in energy (high-energy electrons from food to low-energy electrons in oxygen) is captured, providing energy for the cell. Oxygen-based respiration is a very efficient means of extracting maximum energy from food and has thus superseded the anaerobic metabolism in most higher organisms.
Some anaerobes burn their food in respiration, but not with oxygen. Many minerals (such as sulfate or nitrate) serve as low-energy electron dumps for their respiratory cascade. You may well have noticed that the sand alongside estuary waters is often black and smelly. The bad smell is hydrogen sulfide and the blackness is due to the presence of iron sulfide, both products of anaerobic bacteria’s respiration in these estuarine waters. In fact, a wide range of minerals can be utilized by bacteria for respiration; some bacteria can even breathe iron.
Still other anaerobes do not actually respire at all but derive their energy from chopping up their food molecules into small pieces, usually into simple acids or alcohol, in a process known as fermentation. Fermented foods and beverages like wine, beer, sauerkraut, cheese and even coffee, depend upon the actions of these busy microbes. Anaerobes are of enormous ecological importance since they are often responsible for the final decay of organic matter. A giant compost heap would have long since enveloped the whole planet were it not for these bacteria. Cows and other ruminants have learned to harness the powers of these microbes. Their stomachs house an internal compost heap of plant material decomposing through the activity of billions of fermentative bacteria. One of the by-products of their fermentation is the greenhouse gas, methane. The huge quantities of the gas flatulently emitted by (the bacteria inside) domesticated ruminants is thought to contribute signifycantly to the greenhouse effect.
Far from being the breath of life, life carries on very well in oxygen’s complete absence. This must of course be so, since (as covered further in Chapter 4) life emerged on this planet in an atmosphere completely devoid of oxygen. It was only after photosynthetic plants and microbes began to pour oxygen into the earth’s atmosphere that aerobic life became possible on Earth.
JOURNEY TO THE CENTRE OF THE EARTH
Descend into the crater of Yocul of Sneffels, which the shade of Scartaris caresses, before the kalends of July, audacious traveller, and you will reach the centre of the earth. I did it.
ARNE SAKNUSSEMM
These were the instructions that, in Jules Verne’s famous tale, led Professor Hardwigg and his companions to descend into ‘the great volcano of Sneffels’, following in the footsteps of the intrepid Icelander. Miles below the surface, they crossed a subterranean sea to discover a fabulous world of gigantic mushrooms, giant trees and extinct animals; flora and fauna from a world buried for millions of years. Scientist are now following in Professor Hardwigg’s fictional footsteps to discover a world – though not as fabulous as that created by Jules Verne – that harbours many strange and remarkable creatures.
It was generally believed that terrestrial ecosystems extended just a few metres below either the land surface or the ocean bottom. Deeper than this living organisms were thought to peter out, as nutrients became scarce. However, oil exploration drilling in the 1970s started turning up microbes from deep inside the Earth. It was initially believed that the bacteria represented surface contamination of the drilling equipment. This view changed dramatically in 1987 when the Department of Energy (DOE) in the USA started to explore the storing of nuclear waste below ground. To investigate the stability of potential sites, they drilled three deep holes into the sedimentary rock beneath Savannah in South Carolina and extracted cores from as deep as five hundred metres. To their astonishment, even the deepest cores contained abundant microbial flora with more than four hundred species of bacteria. Similar cores drilled into sedimentary rock seven hundred and fifty metres beneath the ocean have yielded similar numbers of bacteria. Although bad news for the DOE (who want to store their nuclear waste in sterile environments), it has provided microbiologists with a further habitat to explore. The deepest hole so far, a three-and-a-half-kilometre-deep South African gold mine has yielded rock-eating bacteria that can acquire energy from iron, manganese, sulfur, cobalt and possibly even gold.
Remarkably, the deep drilling has not yet hit any level where life peters out. In fact, in some studies the bacteria are more numerous the deeper the drilling. Most of these bacteria are thought to eat the buried organic matter trapped in the rock when the sediments were laid down millions of years ago. However, abundant bacteria have also been found to inhabit deep water-filled cracks of buried volcanic rock where little or no organic material has percolated down from the surface far above. In 1995, another DOE project drilled one thousand five hundred metres deep into basalt beneath the Columbia River valley in Washington State. The bacteria found were mostly methanogens, able to use hydrogen as an energy source to make methane, which they incorporate into their tissue. Methanogens are also common on the Earth’s surface. The intestinal tracts of animals, particularly ruminants, are full of methanogens; as are boggy waters where the methane they generate may spontaneously ignite, causing the ghostly will-o’-the-wisp flames that dance over the water’s surface.
Thriving microbial ecosystems may also be found below permanently covered ice-sheets. I have already mentioned the ice-covered Dry Valley Lakes of Antarctica as ecosystems totally isolated from the surface. A vast ice-buried habitat of Antarctica remains to be explored. Lake Vostok is a liquid-water lake, two hundred kilometres long, with an average depth of one hundred and twenty-five metres, which lies two miles beneath the Antarctic ice sheet. The lake was only discovered in 1974 and, as far as we know, has been buried for at least a million years. There are plans to drill down into the lake and sample its ancient waters. The danger is that the sampling will contaminate its pristine waters, so a drilling programme in 1996 stopped just one hundred and fifty metres above the lake surface whilst scientists consider the best way to proceed.
LIFE WITHOUT WATER?
We have already encountered some of Earth’s driest places, since they are also the hottest (deserts) and coldest (Antarctic Dry Valleys). As we have seen, many organisms, such as lichen, manage to survive drought conditions, but do so in a dormant state awaiting the return of moisture from melting ice, rain, fog or dew. The key to long-term survival appears to be a carefully controlled desiccation – removal of water under conditions avoiding damaging the cell. A commonly used technique for long-term storage of microbes and plant seeds is freeze-drying, in which water is evaporated whilst the cells remain frozen to minimize cell damage. Plants use a similar strategy to make drought-resistant seeds. The seeds undergo a process of controlled desiccation, in which water is replaced by a sugary liquid hardening to vitrify the seed.
Animals and vegetative plants do not tolerate drought. There are however a few plants, known as resurrection plants, which can survive conditions that reduce their moisture content to less than ten per cent. The palm-like fern, Actiniopteris semiflabellata, adorns exposed rock faces throughout East Africa. In times of drought, the plant dries to a crisp brownish-grey discolouration on the rocks; yet, when the next rains arrive, the dehydrated leaves absorb the water, resuming growth. Resurrection plants use a variety of mechanisms to resist the damaging effects of drought. Water is sometimes replaced by sucrose, which encases their cells in a glassy fluid. In other plants, a group of proteins, called dehydrins, appear to protect delicate cellular structures during desiccation.
Survival is, however, not active life. Seeds and drought-resistant plants are never active. Although, paradoxically, removal of water appears to be essential for long-term survival of dormant forms, it remains essential for active life.
SO WHAT ARE THE LIMITS?
The spacecraft’s exploration of life would thus have discovered its extraordinary versatility. Life on Earth, particularly microbial life, knows few limits. The minimal ingredients appear to be simply sources of carbon, nitrogen, oxygen and hydrogen plus
