MONDAY, 3 OCTOBER 2011
To us, the 400°C hydrothermal fluids exiting deepwater fissures at pressures of up to 50,000 kilopascals (when we’ve evolved to enjoy a leisurely 101 kilopascals) may sound ‘extreme’, but over the last few decades scientists have suggested that might be a tad anthropocentric. They are now accepted as havens for a diverse web of crabs, fish and octopuses that are sustained by some unusual bacteria. These bacteria feed off the dissolved sulphur compounds, usually toxic to other organisms, abundant at the mouth of ‘black smokers’. The biodiversity supported by these chemosynthetic organisms, which use heat, methane and sulphur instead of sunlight as sources of energy, is only now being fully revealed. Some single-celled organisms have also been shown to use the dim light generated by black smokers as a source for photosynthesis. This has all been discovered in the 51 years since Jacques Piccard and Don Walsh first navigated 10.9 kilometres down into the Marianas Trench.
Since 1993, further deep sea exploration has also discovered over 300 new species of deep sea gastropod, as well as one quite fascinating species of mollusc. This particular snail has a foot mineralised with iron sulphide armour plating, making it frustratingly magnetic for anyone attempting a dissection with metallic tools. Elsewhere in the water column, the HADEEP collaborative project captured images of the deepest recorded fish in 2008—a suction-feeding snailfish 7.7 kilometres down in the Peru-Chile trench. Two years later they went further with a new species of shrimp, detected in the Izu-Bonin Trench at 9.3 kilometres.
As with many specialisms of science, the study of these remarkable organisms has developed its own terminology. Organisms with a preference for high pressures and temperatures are now termed ‘extremophiles’. This umbrella term also incorporates organisms tolerant of high or low pHs, immense altitudes, elevated radiation levels and extreme salinities. The greatest challenge to barophilic (‘pressure loving’) organisms is the effect pressure has on the physical properties, especially the fluidity, of cellular membranes. At high pressure the phospholipids, which make up the membranes that allow metabolic movement of structures into and out of cells, are crushed together. In response, barophiles universally express a far greater percentage of unsaturated fatty acids within this layer, resulting in an appropriately plastic membrane. The fantastic suite of specialisations being discovered in more and more deep sea organisms means some extremophiles truly are extreme ‘lovers’—‘obligatory barophiles’ are unable to withstand pressure less than 50,000 kilopascals.
Far from being simply zoological oddities, the study of extremophiles is big business—industries have been using ‘extremozymes’ for agricultural and laundry markets for decades. Taq polymerase, ubiquitous in labs around the world as the key enzyme for polymerase chain reactions (PCR), was derived from thermophilic bacteria found in Yellowstone National Park. Now it seems medicine and biotechnology may further benefit from the study of deep sea organisms, with extremophiles being studied in order to find novel chemotherapeutic agents. For instance, the vent bacteria Vibrio diabolicus might be key in studying and delaying blood clotting due to its sugar-residues. So as the fascinating depths of our oceans continue to amaze and provide potentially ground-breaking research, what has the ground right below us got to offer?
For over twenty years biologists have been aware that crammed in between the pores of the consolidated sediments of the Earth’s crust, an entire ecosystem of microorganisms exists. Earlier this year, for example, in borehole water filtered from the Beatrix gold mine in South Africa, a multicellular organism was found tolerating anaerobic conditions and temperatures of up to 41°C—over 1.3 kilometres under the surface of the crust. The discovery of Halicephalobus mephisto, a nemotode worm, not only expanded the range that multicellular life was known to inhabit, but also hinted at the resilience of life within those extreme habitats.
A multitude of animal species may have adapted successfully to life below ground, but what about humans? Today, millions of people across the globe call an underground residence ‘home’, although the vast majority live far closer to the surface than their extremophile counterparts.
The same solution to the problem of staying cool is found in several desert villages around North Africa, most famously the Berber township of Matmata in Tunisia. Here residents live in caves excavated around the perimeter of large circular pits dug from the surrounding sandstone. Troglodyte (cave-dwelling) residences are thought to have been continuously present at Matmata from the 13th century onwards. The township was only ‘discovered’ in 1967, however, when heavy rains flooded the houses forcing residents to seek help from the Tunisian authorities. Today, Matmata survives largely as a result of its international fame—its subterranean hotel, the ‘Siri Diss’, acted as the set for Luke Skywalker’s home in the Star Wars films.
Underground extra-terrestrial accommodation may not be limited to films, however. Using ‘Themis’ (Thermal Energy Imaging System), NASA spacecraft have identified what appear to be caves on the surface of Mars, and are now investigating whether these might be suitable locations for a future base on the red planet. The caves would offer protection from the meteoroids, ultraviolet radiation and solar flares that continuously bombard the Martian surface. Caves are also the most plausible source of minerals, gases and possibly even ice that would be required by any future manned mission.
