Carbon isotopic fractionation in lipids from methanotrophic bacteria: Relevance for interpretation of the geochemical record of biomarkers

• Published in: Geochimica et cosmochimica acta Vol. 58; no. 13; pp. 2853 - 2863
• Main Authors: Summons, Roger E., Jahnke, Linda L., Roksandic, Zarko
• Format: Journal Article
• Language: English
• Published: Ames Research Center Elsevier Ltd 01.07.1994
• Subjects: 02 PETROLEUM; 550700 > Microbiology; BACTERIA; BASIC BIOLOGICAL SCIENCES; BIOLOGICAL MARKERS; BIOLOGICAL PATHWAYS; Biomass; Carbon - metabolism; CARBON 13; CARBON ISOTOPES; Chemical Fractionation; Copper - metabolism; Environmental Microbiology; EVEN-ODD NUCLEI; Exobiology; Geologic Sediments - analysis; Geologic Sediments - chemistry; Geologic Sediments - microbiology; ISOTOPE RATIO; ISOTOPE SEPARATION; ISOTOPES; LIGHT NUCLEI; Lipid Metabolism; Lipids - analysis; Lipids - biosynthesis; Methane - analysis; Methane - metabolism; Methanol - analysis; Methanol - metabolism; METHANOTROPHIC BACTERIA; Methylococcaceae - chemistry; Methylococcaceae - enzymology; Methylococcaceae - growth & development; Methylococcaceae - metabolism; MICROORGANISMS; NUCLEI; Oxygen - analysis; Oxygenases - metabolism; Paleontology; Ribulose-Bisphosphate Carboxylase - metabolism; SEPARATION PROCESSES; Space life sciences; STABLE ISOTOPES 020200 > Petroleum > Reserves, Geology, & Exploration; Sterols - analysis; Sterols - biosynthesis; Sterols - metabolism; Triterpenes - analysis; Triterpenes - chemistry; Triterpenes - metabolism; Nasa Discipline Exobiology; Nasa Center Arc; NASA Center ARC; NASA Discipline Exobiology
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/0016-7037(94)90119-8

Experiments with cultured aerobic methane oxidising bacteria confirm that their biomarker lipids will be significantly depleted in 13C 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 13C fractionation of approximately 30%. 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%. 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%. compared to the substrate. Additional strong 13C fractionation takes place during polyisoprenoid biosynthesis in methanotrophs. The δ 13 C values of individual hopanoid and steroid biomarkers produced by these organisms were as much as 10%. more negative than total biomass. In individual cultures, squalene was 13C-enriched by as much as 14%. 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%. difference between the hopane and 3 β-methylhopane skeletons. Hopane and sterane polyisoprenoids were also 13C-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.