Limits of life on Earth Thermophiles Temperatures up to ~55C are common, but T > 55C is Some archaea and bacteria (extremophiles) can live in associated usually with geothermal features (hot springs, environments that we would consider inhospitable to volcanic activity etc) life (heat, cold, acidity, high pressure etc) Thermophiles are organisms that can successfully live Distinguish between growth and survival: many organisms can survive intervals of harsh conditions but could not at high temperatures live permanently in such conditions (e.g. seeds, spores) Best studied extremophiles: may be relevant to the Interest: origin of life. Very hot environments tolerable for life do not seem to exist elsewhere in the Solar System • analogs for extraterrestrial environments • `extreme’ conditions may have been more common on the early Earth - origin of life? • some unusual environments (e.g. subterranean) are very widespread Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Grand Prismatic Spring, Yellowstone National Park Hydrothermal vents: high pressure in the deep ocean allows liquid water Colors on the edge of the at T >> 100C spring are caused by different colonies of thermophilic Vents emit superheated water (300C or cyanobacteria and algae more) that is rich in minerals Hottest water is lifeless, but `cooler’ ~50 species of such thermophiles - mostly archae with some margins support array of thermophiles: cyanobacteria and anaerobic photosynthetic bacteria oxidize sulphur, manganese, grow on methane + carbon monoxide etc… Sulfolobus: optimum T ~ 80C, minimum 60C, maximum 90C, also prefer a moderately acidic pH. Live by oxidizing sulfur Known examples can grow (i.e. multiply) at temperatures which is abundant near hot springs. as high as 110-120C… but can also survive at much lower temperatures Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Adaptations of thermophiles Low temperatures: growth below the freezing point is very difficult, but some liquid water exists below 0C in Generally, thermophiles are very similar to `ordinary’ thin films or in water under high pressure archaea and bacteria (DNA, same amino acids etc) Series of quite subtle differences, e.g.: Slow multiplication of bacteria living in permafrost has been seen at temperatures as low as -20C • cell membranes are made up of lipids that are more stable to high temperature • additional enzyme (reverse DNA gyrase) causes the DNA to fold up in a way that is more stable against heat Thought that the absolute maximum temperature is around 150C - above this DNA breaks up very readily Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 1 Lake Vostok Atacama desert in Chile is one of the driest locations Large sub-glacial lake on Earth (14,000 km2) at a depth of ~3750m under Average annual rainfall Antarctica in parts of the Atacama is < 1mm / yr Water is not frozen, probably very oxygen rich May have been isolated for around a million years from No plants have lived for more than one million years any other environment Difficult location to find life - the NASA Viking experiments Terrestrial analog for an ocean on Europa? would fail. Microbes can be grown from shallow subsurface layers successfully. Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Other extremes Radiation High pressures: 10m of water adds 1 atmosphere of Radiation exposure is measured in Gray (Gy): pressure. Viable organisms have been recovered from the Mariana trench (10,900m, ~1100 atm). • 5-10 Gray is fatal for humans Main adaptations are in the cell membranes. • some microbes can survive 30,000 Gray Saline environments: haloarchaea thrive in waters with Adaptation: efficient repair mechanisms for DNA that is near saturation levels of salt (e.g. Dead Sea), and damaged by the radiation flux are found in rock salt deposits. Adapted to make use of very little water. Puzzle: such high levels of radiation are not found naturally on Earth - may be that radiation tolerance is Subterranean environments: microbes appear to exist a byproduct of adaptation to very dry environments in dry, hot subterranean environments (several km depth). Properties and origin unclear… Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Travel time from Mars to Earth is typically 1-10 Myr - all the known specimens probably came from a handful of impact events Ejection events likely produced large (30km) craters - but pressures and temperatures in some of the ejecta would have been low enough to allow survival Microbes within the rock would be shielded from ultraviolet radiation and cosmic rays… Interest: some meteorites from Mars have been recovered, many from Antarctica Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 2 Sterilization of space missions to Mars, Europa Implications for extraterrestrial life Galileo orbiter was crashed into Jupiter to avoid any Life can flourish in environments well beyond those that are obviously `habitable’ chance that it might collide with and contaminate Europa Need liquid water, but -20C < T < 120C OK - modest Astrobiology missions to Mars, Europa need to be sterilized to very high standards: expansion of traditional habitable zone requirement • assembly in clean room conditions Subterranean organisms seem of greatest interest for • irradiation with gamma-rays Mars, Lake Vostok may be similar to Europa • heated to 105-124C for several hours • hydrogen peroxide plasma sterlilization… Can only speculate as to whether life on Earth started in an extreme environment… Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 3.