life
In 1977, scientists found the first true hydrothermal vent system in the Galápagos Rift of the Pacific Ocean. The US team of geologists, geochemists, and geophysicists had been hoping to find undersea vents, but they hadn’t been prepared for colossal, chimney-like geological features belching out superheated plumes of water so saturated with dissolved minerals that they appeared to be releasing thick black smoke. They were more surprised still to discover a rich diversity of marine life living on and around these vents: enormous white clams, pale crabs, shrimp, and giant, brilliant-red tube worms. Here in the perpetual night of the abyss, they’d stumbled across an oasis of life despite the darkness and crushing pressures.
Since the discovery of the Galápagos site, many other hydrothermal vent areas have been located throughout the world’s oceans, including at the Mid-Atlantic Ridge. Life can survive in these places without relying on energy from the sun via photosynthesis. Instead, lifeforms here use energy obtained from inorganic molecules produced in geothermal reactions, a process called chemosynthesis. But there is still much we don’t know about these ecosystems, largely because they are so difficult to visit and study in situ.
In 2000, a unique vent field in the Mid-Atlantic Ridge was discovered quite by accident. Named Lost City, the tangle of sixty-metre-high vents resembles a collection of skyscrapers. But these dazzling limestone skyscrapers don’t emit plumes of black smoke and are not located at the bottom of the rift zone. Instead, this vent field is situated nearby on the summit of a submarine mountain: the Atlantis Massif.
Unlike all other known vent sites which produce heat from rising molten rock, Lost City’s heat is generated through a chemical reaction known as serpentinisation. This is triggered by a reaction between seawater and olivine – a mineral uplifted to the seafloor from the deep mantle of the Earth.
The warm water produced by serpentinisation has an extremely high pH (i.e., very alkaline), causes the minerals dissolved in the surrounding seawater to solidify, forming the characteristic geological features known as chimneys. But that’s not all the serpentinisation process does. It also produces dissolved gases – hydrogen and methane – which allow the chimneys to support life. The dissolved gases serve as sources of food and energy for microbial life, which cover the chimneys in thick, snotty mats. This allows the chimneys to support a diverse community of tiny arthropods, crustaceans, and microbes that call this inhospitable environment home.
Though hellish environments like these are some of the most isolated places on the planet, scientists believe they may be key to understanding how life first arose on Earth. If this is true, scientists say similar processes might have also taken place on other celestial bodies, with some clues indicating that similar communities might currently dwell within our own solar system on the moons Europa or Enceladus. Gaining a better understanding of how these microbial ecosystems work could give us insight into our very existence as a species and possibly answer questions for which we have long sought answers: why are we here? Are we alone in the universe?
But studying these remote ecosystems is not an easy task.
The 2018 Lost City expedition
In contrast to what you might imagine, oceanographers do not spend the majority of their careers at sea. There are only a few research vessels available for expeditions at any one time. The United States academic fleet, for example, consists of only eighteen ships currently in operation; Australia, in comparison, has just two.
For Brazelton, an Assistant Professor of Biology at the University of Utah, the 2018 expedition to the Lost City hydrothermal vent field was his first expedition in a decade; it was also his first time serving a leading role as a co-chief scientist along with expedition co-leader, Dr Susan Lang of the University of South Carolina.
The aim of the expedition was to better understand the link between geochemistry and biochemistry at sites like Lost City. The team hope to identify which microbes and which specific metabolic pathways convert chemical energy into biological energy, or, as Brazelton puts it, they are “trying to understand the connection between geology and life”.
The project first began when their funding proposal was accepted in 2015, but three more years would pass before the research vessel Atlantis, operated by the Woods Hole Oceanographic Institution, became available in September 2018. The ship’s schedule also had to align with the availability of the Remotely Operated Vehicle (ROV) Jason, a tethered submersible robot able to descend to a depth of over 6000 metres.
The scientific staff and the ship’s crew were joined by a dedicated group of ten engineers specialising in the vehicle’s maintenance and operation. It was only through the collaboration of this multidisciplinary team, operating an assembly of complex instruments, that a few water samples could be gathered from one of the most isolated geological features on the planet.
The first two expeditions to Lost City (in 2000 and 2003) made dives using the Human Occupied Vehicle (HOV) Alvin, the same three-person submarine used to locate the wreck of the RMS Titanic in 1986. The few samples recovered from these expeditions led to the discovery of novel microbes and revealed the presence of organic molecules generated through serpentinisation.
For the purpose of collecting useful water samples for analysis, however, a submarine can be limiting. Alvin can only spend a few hours on the seafloor before needing to surface to recharge batteries and refill oxygen tanks for the sake of the human occupants. In contrast, Jason can remain at depth for as long as necessary bacause it is unmanned and powered by a tether attached to the research vessel. At the Lost City site, Jason is able to stay under for about 20 hours.
