Carbon stable isotope patterns of cyclic terpenoids: A comparison of cultured alkaliphilic aerobic methanotrophic bacteria and methane-seep environments. (January 2020)

• Record Type: Journal Article
• Authors: Cordova-Gonzalez, Alexmar; Birgel, Daniel; Kappler, Andreas; Peckmann, Jörn
• Is Part Of: Organic geochemistry. Volume 139(2020)
• Publication Date: 2020-01
• Subjects: Aerobic methanotrophic bacteria -- Methylomicrobium -- Methane seeps -- Stable carbon isotopes -- Cyclic terpenoids -- Tetrahymanol -- Hopanoids -- Steroids -- Bacteriohopanepolyols; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2019.103940
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: δ 13 C composition of cyclic terpenoids of M. kenyense and M. alcaliphilum is reported. Average Δδ 13 Cterpenoid-methane is −25‰ for M. kenyense and −16‰ for M. alcaliphilum. 13 C-depleted tetrahymanol is a biomarker of aerobic methanotrophs at seeps. Δδ 13 Cterpenoid-methane characterizes types of aerobic methanotrophy at seeps. Abstract: Aerobic methanotrophic bacteria are known to synthesize a variety of cyclic terpenoids which are typified by 13 C-depleted, methane-derived carbon. This peculiarity facilitates identification of methanotroph biomarkers in natural samples. However, the current biomarker database does not always allow biomarker patterns of marine samples to be assigned to the different types of aerobic methanotrophs. To overcome this shortcoming, the carbon stable isotope composition of cyclic terpenoids of two strains of the Type I methanotroph genus Methylomicrobium was analyzed. Other than aerobic methanotrophs used for biomarker studies in the past, these two strains deriving from soda lake environments are able to tolerate the conditions typifying marine environments including high alkalinity and salinity. The cyclic terpenoid inventory of the two strains comprises 4-methyl steroids, 3-methyl- and desmethyl bacteriohopanepolyols (aminotetrol and aminotriol), and tetrahymanol, all of which are 13 C-depleted. The average carbon isotope fractionation between methane and the respective lipid (Δδ 13 Cterpenoid-methane ) is found to be −25‰ for M. kenyense and −16‰ for M. alcaliphilum . These data shed new light on the previously reported compound and carbon stable isotope patterns of cyclic terpenoids from methane-seep environments. Particularly, 13 C-depleted tetrahymanol and gammacerane are reinterpreted as biomarkers of aerobic methanotrophic bacteria based on their occurrence in methane-seep deposits in association with other biomarkers of aerobic methanotrophs. The use of δ 13 C values of anaerobic methane-oxidizing archaea (ANME) lipids for the reconstruction of the isotopic composition of parent methane allows us to calculate the Δδ 13 Cterpenoid-methane even for ancient seep environments. With this calculation, Type I and Type II methanotrophs can be discriminated, representing a new approach to better characterize past methanotrophy at seeps and possibly other marine environments.