Back here on Earth, the world’s most populous troglodyte residences are, without a doubt, the yaodong caves of China. Stretching across six northern provinces, the ‘yaodongs’ (or ‘arched tunnels’) are thought to house around 40 million people. The caves are carved into hillsides composed of loess—a mixture of silt, sand and clay with a distinctive yellow colour. In summer, the caves are naturally cool, while in winter they are heated by a network of channels that distribute hot air from fireplaces to the rooms. Since the yaodongs are carved into hillsides that would be difficult to farm, they free up valuable arable land, making it possible to house large numbers of people with relatively little environmental impact.
Looking out from such sites onto the barren landscape of Antarctica, one could be forgiven for thinking that you were as isolated as you could possibly be. Indeed, it is this isolation that draws astronomers to the Amundsen-Scott South Pole station, where they can study the stars without interference from artificial light, atmospheric pollution, and noise that invades all but the furthest reaches of the Earth. Isolation is exactly what the scientists at the IceCube neutrino telescope are looking for; buried one kilometre below the ice, it is not the traditional place to have a telescope, but then this is not a traditional telescope. This team of physicists are searching for neutrinos, but instead of looking up into the sky, they are looking down.
Neutrinos are among the smallest particles known to man, so small that their mass has yet to be determined. As their name suggests, they are electrically neutral, and this lack of electrostatic charge means that they barely interact with other forms of matter. In fact 99.99% of the time a neutrino will pass straight through the Earth. They are also incredibly abundant—trillions of them have passed through you in the time it took to read this sentence. But because it is so rare for a neutrino to interact with other matter, experiments to detect them need to be highly sensitive. However, this sensitivity can make them susceptible to interference from cosmic rays coming down through the atmosphere. To combat this interference, detectors are placed one kilometre underground, facing down towards the centre of the Earth. This way the Earth acts as a filter to deflect or absorb particles from the cosmic rays that can interfere with the neutrino telescope. Only particles that can pass straight through the Earth and come out the other side—like neutrinos—can be detected.
Unlike photons or charged particles, neutrinos will not deviate from travelling in a straight line while hurtling from one galaxy to the next. This means we are able to trace the paths of neutrinos back to where they came from and produce a map of neutrino sources in the universe. This can be compared to what we have been able to observe from optical, infrared and X-ray telescopes. Combine that with measuring the energy of the incoming neutrinos and we can begin to pinpoint the locations of black holes, gamma ray bursts and supernova explosions. Understanding these events is all part of cracking the puzzle of dark energy, and buried under a billion tonnes of ice, thousands of detectors are looking for a faint blue flash of enlightenment.
In contrast to the freezing depths of the Antarctic, the ground beneath us also contains phenomenal amounts of heat. No one who has witnessed the eruption of a volcano, or the devastation of earthquakes, can doubt that there are huge amounts of energy stored inside the Earth. Some of this energy, known as geothermal energy, is released as heat. It comes from energy stored as the Earth formed, as well as from naturally occurring radioactive elements that release energy as they decay.
Of course, not all manifestations of this energy are destructive. The Yellowstone region of the USA is famous for its spectacular geysers, having one of the highest concentrations in the world. There are also hot springs and pools with beautiful coloured rocks, where minerals from deep within the Earth are carried to the surface by deep flowing water. The driving force for geysers and hot springs comes from the pool of molten rock (magma), which sits relatively close to the surface in Yellowstone National Park.
In the last 60 years, the development of new technologies means that we are able to exploit geothermal energy more effectively than our ancestors. This has been achieved in two ways: electricity generation and the direct utilisation of heat. While electricity is a more versatile form of energy, generating it from geothermal energy is more difficult, requiring a more favourable site. Approximately 70 countries have the infrastructure in place to use the direct heat from geothermal sources, as opposed to 24 countries that are able to generate electricity.
For electricity generation, there are several types of power plants, all of which are very similar to those that run on more conventional fossil fuels—they base their operation on steam driven turbines. The difference is that the steam is obtained either directly from a geothermal source, such as a very hot underground water reservoir, or by using the heat from deep underground to turn water into steam. This requires drilling a few kilometres into the ground to gain access to the hot underground water, or injecting water into hot rocks, which then comes back to the surface as steam.
As fossil fuel stores start to run low, and with increasing concerns about the damage to our environment, using the energy under our feet is an attractive option. Not only is it available at all times, unlike solar or wind energy, geothermal power plants can also be adapted to make the most efficient use of the geothermal resources available.
We seldom think of what lies beneath our feet as we go about our daily lives. Earth’s internal energy gives us a mechanism for generating electricity; it powers life at ocean depths sunlight can never reach; it provides a sheltered environment for relatively cheap, effective and efficient housing and laboratories; but the most exciting part of all is that we have only hit the tip of the iceberg—as technology advances, we can be prepared to see many more exciting discoveries coming from the world below.
Nick Crumpton is a PhD student in the Department of Zoology
Ian Le Guillou is a PhD student in the Department of Biochemistry
Louisa Lyon is a postdoctoral researcher in the Department of Experimental Psychology
Wendy Mak is a PhD student in the Department of Physics