Brazelton explains that lengthy time spent on the seafloor was needed to acquire large-volume water samples for the key analyses they planned to conduct, spanning geochemistry, microbiology, culturing, and analysis of community genomes. The only way to accomplish this sort of sampling, he says, is “if you’re willing to dedicate twelve hours of the vehicle only sitting in the same spot”.
Jason was connected to a control room (a converted shipping container) on the deck of the Atlantis by a fibre optic cable. This allowed operators to remotely manoeuvre the robot while watching high-definition footage transmitted from the seafloor in real time, the gloom of the abyss illuminated only by the lights attached to the vehicle.
The robot essentially acts as an extension of the scientists’ eyes and limbs, blurring the boundary between human and machine. Scientists hope that in the near future, robotic and semi-autonomous systems will facilitate oceanographic research and allow longer-term in-situ monitoring of marine life. Oceanographers Dr Julie Huber and Dr Christina Preston recently wrote, “It is time to embrace the space analogy for our oceans and be more like NASA: if they can land a scientific laboratory on Mars, we can do it here in Earth’s oceans.”
Still, each new expedition offers a chance to refine instrumentation and equipment. The 2018 Lost City expedition, for example, tested a new water sampling instrument designed by Lang, which meant that enough water was available for each sample to be processed using a variety of different analysis techniques. Previous instruments could only take limited samples at a time; as Brazelton explains, “You would put the sampler into the vent source, pull the trigger, and you were done. If you wanted a second sample, you would have to pick up a second, separate sampler, try to find the same source again, and pull the trigger again”.
Lang’s instrument can take multiple samples without moving the water intake nozzle – which is essential for replicate experiments and for comparing measurements.
“Imagine if you had to study a Yellowstone hot spring by taking each sample in a different part of the spring,” Brazelton explains. “With no two samples looking anything like each other, it would be impossible to say anything with confidence.”
The instrument also filtered the seawater at the time of collection, so instead of retrieving the water sample, the scientists could simply recover the filter.
Pushing the envelope further, the team carried out what Brazelton describes as “test tube experiments on the seafloor”. Before deploying Jason, a selection of organic molecules could be added to the sampling bags as bait, and when the bags were exposed to water coming out of the vents, different microbes would enter looking for a snack. Once the sample was retrieved, the team could determine which microbes consumed which carbon molecules, thus providing a sampling of the diverse microbial community that inhabits the vent site.
Previous expeditions had identified some of the microbes present at Lost City using gene sequencing techniques, but the 2018 expedition conducted experiments in situ in order to determine how microbes were taking up energy and living in this extreme environment. The goal was to collect “really good, and really large, samples from a few sites” rather than “questionable quality samples from many sites”. In contrast, most oceanographic expeditions typically carry out opportunistic sampling and piece together an explanation for the observations – like trying to infer the full picture of a complex puzzle when the majority of the pieces are missing.
An important discovery from the 2018 expedition was the presence of warm water flowing from porous rocks on the eastern side of the Atlantis Massif. Researchers still don’t know what effect this venting has on nearby communities of microbes, but it’s possible that such diffuse venting may be commonplace.
“Lost City could be an extreme example of a coupled geochemical-biochemical interaction that occurs in subtle ways all over the seafloor,” says Brazelton. “We would have never known to look for such a thing before Lost City was discovered.”
But the window of opportunity to explore Lost City was brief. After only six days on station, it was time for the Atlantis to head to port in San Juan, Puerto Rico. When it arrived on October 3rd 2018, the scientific staff disembarked to prepare their samples for shipping back to their university laboratories.
This moment was one of both triumph and sadness – because it might be the last time these researchers undertake such a journey.
It is nearly impossible for marine scientists to obtain research funds for expeditions when the mission is solely exploration. Funding agencies want some guarantee of results, and so scientists generally propose visits to known sites and define their sampling and analysis procedures in advance. And yet so much of the ocean remains unexplored. Places like Lost City – discovered by accident – hint at the wonders that may still be out there.
“We’ll have to get the most we can from these samples, hopefully publish a bunch of papers, and then try to develop a new scientific question for the next expedition,” says Brazelton. “I don't know what that would be, and that’s the point.”
Meanwhile, with the expedition departed, the Lost City vent field has been returned to perpetual darkness once more. Unperturbed by human probes, chemical and biological processes continue as they have for millennia. And somewhere, in the warm waters emanating from a crack in those silent, towering chimneys, a microbe is busy going about its daily habits, metabolising energy and reproducing, unaware of its human-perceived role as a ‘missing link’ in a complex ecosystem.
Though Lost City remains difficult to access and seemingly inhospitable, it is in this isolated part of the deep sea that we may yet discover answers to long-posed questions about the meaning of life and our place in the cosmos.