oa Novel Cyclic Aliphatic Hydrocarbons as Potential Dia- and Catagenetic Products of Ladderane Lipids

The contribution of anammox to ancient nitrogen biogeochemical cycles cannot currently be considered because the most characteristic lipid biomarkers of anammox bacteria, ladderane lipids, do not reach the catagenic window in the sedimentary record. Here, we describe novel cyclic hydrocarbons produced by the simulated thermal maturation of anammox bacterial biomass. Alkyl branched tricyclic hydrocarbons were present in oils generated by hydrous pyrolysis experiments at temperatures >260 °C. A representative product was isolated by preparative gas chromatography but was shown to be composed of two co-eluting components. Its structure was partially identified, by way of two-dimensional nuclear magnetic resonance (NMR), to be a dodecane with the three concatenated rings comprised of 7, 4, and 5 carbons. The isolated components had a stable carbon isotopic composition similarly depleted in 13C as the fatty acids of the original anammox biomass. Molecular mechanic calculations were used to generate the relative structural stability of stereoisomers and the possible conversion steps to the formation of these components. These components were not detected in thermally mature sediment and rocks screened using conventional GC-MS analysis, but perhaps using more advanced analytical methods these lipids may be useful biomarkers for assessing past anammox in ancient sediments and crude oils.