If you were to hit the seafloor and continue to travel down, you’d run into an ecosystem unlike any other on earth. Beneath several hundred meters of seafloor sediment is the Earth’s crust: thick layers of lava rock running with cracks that cover around 70% of the planet’s surface. Seawater flows through the cracks, and this system of rock-bound rivulets is enormous: it’s the largest aquifer on earth, containing 4% of global ocean volume, says Mark Lever, an ecologist who studies anaerobic (no-oxygen) carbon cycling at Aarhus University in Denmark.
The sub-seafloor crust may also be the largest ecosystem on earth, according to a new study by Lever, published this month in Science. For seven years, he incubated 3.5 million-year old basalt rock collected from 565 meters below the ocean floor–the depth of nearly two stacked Eiffel towers–and found living microbes. These microbes live far away from the thriving bacterial communities at mid-ocean ridges, and survive by slowly churning sulfur and other minerals into energy.
But just how big is this chemically-fueled ecosystem that survives entirely without oxygen? If the results from his sample, collected from below the seafloor off the coast of Washington state, are similar to those found across the planet, then diverse microbial communities could survive throughout the ocean’s crust, covering two-thirds of the earth’s surface and potentially going miles deep.
The sub-seafloor crust has plenty of space and energy-rich minerals–a welcoming potential habitat for a large microbial community–“but we have no idea what the ecosystem looks like,” says Julie Huber, a microbial oceanographer at the Marine Biological Laboratory in Woods Hole, Massachusetts. “Mark’s evidence would point to it being a very different world.”
Microbes that get their energy from minerals, rather than from sunlight, are far from rare. The most well known of these so-called chemoautotrophic or chemosynthetic bacteria are those found at hydrothermal vents in the deep sea. Some of these bacteria live symbiotically with giant tubeworms, mussels and clams, providing chemically-produced energy to these larger organisms as they “breathe” the sulfur-rich water erupting from the vent–not unlike how plants convert sunlight into energy at the surface. Chemosynthetic microbes are also found in the rotting and oxygen-poor muck of salt marshes, mangroves and seagrass beds—“any place you’ve got stinky black mud, you can have chemoautotrophy,” says Chuck Fisher, a deep-sea biologist at Pennsylvania State University in College Park.
But what makes Lever’s sub-seafloor microbes different is that they don’t use any oxygen at all. The symbiotic bacteria at hydrothermal vents are often described as “life without sunlight,” but they still rely on sunlight indirectly by using sun-produced oxygen in the chemical reaction to generate energy. Chemosynthetic microbes in salt marshes feed on decomposing plants and animals, which got their energy from sunlight. Even deep-sea sediment is accumulated from an assortment of dead animals, plants, microbes and fecal pellets that relies on light energy.
The oceanic crust microbes, on the other hand, rely entirely on
non-oxygen-containing molecules derived from rock and completely removed from photosynthesis, such as sulfate, carbon dioxide and hydrogen. “In that sense it’s a parallel universe, in that it runs on a different type of energy,” says Lever. These molecules provide a lot less energy than oxygen, creating a sort of microbial slow food movement. So instead of dividing and growing quickly like many oxygen-based bacteria, Fisher suspects that microbes in the Earth’s crust may divide once every hundred or thousand years.
But just because they’re slow doesn’t mean they’re uncommon. “There are lots of data that there is a large, very productive biosphere under the surface,” says Fisher.
In addition, microbial population sizes in different areas of the crust may vary greatly, Huber notes. Through her studies on the fluid found between the cracks in the crust, she says that in some areas the fluid contains about the same number of microbes as standard deep-sea water collected at ocean depths of 4,000 meters (2.5 miles): around 10,000 microbial cells per milliliter. In other regions, such as at the Juan de Fuca Ridge in the Pacific Ocean where Lever found his microbes, there are fewer cells, around 8,000 microbes per milliliter. And in other regions, such as in non-oxygenated fluid deep in hydrothermal vents, there can be around 10 times more.
It’s not just the number of microbes that vary depending on location–it’s possible that different microbial species are found in different types of crust. “Different types of rock and different types of chemistry should result in different types of microbes,” says Andreas Teske, a deep-sea microbial ecologist at the University of North Carolina at Chapel Hill and co-author on Lever’s paper. The Juan de Fuca Ridge is a relatively hot area bursting with new rock, which tends to be made of more reactive minerals and thus able to provide more energy. Other parts of the crust are older, composed of different minerals, and cooler. And, in some regions, oxygenated water reaches down to the cracks.
It’s this infiltrating seawater that keeps this sub-seafloor ecosystem from existing on a completely separate plane from our oxygenated one. “The crust plays a significant role in influencing the chemical composition of the ocean and the atmosphere, ultimately influencing cycles on earth,” says Lever. Some of the compounds created by oceanic crust microbes from rock are water soluble, and will eventually enter the ocean. Sulfur, for example, is present in magma—but after the microbes use it for energy, it’s converted to sulfate. Then it dissolves and becomes an important nutrient in the ocean food chain.
Lever’s find of a microbial community in the crust could catalyze the scientific community to answer these questions. For example, what kinds of microbes are found where, do they interact through interconnected cracks in the rock, and what role do they play in mineral and nutrient cycling? In some ways, it’s very basic exploratory work. “A lot of what we do on the seafloor is similar to what we’re doing on Mars right now,” says Huber. “Controlling Curiosity is very similar to operating an ROV under the ocean.”