Exploring the limits of the microbial world
using biogeochemical signatures
Roger E. Summons
Massachusetts Institute of Technology
Wednesday, December 19, 2001
3:00 p.m.–Pacific Forum
Microscopic organisms are often overlooked as agents of global change.
Most microbes leave no visible record of their former 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 molecular methods, and
others are the analyses of diagnostic lipids from living, recently dead,
and fossilized organisms.
Biological marker compounds (biomarkers), which comprise the lipids of
extant organisms and their hydrocarbon (fossilized) counterparts, carry
diagnostic information in their chemical structures and in their carbon
and hydrogen isotopic compositions. Particularly good examples of such
compounds are bacteriohopanes, which can be used to evaluate processes
such as cyanobacterial photosynthesis and methanotrophy in modern
environments and ancient sediments. Biomarkers also tell us about the
inhabitants of today’s extreme environments—such as hydrothermal vents—and
are proving an effective means to learn about microbes that inhabited the
early Earth and left their remains trapped in rocks as old as 2.7 billion