Mono-, di- and trimethyl-branched alkanes in cultures of the filamentous cyanobacterium Calothrix scopulorum

The extractable lipids of the cyanobacterium Calothrix scopulorum contain a complex suite of mono-, di- and trimethylheptadecanes. We identified 21 isomers; most of them have not been reported previously. The major compounds are 4,13-, 5,13-, and 4,5-dimethyl-, and 4,5,13- and 4,5,14-trimethylheptadecane. Methyl substitution occurs preferentially, but not exclusively, at positions 4 and 5 (or ω-4 and ω-5) at either end of the carbon chain. In mass spectra, the location of methyl groups at opposite ends of the carbon chain can be deduced from the presence of a characteristic secondary CnH2n−1+ fragment ion. The interpretation of mass spectra was confirmed by calculated retention index values which closely match the measured data. Our data provide evidence that branched alkanes with 17 to 20 carbon atoms can be used as biomarkers for cyanobacterial contributions to sedimentary organic matter.