Earth's metabolism

Exploring the deep biosphere: Organisms abound in regions thought devoid of life

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70% of all bacteria live deep in the earth, constituting a reserve of carbon and biodiversity that vastly outnumbers and outweighs all humans combined

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A methane-producing bacterium, found in a coal bed 2 km below the Pacific Ocean floor.

70% of all bacteria live deep in the earth, constituting a reserve of carbon and biodiversity that vastly outnumbers and outweighs all humans combined

Continuing our series on the social and scientific basis of metabolic rift theory.

Introduction, by Ian Angus

In a previous article in this series, I discussed Vladimir Verdadsky’s pioneering work in identifying and defining the biosphere, the “specific, life-saturated envelope of the Earth’s crust,” comprising all living matter on Earth, and all parts of Earth where life exists, from the crust to the upper atmosphere.

In recent years, Earth System science has not only confirmed Vernadsky’s insights, but extended them, showing that the biosphere is much larger and more complex than he could have known. We now know that most of the biosphere is not readily visible, and that there is still much to learn about how it functions and may affect human life and society. Contrary to claims made by some theorists, it is patently obvious that most of the biosphere (let alone the rest of the Earth System) is not constructed by, contained in, or internal to capitalist society.

As a case in point, this week the Deep Life community of the Deep Carbon Observatory project released results of a ten-year study of organisms that live kilometers below us, in rocks once thought to be entirely devoid of life. As the following summary reveals, the inhabitants of the deep biosphere vastly outnumber and outweigh humans, and most of them are still unknown to science. The metabolic complexity of the Earth System is much greater than anyone imagined a few years ago, and any attempt to understand the web of life that ignores this will be incomplete and misleading.

This summary is adapted from materials provided by the Deep Carbon Observatory, comprising some 1200 scientists in 52 countries. The research was conducted by DCO’s Deep Life community, which “explores the evolutionary and functional diversity of Earth’s deep biosphere and its interaction with the carbon cycle.”

Barely living “zombie” bacteria and other forms of life constitute an immense amount of carbon deep within Earth’s subsurface — 245 to 385 times greater than the carbon mass of all humans on the surface.

Scientists with the Deep Carbon Observatory drilled 2.5 kilometers into the seafloor, and sampled microbes from continental mines and boreholes more than 5 km deep. With insights from hundreds of sites under the continents and seas, they have approximated the size of the deep biosphere — 2 to 2.3 billion cubic km (almost twice the volume of all oceans) — as well as the carbon mass of deep life: 15 to 23 billion tonnes (an average of at least 7.5 tonnes of carbon per cubic km subsurface).

Among 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).
  • Two types of microbes — bacteria and archaea — dominate Deep Earth. Among them are millions of distinct types, most yet to be discovered or characterized. This so-called microbial “dark matter” dramatically expands our perspective on the tree of life. Deep Life scientists say about 70% of Earth’s bacteria and archaea live in the subsurface.
  • 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.
  • While subsurface microbial communities differ greatly between environments, certain genera and higher taxonomic groups are ubiquitous — they appear planet-wide.
  • Microbial community richness relates to the age of marine sediments where cells are found — suggesting that in older sediments, food energy has declined over time, reducing the microbial community.
  • 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. A front-runner 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 degrees Celsius (21 degrees hotter than the boiling point of water).
  • Microbial life can survive up to 122°C, the record achieved in a lab culture (by comparison, the record-holding hottest place on Earth’s surface, in an uninhabited Iranian desert, is about 71°C — the temperature of well-done steak).
  • 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 meters depth, for example, the pressure is approximately 400 times greater than at sea level.

The total global Deep Earth biomass is approximately 15 to 23 petagrams (15 to 23 billion tonnes) of carbon.

“Ten years ago, we knew far less about the physiologies of the bacteria and microbes that dominate the subsurface biosphere,” says Karen Lloyd, University of Tennessee at Knoxville, USA. “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.

“Today too, we know that subsurface life is common. Ten years ago, we had sampled only a few sites — the kinds of places we’d expect to find life. Now, thanks to ultra-deep sampling, we know we can find them pretty much everywhere, albeit the sampling has obviously reached only an infinitesimally tiny part of the deep biosphere.”

“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, Oregon State University, USA. “For example, scientists do not yet know all the ways in which deep subsurface life affects surface life and vice versa. And, for now, 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.”