Barely living “zombie” bacteria and other forms of life constitute an immense amount of carbon deep within Earth’s subsurface, according to scientists nearing the end of a 10-year international collaboration to reveal Earth’s innermost secrets.
Scientists with the Deep Carbon Observatory (DCO) say the discovery of this so-called microbial “dark matter” dramatically expands our perspective on the tree of life, and improves our understanding of the impact on life in subsurface locations manipulated by humans (e.g., fracked shales, carbon capture and storage).
Some highlights of the many key discoveries and insights:
The deep biosphere constitutes a world that can be viewed as a sort of “subterranean Galapagos” and includes members of all three domains of life: bacteria and archaea (microbes with no membrane-bound nucleus), and eukarya (microbes or multicellular organisms with cells that contain a nucleus as well as membrane-bound organelles). Among them are millions of distinct types, most yet to be discovered or characterized.
Deep microbes are often very different from their surface cousins, with life cycles on near-geologic timescales, dining in some cases on nothing more than energy from rocks.
The genetic diversity of life below the surface is comparable to or exceeds that above the surface.
Combined with estimates of subsurface life under the oceans, total global Deep Earth biomass is approximately 15-23 billion tonnes of carbon – 245-385 times greater than the carbon mass of all humans on the surface.
Rick Colwell of Oregon State University comments, “We can only marvel at the nature of the metabolisms that allow life to survive under the extremely impoverished and forbidding conditions for life in deep Earth.”
No limits yet to conditions for life
Indeed, these new discoveries underline the fact that the absolute limits of life on Earth in terms of temperature, pressure, and energy availability have yet to be found – the records continually get broken. For example, a frontrunner for Earth’s hottest organism in the natural world is Geogemma barossii, a single-celled organism thriving in hydrothermal vents on the seafloor. Its cells, tiny microscopic spheres, grow and replicate at 121°C (21 degrees hotter than the boiling point of water).
The record depth at which life has been found in the continental subsurface is approximately 5 km; the record in marine waters is 10.5 km from the ocean surface, a depth of extreme pressure; at 4000 metres depth, for example, the pressure is approximately 400 times greater than at sea level.
Many scientists compared deep life to ecosystems such as the Amazon rainforest and the Galapagos Islands
How these deep microbial life forms get the energy to live and reproduce is still an enigma. DCO scientists have yet to understand whether methane, hydrogen, or natural radiation (from uranium and other elements) is the most important energy source for deep life, and which sources of deep energy are most important in different settings. Karen Lloyd, of the University of Tennessee at Knoxville, says “Today, we know that, in many places, they invest most of their energy to simply maintaining their existence and little into growth, which is a fascinating way to live.”
Many other mysteries remain to be investigated, including questions around the origins of deep life, and how microbial populations move from place to place. Scientists are still wondering whether life started deep in Earth (either within the crust, near hydrothermal vents, or in subduction zones) and then migrated upwards toward the sun, or started in a warm little surface pond and migrated down.
Questions and mysteries
Other questions include, how do subsurface microbial zombies reproduce, or live without dividing for millions to tens of millions of years? How does deep life spread – laterally through cracks in rocks? Up, down? How can deep life be so similar in South Africa and Seattle, Washington? Did they have similar origins and were separated by plate tectonics, for example? Or do the communities themselves move? What roles do big geological events (i.e. plate tectonics, earthquakes, creation of large igneous provinces, meteoritic bombardments) play in deep life movements?
“We recently demonstrated the high reactivity of deep biota to CO2 injections [as part of Carbon Capture and Storage], which ultimately led to the bioclogging of the injection well, and surrounding reservoir.”
—Benedicte Menez of the Institut de Physique du Globe in Paris, France
“Our studies of deep biosphere microbes have produced much new knowledge, but also a realization and far greater appreciation of how much we have yet to learn about subsurface life,” says Rick Colwell of Oregon State University. “For example, scientists do not yet know all the ways in which deep subsurface life affects surface life and vice versa.”
Benedicte Menez of the Institut de Physique du Globe in Paris, France, believes deep life has an important impact on global biogeochemical cycles and chemical equilibria in habitable rocks. According to him, “Deep Life plays a role in aquifer quality, for example, or carbon capture and storage (CCS). Unfortunately, the deep biosphere is very poorly considered in engineering operations carried out in the subsurface. We recently demonstrated the high reactivity of deep biota to CO2 injections (CCS), which ultimately led to the bioclogging of the injection well, and surrounding reservoir.”
These discoveries have inspired a sense of wonder, with many scientists comparing deep life to lush, beautifully evolved terrestrial ecosystems such as the Amazon rainforest and the Galapagos Islands. Says Fumio Inagaki of the Japan Agency for Marine-Earth Science and Technology, “Expanding our knowledge of deep life will inspire new insights into planetary habitability, leading us to understand why life emerged on our planet and whether life persists in the Martian subsurface and other celestial bodies.”
—with files from the Deep Carbon Observatory