CARBON ISOTOPIC FRACTIONATION IN LIPIDS FROM METHANOTROPHIC BACTERIA - RELEVANCE FOR INTERPRETATION OF THE GEOCHEMICAL RECORD OF BIOMARKERS

Experiments with cultured aerobic methane oxidising bacteria confirm t hat their biomarker lipids will be significantly depleted in C-13 comp ared to the substrate. The methanotrophic bacteria Methylococcus capsu latus and Methylomonas methanica, grown on methane and using the RuMP cycle for carbon assimilation, show maximum C-13 fractionation of appr oximately 30 parts per thousand in the resultant biomass. In M. capsul atus, the maximum fractionation is observed in the earliest part of th e exponential growth stage and decreases to approximately 16 parts per thousand as cells approach stationary phase. This change may be assoc iated with a shift from the particulate form to the soluble form of th e methane monooxygenase enzyme. Less than maximum fractionation is obs erved when cells are grown with reduced methane availability. Biomass of M. capsulatus grown on methanol was depleted by 9 parts per thousan d compared to the substrate. Additional strong C-13 fractionation take s place during polyisoprenoid biosynthesis in methanotrophs. The delta C-13 values of individual hopanoid and steroid biomarkers produced by these organisms were as much as 10 parts per thousand more negative th an total biomass. In individual cultures, squalene was C-13-enriched b y as much as 14 parts per thousand compared to the triterpane skeleton of bacteriohopaneaminopentol. Much of the isotopic dispersion in lipi d metabolites could be attributed to shifts in their relative abundanc es, combined with an overall reduction in fractionation during the gro wth cycle. In cells grown on methanol, where there was no apparent eff ect of growth stage on overall fractionation there were still signific ant isotopic differences between closely related lipids including a 5. 3 parts per thousand difference between the hopane and 3beta-methylhop ane skeletons. Hopane and sterane polyisoprenoids were also C-13-deple ted compared to fatty acids. These observations have significant impli cations for the interpretation of specific compound isotopic signature s now being measured for hydrocarbons and other lipids present in sedi ments and petroleum. In particular, biomarker lipids produced by a sin gle organism do not necessarily have the same carbon isotopic composit ion.