ANAEROBIC BIOSYNTHESIS OF HOPANOIDS BY A MICROBE PREVALENT IN ANOXIC ENVIRONMENTS

Hopanoids, a class of pentacyclic triterpenoid lipid biomarkers, are abundant in ancient sediments and oils. For the past twenty years, biomarker studies have worked under the assumption that hopanoids are only produced by aerobic organisms. To explicitly test this assumption we searched publicly-available genomic databases using squalene-hopene cyclase (a fundamental enzyme responsible for hopanoid biosynthesis) sequences from known hopanoid producers to find an obligately anaerobic candidate organism capable of producing hopanoids. Here we present evidence that Geobacter sulfurreducens, a bacterium common in anoxic environments, has the appropriate genes for hopanoid biosynthesis and produces a wide variety of complex hopanoids under strictly anaerobic conditions. It is likely that at least some sedimentary hopanoids originated within anoxic sediments and environments. These findings may help resolve a conundrum raised by several biomarker data sets concerning isotopically-depleted hopanoids found in environments mediating the anaerobic oxidation of methane. We point out that isotopically-depleted hopanoids do not necessarily indicate methane hydrate destabilization and the catastrophic release of methane into the water column. Our work also questions one argument for the early appearance of oxygen and aerobic organisms during the late Archean. The biosynthesis of complex hopanoids does not require molecular oxygen and therefore may have evolved before free oxygen appeared in surface environments.