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An international collaboration of researchers from the Deep Carbon Observatory (DCO) has made several important discoveries, including how much and what kinds of life exist in the Earth’s deep subsurface. Drilling 1.55 miles (2.5 km) into the seafloor, and sampling microbes from continental mines and boreholes more than 3.1 miles (5 km) deep, the DCO scientists have used the results to construct models of the ecosystem deep within the planet. With insights from now hundreds of sites under the continents and seas, they have approximated the size of the deep biosphere — 2 to 2.3 billion km3 (almost twice the volume of all oceans) — as well as the carbon mass of deep life: 15 to 23 billion tons.
A ‘Blue Marble’ image of the Earth taken from the VIIRS instrument aboard NASA’s Earth-observing satellite Suomi NPP (named in honor of Verner E. Suomi of the University of Wisconsin). This composite image uses a number of swaths of the Earth’s surface taken on January 4, 2012. Image credit: NASA / NOAA / GSFC / Suomi NPP / VIIRS / Norman Kuring.
To estimate the total mass of Earth’s subcontinental deep life, for example, the researchers compiled data on cell concentration and microbial diversity from locations around the globe.
They factored in a suite of considerations, including global heat flow, surface temperature, depth and lithology — the physical characteristics of rocks in each location — to estimate that the continental subsurface hosts 2 to 6*1029 cells.
Combined with estimates of subsurface life under the oceans, total global Deep Earth biomass is approximately 15 to 23 petagrams (15 to 23 billion tons) of carbon.
“Exploring the deep subsurface is akin to exploring the Amazon rainforest. There is life everywhere, and everywhere there’s an awe-inspiring abundance of unexpected and unusual organisms,” said Marine Biological Laboratory Woods Hole researcher Dr. Mitch Sogin, co-chair of DCO’s Deep Life community.
“Molecular studies raise the likelihood that microbial dark matter is much more diverse than what we currently know it to be, and the deepest branching lineages challenge the three-domain concept introduced by Carl Woese in 1977.”
“Perhaps we are approaching a nexus where the earliest possible branching patterns might be accessible through deep life investigation.”
This image shows a species of methanogenic bacteria; found in samples from a buried coal bed 1.2 miles (2 km) below the Pacific Ocean floor off the coast of Japan, this specimen was retrieved during an Integrated Ocean Drilling Program (now the International Ocean Discovery Program) expedition aboard the Drilling Vessel Chikyu. Scale bar – 10 μm. Image credit: Hiroyuki Imachi, Japan Agency for Marine-Earth Science and Technology.
Among many key discoveries:
(i) 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);
(ii) two types of microbes — bacteria and archaea — dominate Deep Earth; among them are millions of distinct types, most yet to be discovered or characterized;
(iii) 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;
(iv) the genetic diversity of life below the surface is comparable to or exceeds that above the surface;
(v) while subsurface microbial communities differ greatly between environments, certain genera and higher taxonomic groups are ubiquitous — they appear planet-wide;
(vi) 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;
(vii) the absolute limits of life on Earth in terms of temperature, pressure, and energy availability have yet to be found; 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 250 degrees Fahrenheit (121 degrees Celsius); microbial life can survive up to 252 degrees Fahrenheit (122 degrees Celsius), the record achieved in a lab culture;
(viii) the record depth at which life has been found in the continental subsurface is approximately 3.1 miles (5 km); the record in marine waters is 6.5 miles (10.5 km) from the ocean surface, a depth of extreme pressure; at 2.5 miles (4 km) depth, for example, the pressure is approximately 400 times greater than at sea level.
The scientists will present their findings this week at the Annual Fall Meeting of the American Geophysical Union in Washington, D.C.
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