ALL CELLS ARE BOUND BY A LIPID MEMBRANE THAT SEPARATES THE INTERIOR OF THE CELL FROM THE EXTERIOR ENVIRONMENT WHICH IS A UNIVERSAL CHARACTERISTIC OF ALL KNOWN LIFE ON EARTH. SOME TYPES OF MEMBRANE LIPIDS ARE FOUND ONLY IN SPECIFIC GROUPS OF ORGANISMS AND HISTORICALLY HAVE BEEN USED AS BIOMARKERS FOR THESE GROUPS. THE CARBON SKELETONS OF BRANCHED OR CYCLIC LIPIDS ARE MORE RESISTANT TO DEGRADATION AFTER CELL DEATH AND CAN SURVIVE IN THE ROCK RECORD FOR BILLIONS OF YEARS. SUCH GEOLIPIDS PROVIDE IMPORTANT FOSSIL INFORMATION AND CAN HELP RECONSTRUCT THE RECORD OF PAST MICROBIAL ECOSYSTEMS ON THE EARLY EARTH AND POTENTIALLY OTHER PLANETARY BODIES. PREVIOUS LIPID BIOMARKER WORK HAS LARGELY BEEN DIRECTED AT ESTABLISHING TAXONOMIC PROXIES BY IDENTIFYING COMPOUNDS IN PURE CULTURES AND ENVIRONMENTAL SAMPLES. BUT RECENT MOLECULAR WORK IDENTIFYING THE GENES INVOLVED IN THE BIOSYNTHESIS OF GEOLOGICALLY RELEVANT LIPIDS SUCH AS HOPANOIDS HAS OPENED THE DOOR TO SURVEYING GENOMIC AND METAGENOMIC DATA SETS FOR THE ABILITY TO SYNTHESIZE THESE COMPOUNDS. THIS WORK HAS REVEALED A MUCH BROADER DISTRIBUTION OF THE GENES THAN WAS PREVIOUSLY REALIZED VIA THE PURE CULTURE STUDIES. THIS DISCOVERY HAS NECESSITATED A SHIFT FROM TAXONOMIC STUDIES TO INVESTIGATING THE CELLULAR FUNCTION OF THESE COMPOUNDS. THE GOAL IS TO LINK THE BIOLOGICAL FUNCTION OF THE LIPID BIOMARKERS TO SPECIFIC ENVIRONMENTAL CONDITIONS OR BIO(GEO)CHEMICAL PROCESSES. SUCH A RELATIONSHIP WILL OFFER NEW CRITERIA FOR INTERPRETING THE ROCK RECORD.

The Leland Stanford Junior University

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