Deep biosphere
The deep biosphere is the part of the biosphere that resides below the first few meters of the land surface and seafloor. It extends below the continental surface and below the sea surface, at temperatures that may reach beyond which is comparable to the maximum temperature where a metabolically active organism has been found. It includes all three domains of life and the genetic diversity rivals that on the surface.
The first indications of deep life came from studies of oil fields in the 1920s, but it was not certain that the organisms were indigenous until methods were developed in the 1980s to prevent contamination from the surface. Samples are now collected in deep mines and scientific drilling programs in the ocean and on land. Deep observatories have been established for more extended studies.
Near the surface, living organisms primarily consume organic matter and breathe oxygen or photosynthesize energy from sunlight. Lower down, these resources are not available, so organisms make use of "edibles" such as hydrogen, methane, reduced sulfur compounds, and ammonium. They "breathe" electron acceptors such as nitrates and nitrites, manganese and iron oxides, oxidized sulfur compounds and carbon dioxide. There is very little energy at greater depths, so metabolisms are up to a million times slower than at the surface. Cells may live for thousands of years before dividing and there is no known limit to their age.
The subsurface accounts for about 90% of the biomass across two domains of life, Archaea and Bacteria, and 15% of the total for the biosphere. Estimates in fact vary significantly depending on the samples and the measurement methods used, the "15 to 23 billion tons" figure is cited very often. Eukarya are also found, including some multicellular life - fungi and animals. Viruses are also present and infect the microbes.
Definition
The deep biosphere is an ecosystem of organisms and their living space in the deep subsurface. For the seafloor, an operational definition of deep subsurface is the region that is not bioturbated by animals; this is generally about a meter or more below the surface. On continents, it is below a few meters, not including soils. The organisms in this zone are sometimes referred to as intraterrestrials. A subset of the deep biosphere found at depths where pressure and heat greatly exceed that survivable by surface life was delineated and named by Thomas Gold in a 1992 paper titled, "The Deep, Hot Biosphere."Early discoveries and ideas
At the University of Chicago in the 1920s, geologist Edson Bastin enlisted the help of microbiologist Frank Greer in an effort to explain why water extracted from oil fields contained hydrogen sulfide and bicarbonates. These chemicals are normally created by bacteria, but the water came from a depth where the heat and pressure were considered too great to support life. They were able to culture anaerobic sulfate-reducing bacteria from the water, demonstrating that the chemicals had a bacterial origin.Also in the 1920s, Charles Lipman, a microbiologist at the University of California, Berkeley, noticed that bacteria that had been sealed in bottles for 40 years could be reanimated – a phenomenon now known as anhydrobiosis. He wondered whether the same was true of bacteria in coal seams. He sterilized samples of coal, wetted them, crushed them and then succeeded in culturing bacteria from the coal dust. One sterilization procedure, baking the coal at for up to 50 hours, actually encouraged their growth. He published the results in 1931.
The first studies of subsurface life were conducted by Claude E. Zobell, the "father of marine microbiology", in the late 1930s to the 1950s. Although the coring depth was limited, microbes were found wherever the sediments were sampled. With increasing depth, aerobes gave way to anaerobes.
Most biologists dismissed the subsurface microbes as contamination, especially after the submersible Alvin sank in 1968 and the scientists escaped, leaving their lunches behind. When Alvin was recovered, the lunches showed no sign of microbial decay. This reinforced a view of the deep sea as a lifeless desert. The study of the deep biosphere, like many bacteria, was dormant for decades; an exception is a group of Soviet microbiologists who began to refer to themselves as geomicrobiologists.
Interest in subsurface life was renewed when the United States Department of Energy was looking for a safe way of burying nuclear waste, and Frank J. Wobber realized that microbes below the surface could either help by degrading the buried waste or hinder by breaching the sealed containers. He formed the Subsurface Science Program to study deep life. To address the problem of contamination, special equipment was designed to minimize contact between a core sample and the drilling fluid that lubricates the drill bit. In addition, tracers were added to the fluid to indicate whether it penetrated the core. In 1987, several boreholes were drilled near the Savannah River Site, and microorganisms were found to be plentiful and diverse at least 500 metres below the surface.
From 1983 until now, microbiologists have analyzed cell abundances in drill cores from the International Ocean Discovery Program. A group led by John Parkes of the University of Bristol reported concentrations of 104 to 108 cells per gram of sediment down to depths of 500 metres. This was initially met with skepticism, and it took them four years to publish their results.
In 1992, Thomas Gold published a paper titled "The Deep, Hot Biosphere" suggesting that microbial life was widespread throughout the subsurface, existing in pore spaces between grains of rocks. He also published a book similarly titled The Deep Hot Biosphere. According to one paper, he "pioneered" the idea the hydrocarbons could sustain life to "known depths of 10km and possibly down to 300km", if the temperature was not over a hypothetical maximum of 150°C. Gold also suggested that the deep biosphere is sustained by hydrocarbons geologically produced by the subsurface, or their derivatives. According to the paper, Gold's proposals helped to inspire later generations of scientists.
In 1998, William Whitman and colleagues published a summary of twelve years of data in the Proceedings of the National Academy of Sciences. They estimated that up to 95% of all prokaryotes live in the deep subsurface, with 55% in the marine subsurface and 39% in the terrestrial subsurface. In 2002, Ocean Drilling Program Leg 201 was the first to be motivated by a search for deep life. Most of the previous exploration was on continental margins, so the goal was to drill in the open ocean for comparison. In 2016, International Ocean Discovery Program Leg 370 drilled into the marine sediment of the Nankai Accretionary Prism and observed 102 vegetative cells per cm3 at 118 °C.
Scientific methods
The present understanding of subsurface biology was made possible by numerous advances in technology for sample collection, field analysis, molecular science, cultivation, imaging and computation.Sample collection
Microbes from the ocean floor can sampled by drilling boreholes and collecting cores. The methods must be adapted to different types of rock, and the cost of drilling limits the number of holes that can be drilled. Microbiologists have made use of scientific drilling programs: the Ocean Drilling Program, which used the JOIDES Resolution drilling platform, and the Integrated Ocean Drilling Program, which used the Japanese ship Chikyū.Deep underground mines, for example South African gold mines and the Pyhäsalmi copper and zinc mine in Finland, have also provided opportunities to sample the deep biosphere, as have chosen or proposed nuclear waste repository sites. Scientific drilling of the continental deep subsurface has been promoted by the International Continental Scientific Drilling Program.
To allow continuous underground sampling, various kinds of observatories have been developed. On the ocean floor, the Circulation Obviation Retrofit Kit seals a borehole to cut off the influx of seawater. An advanced version of CORK is able to seal off multiple sections of a drill hole using "packers", rubber tubes that inflate to seal the space between the drill string and the wall of the borehole.
In sediments, the Simple Cabled Instrument for Measuring Parameters In-Situ is designed to remain and take measurements after a borehole has collapsed. Packers are also used in the continental subsurface, along with devices such as the flow-through in situ reactor. Various methods are used to extract fluids from these sites, including passive and osmotic gas samplers and U-tube systems. In narrow holes, polyamide tubes with a back-pressure valve can be lowered to sample an entire column of fluid.