SESSION 9

EXTREMOPHILES

SEARCH FOR LIFE

• The search is for two different lines of evidence:

• Presence of present day life on the planet, either on the surface or underground

• Fossil evidence for former life on the planet

ASTROBIOLOGY• It is the science of the possibility of life

on other planets• At least one American university has a

School of Astrobiology• The environmental biologist Jared

Diamond has commented that it must be the only field of science whose subject matter has not yet been shown to exist

LIFE BASED ON CARBON• It is assumed that if life is found

elsewhere in the Universe it will also be based on the element carbon because it is the only element that can form the enormous range of compounds necessary for life

• Carbon atoms attach to each other to form chains, rings, spirals, tubes and even spheres and also bond with a wide range of other elements and compounds – organic chemistry

Science fiction – alien life based on silicon

WHY LIFE ON MARS?

• Early Mars must have been a wetter and warmer planet with a denser atmosphere

• If life was present it would have been unicellular as was life on Earth for most of its history

• Multicellular organisms did not appear on the scene until about 650 MY at the beginning of the Ediacaran period

FAMILY TREE OF LIFE

• Discovery of DNA was first published by Zuckerkandl and Pauling in 1965

• Genetic information required to maintain and propagate species is encoded in their DNA

• DNA code can now be analyzed

• The Human Genome Project has done that for our own species

Computer model of spiral structure of partof DNA molecule

SPECIES RELATIONSHIPS• Biologists have long used similarities

in appearance between species to study their relationships and to classify them but these classifications were often misleading

• DNA studies now enables us to study the evolutionary relationship between species

• Humans and chimpanzees are very closely related because 99 % of their DNA code is identical

NUCLEAR AND MITOCHONDRIALDNA

• Relationships between species are usually studied by comparing mito-chondrial DNA because it has a much simpler structure and is therefore easier to analyze

• Mitochondrial DNA occurs in all plants and animals and, unlike nuclear DNA is passed on from the mother to both sons and daughters

HYPERTHERMOPHILES

• They are an early form of life and are our earliest identifiable common ancestor

• They grow and reproduce best at temperatures of more than 80°C

• The highest temperatures at which some of these organisms can survive may be as much as 150°C

Hot spring at Yellowstone National Park – a favourite haunt of hyperthermophiles

EXTREMOPHILES

• Microscopic forms of life that can live under extreme environmental conditions

• Hyperthermophiles are only one group coping with extreme temperatures

• They have been found in hot springs in volcanic areas and “black smokers” (hydrothermal vents) on the floor of tne oceans

Hydrothermal vent “black smoker”on the ocean floor

EXTREMOPHILES

• They began to be discovered in the 1980’s

• They have been found in caves, in lakes buried under Antarctic ice, the deepest parts of the oceans and up to 600 meters into basalt rock underlying the sea floor at a depth of more than 2,600 meters

• Most are unicellular and belong to the domains of Archaea and Bacteria

EXTREMOPHILES• Apart from temperature extremes they

may be adapted to:• Highly acid or alkaline conditions• Anaerobic conditions• Highly saline conditions• Tolerant of high concentrations of

heavy metals• Tolerant of very high pressures• Highly resistant to nuclear and UV

radiation

PANSPERMIA

• It is the hypothesis that some primitive life forms, such as extremophiles, can survive the effects of space

• If they were trapped in debris such as meteorites, they could be transported from one planet to another

• In this way life could be transported from Mars to Earth or vice versa

Artist’s impression of a meteorite transporting a bacterial life form through space to Earth

FOSSIL EVIDENCE FOR LIFE• Finding fossil evidence of life on Mars

is made more difficult by the fact that we are probably looking for fossils of single-celled organisms to small to be seen with the naked eye

• Until recently, geologists believed that fossil evidence on Earth began at 650 MY when multi-cellular organisms first appeared

MICROSCOPIC EVIDENCE• Microscopic evidence for early life has

only been found in the last decades• The Australian palaeontologist Malcolm

Walter has been a pioneer working for NASA

• The Pilbara region of WA has some of the oldest sedimentary rocks on Earth

• A well known fossil locality in the area is North Pole, named by early miners

STROMATOLITES

• They are structures built in shallow water by colonial cyanobacteria

• They were the first organisms to develop a form of photosynthesis

• Their life processes caused the deposition of calcium carbonate to form the world’s first limestone reefs

• In a few localities with hypersaline water they are still growing today

Stromatolite colonies at Shark Bay, WA

DETECTING UNI-CELLULARORGANISMS

• On Earth cell structures are sometimes preserved in rocks called cherts and can be observed under the microscope

• Another way to identify early life forms is to examine the isotopic ratios of elements such as carbon and sulphur that are modified by biological processes – such clues are known as biomarkers

• Both methods can be applied on Mars

Microscopic fossils in West Australian chert