Occurrence of tetraester and mixed ether/ester-bound iso-diabolic acid membrane-spanning lipids in mineral soils

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a suite of membrane lipids that are widely used as empirical proxies for past temperature and pH. Although the stereochemistry of their glycerol moiety suggests that they are produced by bacteria, the exact producers and the biosynthetic pathway of brGDGTs remain unclear. Here we report the occurrence of tetraester and mixed ester/ether membrane-spanning lipids with a backbone consisting of iso-diabolic acid (iso-DA) containing up to two additional methyl groups in mineral soils from Nepal and Rwanda. These compounds are presumed intermediate products during brGDGT synthesis but had not been detected in cultures or the environment before. Interestingly, while acid hydrolysis of the polar fraction releases iso-DA in the soil from Nepal, monoalkyl glycerol ethers with iso-C15 and iso-C17 chains are released in the soil from Rwanda. These results support both current hypotheses that brGDGT synthesis can occur via tail-to-tail condensation of two iso-C15 fatty acids to form iso-DA, as well as through the reduction of diesters to diethers and subsequent carbon-carbon linking as shown to occur during the synthesis of archaeal GDGTs.