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 a few minerals – elements abundant both on this planet and elsewhere. It doesn’t seem to matter too much how these are supplied; living organisms, particularly bacteria, are able to utilize sources as diverse as air, rock or vegetation. Active life also needs energy but organisms can capture either light energy or a multitude of chemical forms of energy.
Liquid water appears to be the chief limiting factor to life on Earth. Living organisms have a very limited ability to manipulate the freezing or boiling point of water: when the exterior temperature exceeds the limits of this ability, active life ceases. The most barren places on Earth are generally the driest. The relative sterility of the Antarctic Dry Valleys epitomizes the requirement for water but our own homes strikingly illustrate the same principle. Home maintenance is essentially a battle against moisture. We repair roofs and windows, paint surfaces with water-repelling chemicals and make endless trips to the DIY store in our battle to exclude moisture and promote desert conditions inside our houses. If we neglected this then microbes and moulds would quickly invade and undermine our homes.
Why water in its liquid state is so essential to life is a question we will return to in Chapter Five. On Earth, so long as liquid water is available, then life is also possible. Microbial life thrives in a diverse array of (watery) chemical environments from hot to cold, acid to alkaline and every other extreme of chemistry available on this planet. The source chemicals used to make up living cells are incorporated by a wide variety of chemical pathways and transformed inside living cells by a host of diverse metabolic pathways. There appears to be no common core of metabolic chemistry that drives all living cells. The diversity of the chemistry that underpins living cells in different creatures is surprising given the usual chemical explanation for the phenomenon of life: that it is a highly complex self-organizing chemical reaction (we shall be examining this in Chapter Six). If life is a merely a complex chemical reaction then we must explain how such a wide variety of chemical processes generates essentially the same phenomenon: life. This indicates to me that we need to look further than standard chemistry to discover the essential quality of life.
Another curious feature of life apparent from our brief exploration is its invasiveness. The inanimate world is characterized by a flowing down of energy: water flows down a hillside; electrons flow down to lower energy states and complex molecules break down to simpler ones. Yet living organisms will climb to fill any vacant niche. When in 1883, a volcano exploded on Krakatau, it obliterated two thirds of the island, leaving the remainder an inhospitable nutrient-depleted dustbowl. Yet some microbes (mostly cyanobacteria) can survive on a diet of volcanic ash and rapidly colonized the island. The growth of these microbes provided the nutrients to support further colonization, and a few decades later there was abundant plant-life and even a few small animals. Over the course of evolutionary time, life has relentlessly thrust itself in all possible directions to fill the planet’s every available niche. This aspect of life, its ability to direct itself forward, is hard to account for in terms of the perpetual running down that dominates inorganic chemistry. We are again pointed towards the realization that