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Intraterrestrial life has been found at depths of several thousand metres in deep sub-sea floor sediments and in the basement crust beneath the sediments. It has also been found at up to 2800-m depth in continental sedimentary rocks, 5300-m depth in igneous rock aquifers and in fluid inclusions in ancient salt deposits from salt mines. The biomass of these intraterrestrial organisms may be equal to the total weight of all marine and terrestrial plants. The intraterrestrial microbes generally seem to be active at very low but significant rates and several investigations indicate chemolithoautotrophs to form a chemosynthetic base. Hydrogen, methane and carbon dioxide gases are continuously generated in the interior of our planet and probably constitute sustainable sources of carbon and energy for deep intraterrestrial biosphere ecosystems. Several prospective research areas are foreseen to focus on the importance of microbial communities for metabolic processes such as anaerobic utilisation of hydrocarbons and anaerobic methane oxidation.

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This study investigated the distribution of bacteria in groundwater from 16 different levels in five boreholes in granite bedrock down to a maximum of 860 m. Enrichment cultures were used to assay the groups of bacteria present. Autoradiographic studies with(14)C- or(3)H-labeled formate, methanol, acetate, lactate, glucose, sodium bicarbonate, leucine, glutamine, thymidine, orN-acetyl-glucosamine were used to obtain information about bacteria active in substrate uptake. The biofilm formation potential was studied in one borehole. The chemical environment in the groundwater was anaerobic with an Eh between -112 and -383 mV, a pH usually around 8, and a temperature range of 10.2 to 20.5°C, depending on the depth. The organic content ranged between <0.5 and 9.5 mg total organic carbon liter(-1). Carbon dioxide, hydrogen, hydrogen sulfide, and methane were present in the water. The nitrate, nitrite, and phosphate concentrations were close to, or below, the detection limits, while there were detectable amounts of NH4 (+) in the range of 4 to 330 μg liter(-1). The average total number of bacteria was 2.6×10(5) bacteria ml(-1), as determined with an acridine organge direct-count (AODC) technique. The average number of bacteria that grew on a medium with 1.5 g liter(-1) of organic substrate was 7.7×10(3) colony-forming units (CFU) ml(-1). The majority of these were facultatively anaerobic, gram-negative, nonfermenting heterotrophs. Enrichment cultures indicated the presence of anaerobic bacteria capable of growth on C-1 compounds and hydrogen, presumably methanogenic bacteria. Most probable number assays with sulfate and lactate revealed up to 5.6×10(4) viable sulfate-reducing bacteria per ml. A biofilm development experiment indicated an active attached microbial population. Active substrate uptake could not be registered with the bulk water populations, except for an uptake of leucine not associated with growth. The bulk water microbial cells in deep groundwater may be inactive cells detached from active biofilms on the rock surface.

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Granitic rock has aquifers that run through faults and single or multiple fracture systems. They can orientate any way, vertically or horizontally and usually, only parts of hard rock fractures are water conducting. The remaining parts are filled by coatings of precipitated minerals, and clay and gouge material. Sampling hard rock is difficult and the risk of contamination due to intrusion of drilling fluids and cuttings in aquifers is obvious. A recent investigation of the potential for contamination of boreholes in granite during drilling operations, using molecular and growth methods, showed that predominating microorganisms in the drilling equipment were absent in groundwater from the drilled boreholes. The total number of bacteria found in subterranean granitic environments ranges from 103 up to 107 cells per ml groundwater, but the number of cultivable microorganisms is usually much lower. We have used culturing techniques with numeric taxonomy for the identification of cultivable microorganisms and the 16S rRNA gene technique to determine bacterial diversity in granitic groundwater. Members of the genera Bacillus, Desulfovibrio, Desulfomicrobium, Eubacterium, Methanomicrobium, Pseudomonas, Serratia and Shewanella have been found. Several biogeochemical processes in granitic rock have been demonstrated where microorganisms seem to be of major importance. One process is the mobilization of solid phase ferric iron oxy‐hydroxides to liquid phase ferrous iron by iron reducing bacteria with organic carbon as electron donor. Another biogeochemical process found to be important is the reduction of sulfate to sulfide by sulfate reducing bacteria. They frequently appear in granitic aquifers at depths, and seem to prefer a moderate salinity, approximately 1%. When groundwater rich in ferrous iron, manganese(II) and reduced sulfur compounds reaches an oxygenated atmosphere such as an open tunnel, gradients suitable for chemolithotrophic bacteria develop. A third process is the conversion of carbon dioxide to organic material with hydrogen as the source of energy, possibly formed through radiolysis, mineral reactions or by volcanic activity. Recent results show that autotrophic methanogens, acetogenic bacteria and acetoclastic methanogens all are present and active in deep granitic rock. These observations announce the existence of a hydrogen driven deep biosphere in crystalline bedrock that is independent of photosynthesis. If this hypothesis is true, life may have been present and active deep down in the earth for a very long time, and it cannot be excluded that the place for the origin of life was a deep subterranean igneous rock environment (probably hot with a high pressure) rather than a surface environment.