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 years.