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The cycling of carbon between organic and inorganic forms controls levels of O2 in the global environment. Through its relationships to CO2 and CH4, both greenhouse gases, the carbon cycle also influences climate. For such reasons, geoscientists are broadly interested in understanding processes within the carbon cycle, both now and in ancient times. Microbial catalysis is pervasive, so organic geochemists—who once focused largely on petroleum—are taking a great interest in recognizing and interpreting molecular fossils, or “biomarkers,” that reflect the presence and activities of specific microorganisms. As they search, they occasionally come across products that surprise everyone. The report by Stefan Schouten et al. in this issue of PNAS is a recent and particularly interesting example (1). By using a new technique applicable to lipids containing as many as 86 carbon atoms, they found molecules that expand the range of known microbial products. Moreover, they have discovered some carbon skeletons in which archaeal and bacterial structural elements are combined. This work is intriguing because biomarkers are in some cases very specific to a particular microorganism. A few species of haptophyte algae, for example, produce methyl ketones with 37 carbon atoms and 2, 3, or 4 double bonds (2). These alkenones can comprise up to 5% by weight of the biomass, and the number of double bonds varies systematically with the temperature of growth (3). Fortunately for geochemists, …

Lipids as a common interest of microorganisms and geochemists