Biosignatures of Cellular Components and Metabolic Activity

The astrobiological search for biosignatures requires a working concept of the fundamental attributes of life. Life’s basic capabilities of energy harvesting, metabolism, and self-replication can create objects, substances and patterns—biosignatures—that indicate their biological origins. High relative abundances of certain lipids, hydrocarbons, amino acids and polysaccharides are diagnostic products of billions of years of evolution. Lipid assemblages having narrow molecular weight ranges are key constituents of cellular membranes. The molecular structures of lipids provide details of their biosynthetic pathways. Some lipid biosignatures are diagnostic for particular groups of microorganisms. The biosynthesis of organic matter and biochemical oxidation-reduction reactions can discriminate against the heavier isotopes of carbon and sulfur and thereby create molecular isotopic patterns that indicate not only their biological origins, but also key details about biosynthetic pathways. Sulfur isotopic patterns can indicate biological redox reactions. Because microorganisms can greatly enhance, at relatively low to moderate temperatures, reaction rates between oxidized and reduced sulfur compounds, they can create a range in the stable isotopic compositions of these compounds that is substantially larger than nonbiological reactions can achieve under similar conditions. The simultaneous presence of multiple biosignature objects, substances and patterns in a demonstrably habitable earlier environment constitutes the most compelling evidence of past life.