Cyanobacterial Bioamarkers: Triterpenoids plus Steroids?

Reconstructing the biomarker record of the early Earth requires us to make assumptions about the constancy and specificity of lipid biosynthetic pathways through geological time and rests upon our knowledge of the lipid composition of extant organisms.\x90\x90 For example, the three domains of extant life Eukarya, Archaea and Bacteria were present in Archean ecosystems if we are to believe the record of chemical fossils that have been found in rocks as old as 2700Ma.\x90 However, the validity of the steroid signature as an indicator for Eukarya has questioned because of some reports of Ophytosterol, biosynthesis in cyanobacteria and we examined this issue in some new experiments with cyanobacteria grown in culture.\x90 After initial cultures of Phormidium and Chlorogloeopsis proved to contain sterols, in addition to the expected complement of hopanoid and 2-methylhopanoid, we hypothesized that these might be the result of fungal contamination.\x90 Subsequent cultures grown after repeated sub-culturing in the presence of cycloheximide proved to be devoid of sterol. Our results are therefore consistent with the notion that sterol biosynthesis is, with very few exceptions, a primary characteristic of Eukarya.\x90 Consequently, the steroid hydrocarbons that are found in ancient sediments are most likely the remains of algae and other eukaryotes.\x90