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 that their biomarker lipids will be significantly depleted in C-13 compared to the substrate. The methanotrophic bacteria Methylococcus capsulatus and Methylomonas methanica, grown on methane and using the RuMP cycle for carbon assimilation, show maximum C-13 fractionation of approximately 30 parts per thousand in the resultant biomass. In M. capsulatus, the maximum fractionation is observed in the earliest part of the exponential growth stage and decreases to approximately 16 parts per thousand as cells approach stationary phase. This change may be associated with a shift from the particulate form to the soluble form of the methane monooxygenase enzyme. Less than maximum fractionation is observed when cells are grown with reduced methane availability. Biomass of M. capsulatus grown on methanol was depleted by 9 parts per thousand compared to the substrate. Additional strong C-13 fractionation takes place during polyisoprenoid biosynthesis in methanotrophs. The deltaC-13 values of individual hopanoid and steroid biomarkers produced by these organisms were as much as 10 parts per thousand more negative than total biomass. In individual cultures, squalene was C-13-enriched by as much as 14 parts per thousand compared to the triterpane skeleton of bacteriohopaneaminopentol. Much of the isotopic dispersion in lipid metabolites could be attributed to shifts in their relative abundances, combined with an overall reduction in fractionation during the growth cycle. In cells grown on methanol, where there was no apparent effect of growth stage on overall fractionation there were still significant isotopic differences between closely related lipids including a 5.3 parts per thousand difference between the hopane and 3beta-methylhopane skeletons. Hopane and sterane polyisoprenoids were also C-13-depleted compared to fatty acids. These observations have significant implications for the interpretation of specific compound isotopic signatures now being measured for hydrocarbons and other lipids present in sediments and petroleum. In particular, biomarker lipids produced by a single organism do not necessarily have the same carbon isotopic composition.