The search for life: Astrobiology

By Monica Grady

Where on Earth is life now?

Higher organisms have spread to occupy a vast range of environments, from the hot, humid rainforests of the tropics, to the cold, dry tundra of the polar regions. The range of habitats for micro-organisms is even greater, as less-evolved species colonise niches that are hostile to higher forms of life. We will examine such environments in detail, to help give a greater understanding of the possibilities of discovery of a whole host of extraterrestrial lifeforms on neighbouring planets and their satellites.


Hydrothermal vents

Spreading centres--chains of mountains and ridges deep down on the ocean floor--are areas characterised by intense volcanic activity. In these regions molten rock wells up from below and forms new oceanic crust. Hydrothermal vents were discovered close to these areas in the 1970s when specially adapted submersibles took photographs of the ocean floor for the first time. The vents, or 'black smokers', are hot springs where super-heated water (up to 350-400°C), rich in hydrogen, methane and hydrogen sulphide, shoots up from the sea floor. Where the hot water meets the cold, oxygen-rich bottom water, there is an instant chemical reaction and sulphides precipitate out from the water, colouring it black. The sulphides build up rapidly until the chimneys reach heights of several tens of metres.

Discovery of these vents revealed that, despite the depth and darkness, parts of the ocean floor are home to an unusual collection of animals such as mussels, crabs and tubeworms, feeding on hyperthermophilic bacteria and archaea that flourish in these very hot conditions. The ocean floor is too dark to allow photosynthesis so chemical energy is the basis of the foodchain. The discovery of a flourishing ecosystem at vent sites has raised the possibility that life may not have arisen in surface waters, as original theories purport. Discovering communities entirely based on chemosynthesising microbes has given the impetus to the search for life in other deep oceans, especially on Jupiter's satellite, Europa, which may have a liquid water ocean below the visible crust of ice.



Accounting for 10 percent of the Earth's landmass, the desert climate of Antarctica (0 percent humidity) cannot support a complex ecosystem of plants and animals. Even so, there is a significant biomass of psychrophiles within the sandstones and quartzites of the Dry Valleys. Communities of lichens colonise the top layer of the sandstones, whose dark pigments are able to block out excessive sunlight and absorb UV radiation.

Despite the dry conditions of the Antarctic environment, the microscopic communities thrive on water trapped within pore spaces in the sandstones, which permits a relative humidity of up to 100 percent. External temperatures are frequently as low as -30°C, but the warmth generated within the sandstones can reach 10°C. These communities are considered to be useful models for the types of biota that might be found within rocks at the Martian surface, an environment not dissimilar to that of the Dry Valleys themselves.

In addition to the surface species discussed above, viable micro-organisms have been recovered by drilling, using ice cores, into the ice layers (sometimes up to 4km thick) covering Lake Vostok, one of the eighty lakes in the Antarctic plateau. Bacteria, fungi and algae have been discovered sealed in ice that is around 400,000 years old and has presumably always been sealed from the outside environment. Again, scientists believe this to be a useful terrestrial model for Europa, Jupiter's ice-covered satellite, where the survival of organisms in such environments may be possible.


Hot springs and solfataras

These are characterised by hot (sometimes boiling) water, acidity and high sulphur concentrations, and are frequently associated with regions of volcanic activity. Despite these extreme conditions, rich ecosystems are able to survive in hot springs and geysers. Solfataras emit steam and hot gases (especially sulphur dioxide) rather than boiling water, but conditions are still highly acidic.