The phrase “low energy” is normally used as a pejorative meant to imply lethargy or sloth. But for the mysterious lifeforms that lurk deep under the seafloor of Earth’s oceans—a sunless habitat with hardly any fuel sources—being “low energy” is a matter of survival.
Now, a team of scientists has discovered that these otherworldly creatures “subsist at energy fluxes lower than have previously been shown to support life, calling into question the power limit to life,” according to a study published on Wednesday in Science Advances.
To put it in perspective, the energy budget of an average human could power a ceiling fan. These organisms exist on an energy budget about 50 quintillion (a billion billion) times smaller than that, according to the study. In effect, we have just discovered that certain lifeforms can be so incredibly inactive that it has expanded our conception of what life on Earth, and elsewhere in the cosmos, can look like.
“We have this habitat in the sediments underneath the seafloor that previously was unknown to us until just a few decades ago,” said lead author James Bradley, an environmental scientist at Queen Mary University of London, in a call.
“This is thought to be a very energy-limiting environment, but it contains a vast amount of microbial life,” he continued. “The number of cells that are contained in global subseafloor sediments is equivalent to the number of cells in all of Earth’s soils, or all of Earth’s global ocean.”
Scientists first stumbled upon the existence of this deep subsurface biosphere decades ago by drilling cores across the globe, from coastal regions to the open ocean. These expeditions have revealed that life, in the form of intact microbial cells, has found its way into niches that can be kilometers under the seafloor.
Bradley and his colleagues used global datasets from these seafloor surveys to calculate the energy budgets of these frugal lifeforms. The team’s numerical model focused on how the ecosystems digest particles of organic carbon—mostly the remains of dead stuff—that rain down on the seafloor from higher levels of the ocean.
“We have good evidence to believe that the oxidation of organic carbon, the burning of this organic matter, is the primary fuel source for life in the subsurface,” Bradley explained. “It's a system cut off from light and it relies on the input of this sinking organic material onto the seafloor, and then its eventual burial and deposition.”
“We used numerical modeling to predict the flow of energy through the system, the number of cells that are there, and the rate of organic carbon that has degraded, which supplies, in part, the energy,” he said.
Your own energy needs, assuming you are a human, are about equal to the power required to run a ceiling fan. Bradley’s team found that the average seabed cell has an energy budget that’s about 50 quintillion (a billion billion) times smaller than that, according to the study.
As you can imagine, life in these dark regions moves at a very different pace to our lush surface world. In the subsurface, microbial living cells often exist in a kind of suspended animation that may span millions of years.
“It looks like the majority of these organisms are living at energy regimes that are below what we thought was even capable of maintenance—just staying alive—so the idea that there might be widespread growth and cell division seems not very likely,” Bradley said.
“Are these the same cells, or at least only a few generations removed from those that were deposited tens of thousands, hundreds of thousands, or even millions of years ago?” he added. “I think that is still an open question.”
It’s mind-boggling that we share a planet with life forms that can eke out basic biological activities with so little fuel in the tank. But it also sheds light on the potential habitability of other worlds in the solar system, such as Mars or Europa, as well as the odds that exoplanets orbiting other stars might bear life.
“What we’re seeing through this study is that these organisms have a very fundamentally different relationship with energy than most of life that we’re familiar with,” Bradley said. “If it's possible that organisms can survive over extremely long timescales on very little energy, that definitely expands the possible habitats that we might search for life in.”
That’s a tantalizing reality to ponder with regards to worlds like Mars, which scientists think was potentially habitable to microbes more than three billion years ago. Perhaps microbial Martians retreated into the subsurface eons ago, and have laid in wait for eons in case friendlier conditions return to the red planet.
Nobody knows, of course, but it’s an encouraging sign that the deep biosphere, which is as alien an ecosystem as you can get on Earth, is teeming with weird ancient life. The team’s findings cover sediments that date back 2.6 million years, but scientists hope to recover more samples that may push that date back upwards of 50 or 100 million years.
“It would be nice to be able to expand this numerical framework to be able to capture those environments,” Bradley said. “We would expect to find that the more ancient environments are subsisting at the same or lower energy regimes to what we find here.”
“As we continue to explore sites that are more difficult to access, sites that are hotter, sites that have been cut off from the sort of dynamic parts of the Earth for longer timescales, we might start to see areas where we don’t find life,” he concluded. “But so far, the majority of the places that we’ve looked we’ve found these zombie-like cells.”