Astrobiology Workshop, Macquarie University July 12-13 2001

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Exploring the Limits of the Microbial World using Biogeochemical Signatures

Roger Summons (Australian Geolgical Survey Organization)

Microscopic organisms are often overlooked as agents of global change. Microbes are geologically significant in that they have helped shape and reshape the Earth's external surface continuously for 3.8 billion years or more. They are essential for maintaining this planet in a habitable state and yet we are only just discovering the true extent of their domain. Most microbes leave no visible record of their presence and we can only infer their involvement in geological processes from chemical clues left in the ocean, the atmosphere and rocks. Molecular methods are providing new ways to track microbiologically driven geological processes. DNA cloned from living microbes is one component of this and analyses of diagnostic lipids of living, recently dead and fossilised organisms is another. Biological marker compounds (biomarkers), which comprise the lipids of extant organisms and their hydrocarbon (i.e. fossil) counterparts carry diagnostic information in their chemical structures and in their carbon and hydrogen isotopic compositions. Particularly good examples of such diagnostic compounds are 2-methyl- and 3-methylbacteriohopanes which are diagnostic for oxygenic photosynthesis and for methane oxidation respectively. These biomarkers can be used to evaluation photosynthesis and methanotrophy in modern environments and ancient sediments. Biomarkers also tell us about the inhabitants of today's extreme environments, for example, hydrothermal vents, and are proving an effective means to learn about microbes that inhabited the early Earth and whose remains are found trapped in rocks as old as 2.7 billion years.