You certainly wouldn't expect to find life in rocks a couple of miles deep in the Earth but this assumption would be wrong. Life does exist there.
So many things about life are so wonderful that one has to be sparing in the use of the word amazing. However, the fact that life can exist under such inhospitable circumstances must surely classify as amazing. It also greatly increases estimates that life exists on other planets.
The basic unit of life is the cell. The simplest life forms, bacteria and algae, are unicellular, i.e. individual organisms composed of a single cell. Complex organisms, such as animals, are multi-cellular.
A bacterial cell is about two microns wide. A micron is a millionth of a metre. Animal cells are 50 microns wide. The adult human body contains about 50 trillion cells.
Life is thought to have arisen on Earth 3.5 billion years ago as a form of bacteria. Bacteria are the most successful form of life on earth. Today there is scarcely an ecological niche on the planet which is not colonised by bacteria.
Science assumed until recently that the sub-surface of the Earth was sterile. The first serious modern proposal that life exists deep beneath the surface was made in the 1920s.
Edson Bastin, a geologist at the University of Chicago, wondered at the presence of hydrogen-sulphide and bicarbonate in water extracted from oil fields. He concluded that these chemicals were produced by a sulphate-reducing class of bacteria. These bacteria reduce sulphate chemicals as they extract energy from food, producing hydrogen-sulphide.
Bastin and Frank Greer, a microbiologist, cultured sulphate-reducing bacteria from water extracted from an oil deposit located hundreds of metres below the surface. They speculated that these bacteria were descended from organisms buried over 300 million years ago when the sediments containing the oil reservoir were deposited. Most scientists rejected this interpretation, explaining the results as sample contamination by surface micro-organisms.
Further work had to wait until the 1980s. The US Department of Energy (DOE) was interested in deep sub-surface repositories for radioactive waste. If these repositories contained bacteria, such biological activity might compromise the long-term integrity of the containment. A DOE programme was initiated to investigate the possibility that life exists deep in the earth.
Special drilling equipment was designed and elaborate precautions introduced to prevent contamination of samples by surface micro-organisms. In 1987, test bore-holes were drilled in South Carolina. The results showed that many types of micro-organisms live at least as deep as 500 metres.
Since then, sub-surface bacteria have been found at many sites. Organisms have been recovered from rocks with temperatures as high as 75C and from depths of 1.7 miles.
Pressure and temperature increase with increasing depth. Increasing pressure has little direct impact on bacteria but increasing temperature does place a limit on the depth of sub-surface life. The maximum temperature which can be tolerated is unknown. However, bacteria growing at 110 C in deep-sea volcanic vents have been found. This would allow bacteria to live up to seven kilometres below the sea floor or four miles below land surfaces.
Every gramme of agricultural topsoil contains over a billion bacteria. Deep beneath the surface, life is less abundant. Samples taken 400 metres deep can have as few as a hundred and as many as 10 million bacteria in each gramme of rock. This is because conditions necessary for life are distributed unevenly. These conditions include the presence of water, carbon, hydrogen, oxygen, nitrogen and sulphur. These are necessary for the bacterium to build its body parts, and a means by which the organism can extract energy from nutrients.
The nourishment for bacteria in sedimentary rocks is easily explained. The sediments laid down contained plant and animal bodies and these organic substances remain in the rock to nourish microbial life.
Most life on the surface of the Earth extracts energy from food by "burning" it in the presence of oxygen. Subterranean bacteria which live in the absence of oxygen must use other means. For example, the sulphate-reducing bacteria "breathe" sulphate which chemically serves the same function as oxygen.
Subterranean bacteria are also found in igneous rocks. Igneous rocks, e.g. granite and basalt, formed when molten magma from the earth's interior welled onto the surface, cooled and solidified. There is little organic nutrient in these rocks to nourish bacteria.
These rocks contain two types of bacteria: autotrophs and heterotrophs. Autotrophic organisms can synthesise their complex organic bodies from the simplest inorganic precursor molecules - water, carbon dioxide, nitrogen, etc.
Heterotrophic bacteria cannot sustain themselves on such simple fare and need carbon supplied to them in more complex form. They live on compounds excreted by the autotrophic organisms.
We are so used to the concept of life intimately dependent on light and oxygen that the idea of living organisms separated by two miles from the sunlight and the air sounds strange and raises many intriguing possibilities.
For example, the search for life on Mars in the 1970s is now seen as being an extremely limited probe. Some surface soil was tested and the results provided no evidence for life. But what about the sub-surface?
Also, the fact that autotrophic organisms exist deep in the earth, without light, raises the possibility that life might arise elsewhere under such circumstances. In that event, the number of potential environments for life in the universe becomes truly infinite.
William Reville is a senior lecturer in biochemistry at UCC.